<?xml version="1.0" encoding="UTF-8"?>
<chapter id="plugin-implement">
    <!-- jEdit buffer-local properties: -->
    <!-- :xml.root=users-guide.xml: -->
    <!-- :maxLineLen=90:wrap=soft: -->
    <title>Implementing a Simple Plugin</title>
    <!-- jEdit Plugin Guide, (C) 2001-2013 John Gellene, Alan Ezust      -->


    <para>There are many applications for the leading operating systems that
    provide a <quote>scratch-pad</quote> or <quote>sticky note</quote> facility
    for the desktop display. A similar type of facility operating within the
    jEdit display would be a convenience. The use of dockable windows would
    allow the notepad to be displayed or hidden with a single mouse click or
    keypress (if a keyboard shortcut were defined). The contents of the notepad
    could be saved at program exit (or, if earlier, deactivation of the plugin)
    and retrieved at program startup or plugin activation.</para>

    <para>We will keep the capabilities of this plugin modest, but a few other
    features would be worthwhile. The user should be able to write the contents
    of the notepad to storage on demand. It should also be possible to choose
    the name and location of the file that will be used to hold the notepad
    text. This would allow the user to load other files into the notepad
    display. The path of the notepad file should be displayed in the plugin
    window, but will give the user the option to hide the file name. Finally,
    there should be an action by which a single click or keypress would cause
    the contents of the notepad to be written to the new text buffer for further
    processing.</para>

    <para>The full source code for QuickNotepad is contained in jEdit's source
    code distribution. We will provide excerpts in this discussion where it is
    helpful to illustrate specific points. You are invited to obtain the source
    code for further study or to use as a starting point for your own
    plugin.</para>

    <section id="plugin-load">
        <title><indexterm>
                <primary>Plugin API</primary>

                <secondary>loading at startup</secondary>
            </indexterm> How Plugins are Loaded</title>

        <para>We will discuss the implementation of the
        <application>QuickNotepad</application> plugin, along with the jEdit
        APIs it makes use of. But first, we describe how plugins are
        loaded.</para>

        <para>As part of its startup routine, jEdit's <function>main</function>
        method calls various methods to load and initialize plugins.</para>

        <para>Additionally, plugins using the jEdit 4.2 plugin API can be loaded
        and unloaded at any time. This is a great help when developing your own
        plugins -- there is no need to restart the editor after making changes
        (see <xref linkend="plugin-implement-reloading" /> ).</para>

        <para>Plugins are loaded from files with the <filename>.jar</filename>
        filename extension located in the <filename>jars</filename>
        subdirectories of the jEdit installation and user settings directories
        (see <xref linkend="settings-directory" />).</para>

        <para>For each JAR archive file it finds, jEdit scans its entries and
        performs the following tasks:</para>

        <itemizedlist>
            <listitem>
                <para>Adds to a collection maintained by jEdit a new object of
                type <ulink url="../api/org/gjt/sp/jedit/PluginJAR.html">
                <classname>PluginJAR</classname></ulink>. This is a data
                structure holding the name of the JAR archive file, a reference
                to the <ulink url="../api/org/gjt/sp/jedit/JARClassLoader.html">
                <classname>JARClassLoader</classname></ulink>, and a collection
                of plugins found in the archive file.</para>
            </listitem>

            <listitem>
                <para>Loads any properties defined in files ending with the
                extension <filename>.props</filename> that are contained in the
                archive. See <xref
                linkend="plugin-implement-properties" />.</para>
            </listitem>

            <listitem>
                <para>Reads action definitions from any file named
                <filename>actions.xml</filename> in the archive (the file need
                not be at the top level). See <xref
                linkend="plugin-implement-actions" />.</para>
            </listitem>

            <listitem>
                <para>Parses and loads the contents of any file named
                <filename>dockables.xml</filename> in the archive (the file need
                not be at the top level). This file contains BeanShell code for
                creating docking or floating windows that will contain the
                visible components of the plugin. Not all plugins define
                dockable windows, but those that do need a
                <filename>dockables.xml</filename> file. See <xref
                linkend="plugin-implement-dockables" />.</para>
            </listitem>

            <listitem>
                <para>Checks for a class name with a name ending with
                <filename>Plugin.class</filename>.</para>

                <para>Such a class is known as a <firstterm>plugin core
                class</firstterm> and must extend jEdit's abstract <ulink
                url="../api/org/gjt/sp/jedit/EditPlugin.html">
                <classname>EditPlugin</classname></ulink> class.</para>

                <para>The initialization routine checks the plugin's properties
                to see if it is subject to any dependencies. For example, a
                plugin may require that the version of the Java runtime
                environment or of jEdit itself be equal to or above some
                threshold version. A plugin can also require the presence of
                another plugin.</para>

                <para>If any dependency is not satisfied, the loader marks the
                plugin as <quote>broken</quote> and logs an error
                message.</para>
            </listitem>
        </itemizedlist>

        <para>After scanning the plugin JAR file and loading any resources, a
        new instance of the plugin core class is created and added to the
        collection maintained by the appropriate <ulink
        url="../api/org/gjt/sp/jedit/PluginJAR.html">
        <classname>PluginJAR</classname></ulink>. jEdit then calls the
        <function>start()</function> method of the plugin core class. The
        <function>start()</function> method can perform initialization of the
        object's data members. Because this method is defined as an empty
        <quote>no-op</quote> in the <ulink
        url="../api/org/gjt/sp/jedit/EditPlugin.html">
        <classname>EditPlugin</classname></ulink> abstract class, a plugin need
        not provide an implementation if no unique initialization is
        required.</para>

    </section>

    <section id="plugin-implement-quicknotepadplugin">
        <title>The QuickNotepadPlugin Class</title>

        <para>The major issues encountered when writing a plugin core class
        arise from the developer's decisions on what features the plugin will
        make available. These issues have implications for other plugin elements
        as well.</para>

        <itemizedlist>
            <listitem>
                <para>Will the plugin provide for actions that the user can
                trigger using jEdit's menu items, toolbar buttons and keyboard
                shortcuts?</para>
            </listitem>

            <listitem>
                <para>Will the plugin have its own visible interface?</para>
            </listitem>

            <listitem>
                <para>Will the plugin have settings that the user can
                configure?</para>
            </listitem>

            <listitem>
                <para>Will the plugin respond to any messages reflecting changes
                in the host application's state?</para>
            </listitem>

            <listitem>
                <para>Should the plugin do something special when it gets
                focus?</para>
            </listitem>
        </itemizedlist>

        <para>Recall that the plugin core class must extend <ulink
        url="../api/org/gjt/sp/jedit/EditPlugin.html">
        <classname>EditPlugin</classname></ulink>. In QuickNotepad's plugin core
        class, there are no special initialization or shutdown chores to
        perform, so we will not need a <function>start()</function> or
        <function>stop()</function> method.</para>

        <para>The resulting plugin core class is lightweight and straightforward
        to implement:</para>

        <itemizedlist>
            <listitem>
                <informalexample>
                    <programlisting>public class QuickNotepadPlugin extends EditPlugin {
  public static final String NAME = "quicknotepad";
  public static final String OPTION_PREFIX = "options.quicknotepad.";
}
</programlisting>
                </informalexample>

                <para>The class has been simplified since 4.1, and all we
                defined here were a couple of <classname>String</classname> data
                members to enforce consistent syntax for the name of properties
                we will use throughout the plugin.</para>
            </listitem>

            <listitem>
                <para>These names are used in <filename>actions.xml</filename>
                for each of the menu choices. This file is discussed in more
                detail in <xref linkend="plugin-implement-actions" />. Each
                action is a beanshell script.</para>

                <informalexample>
                    <programlisting>
&lt;!DOCTYPE ACTIONS SYSTEM "actions.dtd"&gt;
&lt;ACTIONS&gt;
  &lt;ACTION NAME="quicknotepad.choose-file"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).chooseFile();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;

  &lt;ACTION NAME="quicknotepad.save-file"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).saveFile();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;

  &lt;ACTION NAME="quicknotepad.copy-to-buffer"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).copyToBuffer();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;
&lt;/ACTIONS&gt;
</programlisting>
                </informalexample>
            </listitem>

            <listitem>
                <para>The names also come up in the properties file,
                <filename>QuickNotePad.props</filename> file. The properties
                define option panes and strings used by the plugin. It is
                explained in more detail in <xref
                linkend="plugin-implement-properties" /> and the <ulink
                url="../api/org/gjt/sp/jedit/EditPlugin.html">
                <classname>EditPlugin</classname></ulink> API docs.</para>

                <informalexample>
                    <programlisting>
# jEdit only needs to load the plugin the first time the user accesses it
# the presence of this property also tells jEdit the plugin is using the new API
plugin.QuickNotepadPlugin.activate=defer

# Even if you don't store additional files, this is a good idea to set:
plugin.QuickNotepadPlugin.usePluginHome=true

# Required for all plugins:
plugin.QuickNotepadPlugin.name=QuickNotepad
plugin.QuickNotepadPlugin.author=John Gellene

# version number == jEdit version number
plugin.QuickNotepadPlugin.version=4.5

# online help
plugin.QuickNotepadPlugin.docs=index.html

# we only have one dependency, jEdit 4.5
# See jEdit.getBuild() to understand version numbering scheme.
plugin.QuickNotepadPlugin.depend.0=jedit 4.05.99.00

# quicknotepad's plugin menu - a list of actions or separators
plugin.QuickNotepadPlugin.menu=quicknotepad \
  - \
  quicknotepad.choose-file \
  quicknotepad.save-file \
  quicknotepad.copy-to-buffer

# action labels for actions supplied by dockables.xml
quicknotepad.label=QuickNotepad

# action labels for actions supplied by actions.xml
quicknotepad.choose-file.label=Choose notepad file
quicknotepad.save-file.label=Save notepad file
quicknotepad.copy-to-buffer.label=Copy notepad to buffer

# plugin option pane
plugin.QuickNotepadPlugin.option-pane=quicknotepad

# Option pane activation BeanShell snippet
options.quicknotepad.code=new QuickNotepadOptionPane();

# Option pane labels
options.quicknotepad.label=QuickNotepad
options.quicknotepad.file=File:
options.quicknotepad.choose-file=Choose
options.quicknotepad.choose-file.title=Choose a notepad file
options.quicknotepad.choose-font=Font:
options.quicknotepad.show-filepath.title=Display notepad file path

# window title
quicknotepad.title=QuickNotepad

# window toolbar buttons
quicknotepad.choose-file.icon=Open.png
quicknotepad.save-file.icon=Save.png
quicknotepad.copy-to-buffer.icon=CopyToBuffer.png

# default settings
options.quicknotepad.show-filepath=true
options.quicknotepad.font=Monospaced
options.quicknotepad.fontstyle=0
options.quicknotepad.fontsize=14

# Setting not defined but supplied for completeness
options.quicknotepad.filepath=
</programlisting>
                </informalexample>
            </listitem>
        </itemizedlist>
    </section>

    <section id="plugin-implement-properties">
        <title>The Property Files</title>

        <para>jEdit maintains a list of <quote>properties</quote>, which are
        name/value pairs used to store human-readable strings, user settings,
        and various other forms of meta-data. During startup, jEdit loads the
        default set of properties, followed by plugin properties stored in
        plugin JAR files, finally followed by user properties.</para>

        <para>Some properties are used by the plugin API itself. Others are
        accessed by the plugin using methods in the <ulink
        url="../api/org/gjt/sp/jedit/jEdit.html">
        <classname>jEdit</classname></ulink> class. Others are accessed by the
        scripts used by plugin packagers <footnote>
                <para>See the <guimenuitem>Macros/Properties/Create Plugin
                Announcement</guimenuitem> macro for an example.</para>
            </footnote>.</para>

        <para>Property files contained in plugin JARs must end with the filename
        extension <filename>.props</filename>, and have a very simple syntax,
        which the following example illustrates:</para>

        <informalexample>
            <programlisting># Lines starting with '#' are ignored.
name=value
another.name=another value
long.property=Long property value, split over \
    several lines
escape.property=Newlines and tabs can be inserted \
    using the \t and \n escapes
backslash.property=A backslash can be inserted by writing \\.</programlisting>
        </informalexample>

        <para>Now we look at a fragment from the
        <filename>QuickNotepad.props</filename> file <footnote>
                <para>Examine the actual file for a more complete example</para>
            </footnote> which contains properties for the QuickNotepad plugin.
        The first type of property data is information about the plugin itself;
        these are the only properties that must be specified in order for the
        plugin to load:</para>

        <informalexample>
            <programlisting># general plugin information
plugin.QuickNotepadPlugin.activate=defer
plugin.QuickNotepadPlugin.usePluginHome=true
plugin.QuickNotepadPlugin.name=QuickNotepad
plugin.QuickNotepadPlugin.author=John Gellene
plugin.QuickNotepadPlugin.version=4.5
plugin.QuickNotepadPlugin.docs=QuickNotepad.html
# depends on jEdit 4.5
plugin.QuickNotepadPlugin.depend.0=jedit 04.05.99.00
plugin.QuickNotepadPlugin.description=A demo jEdit plugin that provides a notepad dockable.
plugin.QuickNotepadPlugin.longdescription=description.html
</programlisting>
        </informalexample>

        <para>These properties are each described in detail in the documentation
        for the <ulink url="../api/org/gjt/sp/jedit/EditPlugin.html">
        <classname>EditPlugin</classname></ulink> class and do not require
        further discussion here.</para>

        <para>Next in the file comes a property that sets the title of the
        plugin's dockable window. Dockable windows are discussed in detail in
        <xref linkend="plugin-implement-dockables" />.</para>

        <informalexample>
            <programlisting># dockable window name
quicknotepad.title=QuickNotepad</programlisting>
        </informalexample>

        <para>Next, we see menu item labels for the plugin's actions. All of
        these but the first are defined in <literal>actions.xml</literal> file,
        and that is because the dockable itself has its own actions. Actions are
        discussed further in <xref linkend="plugin-implement-actions" />.</para>

        <informalexample>
            <programlisting># action labels
# Dockable label
quicknotepad.label=QuickNotepad
# Additional strings extracted from the plugin java source
quicknotepad.choose-file.label=Choose notepad file
quicknotepad.save-file.label=Save notepad file
quicknotepad.copy-to-buffer.label=Copy notepad to buffer
</programlisting>
        </informalexample>

        <para>Next, the plugin's menu is defined. See <xref
        linkend="plugin-implement-quicknotepadplugin" />.</para>

        <informalexample>
            <programlisting># application menu items
quicknotepad.menu.label=QuickNotepad
quicknotepad.menu=quicknotepad - quicknotepad.choose-file \
  quicknotepad.save-file quicknotepad.copy-to-buffer</programlisting>
        </informalexample>

        <para>We have created a small toolbar as a component of QuickNotepad, so
        file names for the button icons follow:</para>

        <informalexample>
            <programlisting># plugin toolbar buttons
quicknotepad.choose-file.icon=Open.png
quicknotepad.save-file.icon=Save.png
quicknotepad.copy-to-buffer.icon=Edit.png</programlisting>
        </informalexample>

        <para>The menu item labels corresponding to these icons will also serve
        as tooltip text.</para>

        <para>Finally, the properties file set forth the labels and settings
        used by the option pane:</para>

        <informalexample>
            <programlisting># Option pane labels
options.quicknotepad.label=QuickNotepad
options.quicknotepad.file=File:
options.quicknotepad.choose-file=Choose
options.quicknotepad.choose-file.title=Choose a notepad file
options.quicknotepad.choose-font=Font:
options.quicknotepad.show-filepath.title=Display notepad file path

# Initial default font settings
options.quicknotepad.show-filepath=true
options.quicknotepad.font=Monospaced
options.quicknotepad.fontstyle=0
options.quicknotepad.fontsize=14

# Setting not defined but supplied for completeness
options.quicknotepad.filepath=</programlisting>
        </informalexample>

        <tip>
            <title>PropertySideKick</title>

            <para>There is a SideKick for Property files, provided in the
            JavaSideKick plugin. This gives you a compact and sorted tree view
            of property files.</para>
        </tip>


      <section id="localizationfiles">
      <title> Localization Files </title>

      <para> In addition to property files ending in <literal>.props</literal>,
      you will find property files with the names like       <literal>lang_de.properties</literal>. Each of these files provides
      localized strings for a particular locale. In the example above, it is for the German locale. These files are loaded by jEdit automatically when that locale is in use. They need to have a different filename extension from the other property files so they can be treated differently from
      the regular properties. </para>
      </section>

    </section>
    <section id="plugin-implement-editbus">
        <title>The EditBus</title>

        <para>jEdit (and some plugins) generate several kinds of messages to
        alert plugins and other components of jedit-specific events. The message
        classes, all derived from <ulink
        url="../api/org/gjt/sp/jedit/EBMessage.html">
        <classname>EBMessage</classname></ulink> cover the opening and closing
        of the application, changes in the status of buffers and views, changes
        in user settings, as well as changes in the state of other program
        features. A full list of messages can be found in the <ulink
        url="../api/org/gjt/sp/jedit/msg/package-summary.html">org.gjt.sp.jedit.msg</ulink>
        package.</para>



        <para>For example, the ViewUpdate messages are all related to the jEdit
        View, or the top-level window. If the user creates multiple Views, a
        plugin may need to know when they are created or destroyed, so it would
        monitor ViewUpdate messages.</para>

        <para>BufferUpdate messages are all related to jEdit buffers. They let
        plugins know when a buffer has become dirty, when it is about to be
        closed, after it is closed, created, loaded, or saved. Each of these
        messages are described in further detail in the API docs.</para>

        <para>As another example, The Navigator plugin monitors an EBMessage of the kind <ulink
        url="../api/org/gjt/sp/jedit/BufferChanging.html">BufferChanging</ulink>.
        The BufferChanging event provides Navigator enough advance notice to
        save the TextArea's caret just before the current EditPane changes its
        active Buffer. The <literal>BufferChanged</literal> event, another
        <literal>EditPaneUpdate</literal> message, is thrown shortly afterward.
        This is not used by Navigator, but it is used by SideKick to determine
        when it is time to reparse the buffer.</para>

        <para>Plugins register <ulink
        url="../api/org/gjt/sp/jedit/EBComponent.html">
        <function>EBComponent</function></ulink> instances with the <ulink
        url="../api/org/gjt/sp/jedit/EditBus.html">
        <classname>EditBus</classname></ulink> to receive messages reflecting
        changes in jEdit's state.</para>

        <para><ulink url="../api/org/gjt/sp/jedit/EBComponent.html">
        <function>EBComponent</function></ulink>s are added and removed with the
        <ulink
        url="../api/org/gjt/sp/jedit/EditBus.html#addToBus(org.gjt.sp.jedit.EBComponent)">
        <function>EditBus.addToBus()</function></ulink> and <ulink
        url="../api/org/gjt/sp/jedit/EditBus.html#removeFromBus(org.gjt.sp.jedit.EBComponent)">
        <function>EditBus.removeFromBus()</function></ulink> methods.</para>

        <para>Typically, the <ulink
        url="../api/org/gjt/sp/jedit/EBComponent.html#handleMessage(org.gjt.sp.jedit.EBMessage)">
        <function>EBComponent.handleMessage()</function></ulink> method is
        implemented with one or more <function>if</function> blocks that test
        whether the message is an instance of a derived message class in which
        the component has an interest.</para>

        <programlisting>if(msg instanceof BufferUpdate) {
    // a buffer's state has changed!
}
else if(msg instanceof ViewUpdate) {
    // a view's state has changed!
}
// ... and so on</programlisting>

        <para>If a plugin core class will respond to EditBus messages, it can be
        derived from <ulink url="../api/org/gjt/sp/jedit/EBPlugin.html">
        <classname>EBPlugin</classname></ulink>, in which case no explicit
        <function>addToBus()</function> call is necessary. Otherwise, <ulink
        url="../api/org/gjt/sp/jedit/EditPlugin.html">
        <classname>EditPlugin</classname></ulink> will suffice as a plugin base
        class. Note that QuickNotepad uses the latter.</para>

        <tip>
        <title>Using the Activity Log to see the EditBus</title>

        <para> To determine precisely which EditBus messages are being sent by
        jEdit or the plugins, start up jEdit with an additional argument,
        <literal>-log=5</literal>. You can set an even lower log level to see
        further details (the default is 7). With a log level of 5 or lower, the
        Activity Log will include [notice]s, which will show us exactly which
        EditBus message is sent and when. See <xref linkend="activity-log" />
        for more details. </para> </tip>

    </section>

    <section id="plugin-implement-actions">
        <title>The Actions.xml Catalog</title>

        <para>In jEdit as well as in Plugins, actions define procedures that can be bound to a menu item, a
        toolbar button or a keyboard shortcut. Most plugin Actions <footnote>
                <para>Some plugins, such as Sidekick, Console, and
                ProjectViewer, create pure Java EditAction-derived Actions,
                based which services are available, or which files are found in
                a certain path. However, this is an advanced topic you can
                explore further in the source and API docs of those
                plugins.</para>
            </footnote> are short scripts written in BeanShell, jEdit's macro
        scripting language. These scripts either direct the action themselves,
        delegate to a method in one of the plugin's classes that encapsulates
        the action, or do a little of both. The scripts are usually short;
        elaborate action protocols are usually contained in compiled code,
        rather than an interpreted macro script, to speed execution.</para>

        <para>Actions are defined by creating an XML file entitled
        <filename>actions.xml</filename> and placing it in the plugin JAR
        file.</para>

        <para>The <filename>actions.xml</filename> file from the
        <application>QuickNotepad</application> plugin looks as follows:</para>

        <informalexample>
            <programlisting>&lt;ACTIONS&gt;
  &lt;ACTION NAME="quicknotepad.choose-file"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).chooseFile();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;

  &lt;ACTION NAME="quicknotepad.save-file"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).saveFile();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;

  &lt;ACTION NAME="quicknotepad.copy-to-buffer"&gt;
    &lt;CODE&gt;
      wm.addDockableWindow(QuickNotepadPlugin.NAME);
      wm.getDockableWindow(QuickNotepadPlugin.NAME).copyToBuffer();
    &lt;/CODE&gt;
  &lt;/ACTION&gt;
&lt;/ACTIONS&gt;</programlisting>
        </informalexample>

        <tip> <title> Actions in jEdit core </title>
        <para> You can see how each action in jEdit core is implemented by
        inspecting the <literal>actions.xml</literal> file that is there. </para>
        </tip>

        <para>This file defines three actions. They each use a built-in variable
        <literal>wm</literal>, which refers to the current view's <ulink
        url="../api/org/gjt/sp/jedit/gui/DockableWindowManager.html">
        <classname>DockableWindowManager</classname></ulink>. Whenever you need
        to obtain a reference to the current dockable, or create a new one, this
        is the class to use. We use the method <filename>addDockable() followed
        by getDockable()</filename> to create if necessary, and then bring up
        the QuickNotepad plugin dockable. This will be docked or floating,
        depending on how it was last used.</para>

        <para>When an action is invoked, the BeanShell scripts address the
        plugin through static methods, or if instance data is needed, the
        current <ulink url="../api/org/gjt/sp/jedit/View.html">
        <classname>View</classname></ulink>, its <ulink
        url="../api/org/gjt/sp/jedit/gui/DockableWindowManager.html">
        <classname>DockableWindowManager</classname></ulink>, and the plugin
        object return by the <filename>getDockable()</filename> method.</para>

        <para>If you are unfamiliar with BeanShell code, you may nevertheless
        notice that the code statements bear a strong resemblance to Java code,
        with one exception: the variable <varname>view</varname> is never
        assigned any value.</para>

        <para>For complete answers to this and other BeanShell mysteries, see
        <xref linkend="writing-macros-part" />; two observations will suffice
        here. First, the variable <varname>view</varname> is predefined by
        jEdit's implementation of BeanShell to refer to the current
        <classname>View</classname> object. Second, the BeanShell scripting
        language is based upon Java syntax, but allows variables to be typed at
        run time, so explicit types for variables need not be declared.</para>

        <para>A formal description of each element of the
        <filename>actions.xml</filename> file can be found in the documentation
        of the <ulink url="../api/org/gjt/sp/jedit/ActionSet.html">
        <classname>ActionSet</classname></ulink> class.</para>
    </section>

    <section id="plugin-implement-dockables">
        <title>The dockables.xml Window Catalog</title>

        <para>A Dockable is a window that can float like a dialog, or dock into
        jEdit's docking area. Each dockable needs a label (for display in menus,
        and on small buttons) and a title (for display in the floating window's
        title bar).</para>

        <para>The jEdit API uses BeanShell to create the top-level
        visible container of a plugin's interface. The BeanShell code is
        contained in a file named <filename>dockables.xml</filename>. It usually
        is quite short, providing only a single BeanShell expression used to
        create a visible plugin window.</para>

        <para>The following example from the QuickNotepad plugin illustrates the
        requirements of the data file:</para>

        <informalexample>
            <programlisting>&lt;?xml version="1.0"?&gt;

&lt;!DOCTYPE DOCKABLES SYSTEM "dockables.dtd"&gt;

&lt;DOCKABLES&gt;
  &lt;DOCKABLE NAME="quicknotepad"&gt;
    new QuickNotepad(view, position);
  &lt;/DOCKABLE&gt;
&lt;/DOCKABLES&gt;</programlisting>
        </informalexample>

        <para>In this example, the <classname>&lt;DOCKABLE&gt;</classname>
        element has a single attribute, the dockable window's identifier. This
        attribute is used to key a property where the window title is stored;
        see <xref linkend="plugin-implement-properties" />.</para>

        <para>For each dockable, jedit defines an action with the same name.
        This means you do not need to define an explicit action to create your
        dockable - in fact, jEdit defines three actions: "toggle", "get" and
        "new floating instance" for each.</para>

        <para>The contents of the <classname>&lt;DOCKABLE&gt;</classname>
        element itself is a BeanShell expression that constructs a new
        <classname>QuickNotepad</classname> object. The <varname>view</varname>
        and <varname>position</varname> are predefined by the plugin API as the
        view in which the plugin window will reside, and the docking position of
        the plugin. You can use <varname>position</varname> to customize the
        layout of your plugin depending on whether it appears on the sides, or
        the top/bottom, or as a floating dockable.</para>

        <para>A formal description of each element of the
        <filename>dockables.xml</filename> file can be found in the
        documentation of the <ulink
        url="../api/org/gjt/sp/jedit/gui/DockableWindowManager.html">
        <classname>DockableWindowManager</classname></ulink> class. This class
        also contains the public interface you should use for getting, showing,
        hiding, and other interactions with the plugin's top-level
        windows.</para>


        <tip> <title> jEdit's dockables </title> <para> jEdit has its own
        <literal>dockables.xml</literal> file that you can inspect to see how
        jEdit creates its own dockables. </para> </tip>

    </section>

    <section id="plugin-implement-services">
        <title>The services.xml file</title>

        <para>A "service" is a mechanism by which one plugin can work with other
        plugins and avoid a bidirectional build-dependency. For example, the XML
        plugin "depends" on Sidekick, but in fact, it is SideKick which creates
        and operates on an object (a <literal>SideKickParser</literal>, in fact)
        defined in the XML plugin. In a way, the dependency is
        bidirectional.</para>

        <para>Similarly, the SshConsole plugin defines but does not instantiate
        a <literal>Shell</literal> object. It is the Console plugin which
        creates a specific shell for each available service. SideKick and
        Console use the <literal>ServiceManager</literal> to search for services
        offered by other plugins.</para>

        <para>Here is an example of a service from the XML plugin, which extends
        Sidekick:</para>

        <informalexample>
            <programlisting>
&lt;!DOCTYPE SERVICES SYSTEM "services.dtd"&gt;
&lt;SERVICES&gt;
    &lt;SERVICE CLASS=&quot;sidekick.SideKickParser&quot; NAME=&quot;html&quot;&gt;
        new sidekick.html.HtmlParser();
    &lt;/SERVICE&gt;
  [...]
&lt;/SERVICES&gt;

</programlisting>
        </informalexample>

        <para> The value of the <literal>CLASS=</literal> should be a
        base-class or interface of the object that is returned by
        executing the beanshell factory method enclosed in the
        <literal>&lt;SERVICE&gt;</literal> tag.
        </para>

        <para>In the case above, the returned object
        tells Sidekick how it can parse files of a specific type (HTML).
        The API docs for <literal>SideKickParser</literal>
        should indicate precisely which methods must be implemented in a plugin
        which offers this service. </para>

        <para>For more information about services, refer to the <ulink
        url="../api/org/gjt/sp/jedit/ServiceManager.html">ServiceManager</ulink>
        class API documentation. There, you can find out what the tags and
        attributes mean, as well as how to register and use services.
        You can also inspect the <literal>services.xml</literal> file of jEdit
        core to see what services are offered by jEdit itself. </para>

    </section>

    <section id="plugin-implement-quicknotepad">
        <title>The QuickNotepad Class</title>

        <para>Here is where most of the features of the plugin will be
        implemented. To work with the dockable window API, the top level window
        will be a <classname>JPanel</classname>. The visible components reflect
        a simple layout. Inside the top-level panel we will place a scroll pane
        with a text area. Above the scroll pane we will place a panel containing
        a small tool bar and a label displaying the path of the current notepad
        file.</para>

        <para>We have identified three user actions that need implementation
        here: <function>chooseFile()</function>,
        <function>saveFile()</function>, and
        <function>copyToBuffer()</function>. As noted earlier, we also want the
        text area to change its appearance in immediate response to a change in
        user options settings. In order to do that, the window class must
        respond to a <classname>PropertiesChanged</classname> message from the
        EditBus.</para>

        <!-- <para>
  We could have the plugin core class receive and delegate
  <classname>PropertiesChanged</classname> messages to the window class.
  However, this would require the plugin core class to hold a reference
  to either the plugin window class or the visible window class and to
  update that reference when the user activates or deactivates the
  plugin.  It is simpler to have the plugin window class subscribe to the
  EditBus directly; many plugins take this approach.  This means that
  <classname>QuickNotepad</classname> must implement the
  <classname>EBComponent</classname> interface.
</para> -->

        <para>Unlike the <classname>EBPlugin</classname> class, the
        <classname>EBComponent</classname> interface does not deal with the
        component's actual subscribing and unsubscribing to the EditBus. To
        accomplish this, we use a pair of methods inherited from the Java
        platform's <classname>JComponent</classname> class that are called when
        the window is made visible, and when it is hidden. These two methods,
        <function>addNotify()</function> and
        <function>removeNotify()</function>, are overridden to add and remove
        the visible window from the list of EditBus subscribers.</para>

        <para>We will provide for two minor features when the notepad is
        displayed in the floating window. First, when a floating plugin window
        is created, we will give the notepad text area input focus. Second, when
        the notepad if floating and has input focus, we will have the
        <keycap>Escape</keycap> key dismiss the notepad window. An
        <classname>AncestorListener</classname> and a
        <classname>KeyListener</classname> will implement these details.</para>

        <para>Here is the listing for the data members, the constructor, and the
        implementation of the <classname>EBComponent</classname>
        interface:</para>

        <informalexample>
            <programlisting>public class QuickNotepad extends JPanel
    implements EBComponent
{
    private String filename;
    private String defaultFilename;
    private View view;
    private boolean floating;

    private QuickNotepadTextArea textArea;
    private QuickNotepadToolPanel toolPanel;

    //
    // Constructor
    //

    public QuickNotepad(View view, String position)
    {
        super(new BorderLayout());

        this.view = view;
        this.floating = position.equals(
            DockableWindowManager.FLOATING);

        this.filename = jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX
            + "filepath");
        if(this.filename == null || this.filename.length() == 0)
        {
            this.filename = new String(jEdit.getSettingsDirectory()
                + File.separator + "qn.txt");
            jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
                + "filepath",this.filename);
        }
        this.defaultFilename = new String(this.filename);

        this.toolPanel = new QuickNotepadToolPanel(this);
        add(BorderLayout.NORTH, this.toolPanel);

        if(floating)
            this.setPreferredSize(new Dimension(500, 250));

        textArea = new QuickNotepadTextArea();
        textArea.setFont(QuickNotepadOptionPane.makeFont());
        textArea.addKeyListener(new KeyHandler());
        textArea.addAncestorListener(new AncestorHandler());
        JScrollPane pane = new JScrollPane(textArea);
        add(BorderLayout.CENTER, pane);

        readFile();
    }

    //
    // Attribute methods
    //

    // for toolBar display
    public String getFilename()
    {
        return filename;
    }

    //
    // EBComponent implementation
    //

    public void handleMessage(EBMessage message)
    {
        if (message instanceof PropertiesChanged)
        {
            propertiesChanged();
        }
    }


    private void propertiesChanged()
    {
        String propertyFilename = jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX + "filepath");
        if(!defaultFilename.equals(propertyFilename))
        {
            saveFile();
            toolPanel.propertiesChanged();
            defaultFilename = propertyFilename.clone();
            filename = defaultFilename.clone();
            readFile();
        }
        Font newFont = QuickNotepadOptionPane.makeFont();
        if(!newFont.equals(textArea.getFont()))
        {
            textArea.setFont(newFont);
            textArea.invalidate();
        }
    }

    // These JComponent methods provide the appropriate points
    // to subscribe and unsubscribe this object to the EditBus

    public void addNotify()
    {
        super.addNotify();
        EditBus.addToBus(this);
    }


    public void removeNotify()
    {
        saveFile();
        super.removeNotify();
        EditBus.removeFromBus(this);
    }

    ...

}</programlisting>
        </informalexample>

        <para>This listing refers to a
        <classname>QuickNotebookTextArea</classname> object. It is currently
        implemented as a <classname>JTextArea</classname> with word wrap and tab
        sizes hard-coded. Placing the object in a separate class will simply
        future modifications.</para>
    </section>

    <section id="plugin-implement-quicknotepadtoolbar">
        <title>The QuickNotepadToolBar Class</title>

        <para>There is nothing remarkable about the toolbar panel that is placed
        inside the <classname>QuickNotepad</classname> object. The constructor
        shows the continued use of items from the plugin's properties
        file.</para>

        <informalexample>
            <programlisting>public class QuickNotepadToolPanel extends JPanel
{
    private QuickNotepad pad;
    private JLabel label;

    public QuickNotepadToolPanel(QuickNotepad qnpad)
    {
        pad = qnpad;
        JToolBar toolBar = new JToolBar();
        toolBar.setFloatable(false);

        toolBar.add(makeCustomButton("quicknotepad.choose-file",
            new ActionListener() {
                public void actionPerformed(ActionEvent evt) {
                    QuickNotepadToolPanel.this.pad.chooseFile();
                }
            }));
        toolBar.add(makeCustomButton("quicknotepad.save-file",
            new ActionListener() {
                public void actionPerformed(ActionEvent evt) {
                    QuickNotepadToolPanel.this.pad.saveFile();
                }
            }));
        toolBar.add(makeCustomButton("quicknotepad.copy-to-buffer",
            new ActionListener() {
                public void actionPerformed(ActionEvent evt) {
                    QuickNotepadToolPanel.this.pad.copyToBuffer();
                }
            }));
        label = new JLabel(pad.getFilename(),
            SwingConstants.RIGHT);
        label.setForeground(Color.black);
        label.setVisible(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX
            + "show-filepath").equals("true"));
        this.setLayout(new BorderLayout(10, 0));
        this.add(BorderLayout.WEST, toolBar);
        this.add(BorderLayout.CENTER, label);
        this.setBorder(BorderFactory.createEmptyBorder(0, 0, 3, 10));
    }

    ...

}</programlisting>
        </informalexample>

        <para>The method <classname>makeCustomButton()</classname> provides
        uniform attributes for the three toolbar buttons corresponding to three
        of the plugin's use actions. The menu titles for the user actions serve
        double duty as tooltip text for the buttons. There is also a
        <function>propertiesChanged()</function> method for the toolbar that
        sets the text and visibility of the label containing the notepad file
        path.</para>
    </section>

    <section id="plugin-implement-options">
        <title>The QuickNotepadOptionPane Class</title>

        <para>Using the default implementation provided by
        <classname>AbstractOptionPane</classname> reduces the preparation of an
        option pane to two principal tasks: writing a
        <function>_init()</function> method to layout and initialize the pane,
        and writing a <function>_save()</function> method to commit any settings
        changed by user input. If a button on the option pane should trigger
        another dialog, such as a <classname>JFileChooser</classname> or jEdit's
        own enhanced <classname>VFSFileChooserDialog</classname>, the option
        pane will also have to implement the
        <classname>ActionListener</classname> interface to display additional
        components.</para>

        <para>The QuickNotepad plugin has only three options to set: the path
        name of the file that will store the notepad text, the visibility of the
        path name on the tool bar, and the notepad's display font. Using the
        shortcut methods of the plugin API, the implementation of
        <function>_init()</function> looks like this:</para>

        <informalexample>
            <programlisting>public class QuickNotepadOptionPane extends AbstractOptionPane
      implements ActionListener
{
    private JTextField pathName;
    private JButton pickPath;
    private FontSelector font;

    ...

    public void _init()
    {
        showPath = new JCheckBox(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX
            + "show-filepath.title"),
        jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX +  "show-filepath")
            .equals("true"));
        addComponent(showPath);

        pathName = new JTextField(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX
            + "filepath"));
        JButton pickPath = new JButton(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX
            + "choose-file"));
        pickPath.addActionListener(this);

        JPanel pathPanel = new JPanel(new BorderLayout(0, 0));
        pathPanel.add(pathName, BorderLayout.CENTER);
        pathPanel.add(pickPath, BorderLayout.EAST);

        addComponent(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX + "file"),
            pathPanel);

        font = new FontSelector(makeFont());
        addComponent(jEdit.getProperty(
            QuickNotepadPlugin.OPTION_PREFIX + "choose-font"),
            font);
    }

    ...

}</programlisting>
        </informalexample>

        <para>Here we adopt the vertical arrangement offered by use of the
        <function>addComponent()</function> method with one embellishment. We
        want the first <quote>row</quote> of the option pane to contain a text
        field with the current notepad file path and a button that will trigger
        a file chooser dialog when pressed. To place both of them on the same
        line (along with an identifying label for the file option), we create a
        <classname>JPanel</classname> to contain both components and pass the
        configured panel to <function>addComponent()</function>.</para>

        <para>The <function>_init()</function> method uses properties from the
        plugin's property file to provide the names of label for the components
        placed in the option pane. It also uses a property whose name begins
        with <function>PROPERTY_PREFIX</function> as a persistent data item -
        the path of the current notepad file. The elements of the notepad's font
        are also extracted from properties using a static method of the option
        pane class.</para>

        <para>The <function>_save()</function> method extracts data from the
        user input components and assigns them to the plugin's properties. The
        implementation is straightforward:</para>

        <informalexample>
            <programlisting>public void _save()
{
    jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
        + "filepath", pathName.getText());
    Font _font = font.getFont();

    jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
        + "font", _font.getFamily());
    jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
        + "fontsize", String.valueOf(_font.getSize()));
    jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
        + "fontstyle", String.valueOf(_font.getStyle()));
    jEdit.setProperty(QuickNotepadPlugin.OPTION_PREFIX
        + "show-filepath", String.valueOf(showPath.isSelected()));
}</programlisting>
        </informalexample>

        <para>The class has only two other methods, one to display a file
        chooser dialog in response to user action, and the other to construct a
        <classname>Font</classname> object from the plugin's font properties.
        They do not require discussion here.</para>
    </section>

    <section id="plugin-implement-docs">
        <title>Plugin Documentation</title>

        <para>While not required by the plugin API, a help file is an essential
        element of any plugin written for public release. A single web page is
        often all that is required. There are no specific requirements on
        layout, but because of the design of jEdit's help viewer, the use of
        frames should be avoided. Topics that would be useful include the
        following:</para>

        <itemizedlist>
            <listitem>
                <para>a description of the purpose of the plugin;</para>
            </listitem>

            <listitem>
                <para>an explanation of the type of input the user can supply
                through its visible interface (such as mouse action or text
                entry in controls);</para>
            </listitem>

            <listitem>
                <para>a listing of available user actions that can be taken when
                the plugin does not have input focus;</para>
            </listitem>

            <listitem>
                <para>a summary of configuration options;</para>
            </listitem>

            <listitem>
                <para>information on development of the plugin (such as a change
                log, a list of <quote>to do</quote> items, and contact
                information for the plugin's author); and</para>
            </listitem>

            <listitem>
                <para>licensing information, including acknowledgments for any
                library software used by the plugin.</para>
            </listitem>
        </itemizedlist>

        <para>The location of the plugin's help file is stored in the
        <literal>plugin.QuickNotepad.docs</literal> property; see <xref
        linkend="plugin-implement-properties" />.</para>
    </section>

    <section id="plugin-implement-building">
        <title>The build.xml Ant build file</title>

        <para>We have already outlined the contents of the user action catalog,
        the properties file and the documentation file in our earlier
        discussion. The final step is to compile the source file and build the
        archive file that will hold the class files and the plugin's other
        resources.</para>

        <para>Publicly released plugins include with their source a makefile in
        XML format for the <application>Ant</application> utility. The format
        for this file requires few changes from plugin to plugin. Here is a
        version of <filename>build.xml</filename> that could be used by
        QuickNotepad:</para>

        <informalexample>
            <programlisting>
&lt;project name=&quot;QuickNotepad&quot; default=&quot;build&quot;&gt;
  &lt;description&gt;
    This is an ant build.xml file for building the QuickNotepad plugin for jEdit.
  &lt;/description&gt;

  &lt;property file=&quot;build.properties&quot;/&gt;
  &lt;property file=&quot;../build.properties&quot;/&gt;
  &lt;property name=&quot;user-doc.xml&quot; location = &quot;users-guide.xml&quot;/&gt;
  &lt;import file=&quot;${build.support}/plugin-build.xml&quot; /&gt;

  &lt;!-- Extra files that should be included in the jar --&gt;
  &lt;selector id=&quot;packageFiles&quot;&gt;
    &lt;or&gt;
        &lt;filename name=&quot;*.txt&quot; /&gt;
    &lt;/or&gt;
  &lt;/selector&gt;
&lt;/project&gt;
</programlisting>
        </informalexample>

        <para>This build file imports another modular build file,
        <literal>plugin-build.xml</literal> from the <literal>build-support</literal>
        project. It is available as a package you can check out from subversion, or found online in the <ulink
        url="https://jedit.svn.sourceforge.net/svnroot/jedit/build-support/trunk/">jEdit's
        SVN repository</ulink>. It contains the common build steps used to build the core jEdit plugins, and some example <literal>build.properties.sample</literal> files which you can adapt for use with your development environment.</para>


        <para>Customizing this build file for a different plugin will likely
        only require three changes to build.xml file:</para>

        <itemizedlist>
            <listitem>
                <para>the name of the project</para>
            </listitem>

            <listitem>
                <para>the dependencies of the plugin</para>
            </listitem>

            <listitem>
                <para>The extra files that need to be copied into the
                jar.</para>
            </listitem>
        </itemizedlist>

        <para>Because this build file and those used by most plugins import a
        <literal>build.properties</literal> file from the current and the parent
        directories, it is possible to build most of jEdit's plugins in a
        uniform way by setting the common properties in a single
        <literal>build.properties</literal> file, placed in the plugin source's
        parent directory. </para>

        <tip> <para>For a full discussion of the <filename>Ant</filename> file
        format and command syntax, you should consult the <ulink
        url="http://jakarta.apache.org/ant/manual/index.html">Ant
        documentation</ulink>, also available through jEdit's help system if you
        installed the Ant Plugin. When editing Ant build files, the XML plugin
        gives you completion tips for both elements <emphasis>and</emphasis>
        attributes. The Console plugin provides you with an ANT button which you
        can bind to keyboard actions. In addition, there are the AntFarm and
        Antelope plugins which also proivde you with alternate means to execute
        Ant targets through the Console.</para> </tip> </section>

    <section id="plugin-implement-reloading">
        <title>Reloading the Plugin</title>

        <para>Once you have compiled your plugin, you will need to test its
        behavior when it is reloaded. Follow these steps to reload your plugin
        without restarting jEdit:</para>

        <itemizedlist>
            <listitem>
                <para>From the Plugins menu open the Plugin Manager.</para>
            </listitem>

            <listitem>
                <para>On the Manage tab uncheck Hide libraries. This will allow
                you to see plugins that are not loaded.</para>
            </listitem>

            <listitem>
                <para>Recheck the plugin to reload it.</para>
            </listitem>
        </itemizedlist>

        <tip>
            <para>The Activator plugin provides a very convenient (dockable) way
            to test the activating and reloading behavior of your plugin. Be
            sure to test your plugin's reloading behavior with both the
            Activator and the Reloader tabs.</para>
        </tip>

        <para>If you have reached this point in the text, you are probably
        serious about writing a plugin for jEdit. Good luck with your efforts,
        and thank you for contributing to the jEdit project.</para>
    </section>

    <section id="plugin-debugging" >
    <title> Tips for debugging plugins </title>
      <bridgehead> BeanShell </bridgehead>
      <para> jEdit includes a Beanshell interface into its currently running JVM at all times. You can access it a variety of ways, but one way is from <literal>Plugins - Console - Shells - BeanShell</literal>. From here, you can interactively inspect the values of any object in memory, call any of its member functions, or create new instances of any class that is currently loaded by jEdit or any of its plugins. All this, without setting any breakpoints!
      </para>
      <para> If you're too lazy to type each Beanshell statement interactively, you can also create debugging code snippets as macros and invoke them from <literal>utilities - beanshell - evaluate selection</literal>, or <literal>Macros - Misc - Evaluate Buffer in Beanshell</literal>, or place the file in your own macros directory and bind it to its own keyboard shortcut. </para>
      <bridgehead> Other useful tips </bridgehead>
      <para> This section is new but will be expanded shortly. Please post suggestions to the <literal>jedit-devel</literal> mailing list. </para>
      </section>

</chapter>