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Datei: T6622260.java Sprache: XML

Original von: Isabelle^©

<?xml version="1.0" encoding="UTF-8"?>
<chapter id="macro-tips">
    <title>Macro Tips and Techniques</title>
    
    
    

    <section id="macro-tips-input">
        <title>Getting Input for a Macro</title>

        <para>The dialog-based macro discussed in <xref
        linkend="dialog-macro" /> reflects a conventional approach to obtaining
        input in a Java program. Nevertheless, it can be too lengthy or tedious
        for someone trying to write a macro quickly. Not every macro needs a
        user interface specified in such detail; some macros require only a
        single keystroke or no input at all. In this section we outline some
        other techniques for obtaining input that will help you write macros
        quickly.</para>

        <section id="macro-tips-input-single-line">
            <title>Getting a Single Line of Text</title>

            <para>As mentioned earlier in <xref linkend="helpful-methods" />,
            the method <function>Macros.input()</function> offers a convenient
            way to obtain a single line of text input. Here is an example that
            inserts a pair of HTML markup tags specified by the user.</para>

            <informalexample>
                <programlisting>// Insert_Tag.bsh

void insertTag()
{
    caret = textArea.getCaretPosition();
    tag = Macros.input(view, <quote>Enter name of tag:</quote>);
    if( tag == null || tag.length() == 0) return;
    text = textArea.getSelectedText();
    if(text == null) text = <quote></quote>;
    sb = new StringBuffer();
    sb.append(<quote><</quote>).append(tag).append(<quote>></quote>);
    sb.append(text);
    sb.append(<quote></</quote>).append(tag).append(<quote>></quote>);
    textArea.setSelectedText(sb.toString());
    if(text.length() == 0)
        textArea.setCaretPosition(caret + tag.length() + 2);
}

insertTag();

// end Insert_Tag.bsh</programlisting>
            </informalexample>

            <para>Here the call to <function>Macros.input()</function> seeks the
            name of the markup tag. This method sets the message box title to a
            fixed string, <quote>Macro input</quote>, but the specific message
            <guilabel>Enter name of tag</guilabel> provides all the information
            necessary. The return value <varname>tag</varname> must be tested to
            see if it is null. This would occur if the user presses the
            <guilabel>Cancel</guilabel> button or closes the dialog window
            displayed by <function>Macros.input()</function>.</para>
        </section>

        <section id="macro-tips-input-multiple-data">
            <title>Getting Multiple Data Items</title>

            <para>If more than one item of input is needed, a succession of
            calls to <function>Macros.input()</function> is a possible, but
            awkward approach, because it would not be possible to correct early
            input after the corresponding message box is dismissed. Where more
            is required, but a full dialog layout is either unnecessary or too
            much work, the Java method
            <function>JOptionPane.showConfirmDialog()</function> is available.
            The version to use has the following prototype:</para>

            <itemizedlist>
                <listitem>
                    <funcsynopsis>
                        <funcprototype>
                            <funcdef>public static int
                            <function>showConfirmDialog</function></funcdef>

                            <paramdef>Component
                            <parameter>parentComponent</parameter></paramdef>

                            <paramdef>Object
                            <parameter>message</parameter></paramdef>

                            <paramdef>String
                            <parameter>title</parameter></paramdef>

                            <paramdef>int
                            <parameter>optionType</parameter></paramdef>

                            <paramdef>int
                            <parameter>messageType</parameter></paramdef>
                        </funcprototype>
                    </funcsynopsis>
                </listitem>
            </itemizedlist>

            <para>The usefulness of this method arises from the fact that the
            <varname>message</varname> parameter can be an object of any Java
            class (since all classes are derived from
            <classname>Object</classname>), or any array of objects. The
            following example shows how this feature can be used.</para>

            <informalexample>
                <programlisting>// excerpt from Write_File_Header.bsh

title = <quote>Write file header</quote>;

currentName = buffer.getName();

nameField = new JTextField(currentName);
authorField = new JTextField(<quote>Your name here</quote>);
descField = new JTextField(<quote></quote>, 25);

namePanel = new JPanel(new GridLayout(1, 2));
nameLabel = new JLabel(<quote>Name of file:</quote>, SwingConstants.LEFT);
saveField = new JCheckBox(<quote>Save file when done</quote>,
    !buffer.isNewFile());
namePanel.add(nameLabel);
namePanel.add(saveField);

message = new Object[9];
message[0] = namePanel;
message[1] = nameField;
message[2] = Box.createVerticalStrut(10);
message[3] = <quote>Author's name:;
message[4] = authorField;
message[5] = Box.createVerticalStrut(10);
message[6] = <quote>Enter description:</quote>;
message[7] = descField;
message[8] = Box.createVerticalStrut(5);

if( JOptionPane.OK_OPTION !=
    JOptionPane.showConfirmDialog(view, message, title,
        JOptionPane.OK_CANCEL_OPTION,
        JOptionPane.QUESTION_MESSAGE))
    return null;

// *****remainder of macro script omitted*****

// end excerpt from Write_File_Header.bsh</programlisting>
            </informalexample>

            <para>This macro takes several items of user input and produces a
            formatted file header at the beginning of the buffer. The full macro
            is included in the set of macros installed by jEdit. There are a
            number of input features of this excerpt worth noting.</para>

            <itemizedlist>
                <listitem>
                    <para>The macro uses a total of seven visible components.
                    Two of them are created behind the scenes by
                    <function>showConfirmDialog()</function>, the rest are made
                    by the macro. To arrange them, the script creates an array
                    of <classname>Object</classname> objects and assigns
                    components to each location in the array. This translates to
                    a fixed, top-to-bottom arrangement in the message box
                    created by <function>showConfirmDialog()</function>.</para>
                </listitem>

                <listitem>
                    <para>The macro uses <classname>JTextField</classname>
                    objects to obtain most of the input data. The fields
                    <varname>nameField</varname> and
                    <varname>authorField</varname> are created with constructors
                    that take the initial, default text to be displayed in the
                    field as a parameter. When the message box is displayed, the
                    default text will appear and can be altered or deleted by
                    the user.</para>
                </listitem>

                <listitem>
                    <para>The text field <varname>descField</varname> uses an
                    empty string for its initial value. The second parameter in
                    its constructor sets the width of the text field component,
                    expressed as the number of characters of
                    <quote>average</quote> width. When
                    <function>showConfirmDialog()</function> prepares the layout
                    of the message box, it sets the width wide enough to
                    accommodate the designated with of
                    <varname>descField</varname>. This technique produces a
                    message box and input text fields that are wide enough for
                    your data with one line of code.</para>
                </listitem>

                <listitem>
                    <para>The displayed message box includes a
                    <classname>JCheckBox</classname> component that determines
                    whether the buffer will be saved to disk immediately after
                    the file header is written. To conserve space in the message
                    box, we want to display the check box to the right of the
                    label <guilabel>Name of file:</guilabel>. To do that, we
                    create a <classname>JPanel</classname> object and populate
                    it with the label and the checkbox in a left-to-right
                    <classname>GridLayout</classname>. The
                    <classname>JPanel</classname> containing the two components
                    is then added to the beginning of <varname>message</varname>
                    array.</para>
                </listitem>

                <listitem>
                    <para>The two visible components created by
                    <function>showConfirmDialog()</function> appear at positions
                    3 and 6 of the <varname>message</varname> array. Only the
                    text is required; they are rendered as text labels.</para>
                </listitem>

                <listitem>
                    <para>There are three invisible components created by
                    <function>showConfirmDialog()</function>. Each of them
                    involves a call to
                    <function>Box.createVerticalStrut()</function>. The
                    <classname>Box</classname> class is a sophisticated layout
                    class that gives the user great flexibility in sizing and
                    positioning components. Here we use a
                    <function>static</function> method of the
                    <classname>Box</classname> class that produces a vertical
                    <glossterm>strut</glossterm>. This is a transparent
                    component whose width expands to fill its parent component
                    (in this case, the message box). The single parameter
                    indicates the height of the strut in pixels. The last call
                    to <function>createVerticalStrut()</function> separates the
                    description text field from the <guilabel>OK</guilabel> and
                    <guilabel>Cancel</guilabel> buttons that are automatically
                    added by <function>showConfirmDialog()</function>.</para>
                </listitem>

                <listitem>
                    <para>Finally, the call to
                    <function>showConfirmDialog()</function> uses defined
                    constants for the option type and the message type. The
                    constants are the same as those used with the
                    <function>Macros.confirm()</function> method; see <xref
                    linkend="helpful-methods" />. The option type signifies the
                    use of <guilabel>OK</guilabel> and
                    <guilabel>Cancel</guilabel> buttons. The
                    <constant>QUERY_MESSAGE</constant> message type causes the
                    message box to display a question mark icon.</para>

                    <para>The return value of the method is tested against the
                    value <constant>OK_OPTION</constant>. If the return value is
                    something else (because the <guilabel>Cancel</guilabel>
                    button was pressed or because the message box window was
                    closed without a button press), a <constant>null</constant>
                    value is returned to a calling function, signaling that the
                    user canceled macro execution. If the return value is
                    <constant>OK_OPTION</constant>, each of the input components
                    can yield their contents for further processing by calls to
                    <function>JTextField.getText()</function> (or, in the case
                    of the check box,
                    <function>JCheckBox.isSelected()</function>).</para>
                </listitem>
            </itemizedlist>
        </section>

        <section id="tips-macro-input-combo">
            <title>Selecting Input From a List</title>

            <para>Another useful way to get user input for a macro is to use a
            combo box containing a number of pre-set options. If this is the
            only input required, one of the versions of
            <function>showInputDialog()</function> in the
            <classname>JOptionPane</classname> class provides a shortcut. Here
            is its prototype:</para>

            <itemizedlist>
                <listitem>
                    <funcsynopsis>
                        <funcprototype>
                            <funcdef>public static Object
                            <function>showInputDialog</function></funcdef>

                            <paramdef>Component
                            <parameter>parentComponent</parameter></paramdef>

                            <paramdef>Object
                            <parameter>message</parameter></paramdef>

                            <paramdef>String
                            <parameter>title</parameter></paramdef>

                            <paramdef>int
                            <parameter>messageType</parameter></paramdef>

                            <paramdef>Icon <parameter>icon</parameter></paramdef>

                            <paramdef>Object[]
                            <parameter>selectionValues</parameter></paramdef>

                            <paramdef>Object
                            <parameter>initialSelectionValue</parameter></paramdef>
                        </funcprototype>
                    </funcsynopsis>
                </listitem>
            </itemizedlist>

            <para>This method creates a message box containing a drop-down list
            of the options specified in the method's parameters, along with
            <guilabel>OK</guilabel> and <guilabel>Cancel</guilabel> buttons.
            Compared to <function>showConfirmDialog()</function>, this method
            lacks an <varname>optionType</varname> parameter and has three
            additional parameters: an <varname>icon</varname> to display in the
            dialog (which can be set to <constant>null</constant>), an array of
            <varname>selectionValues</varname> objects, and a reference to one
            of the options as the <varname>initialSelectionValue</varname> to be
            displayed. In addition, instead of returning an
            <classname>int</classname> representing the user's action,
            <function>showInputDialog()</function> returns the
            <classname>Object</classname> corresponding to the user's selection,
            or <constant>null</constant> if the selection is canceled.</para>

            <para>The following macro fragment illustrates the use of this
            method.</para>

            <informalexample>
                <programlisting>// fragment illustrating use of showInputDialog()
options = new Object[5];
options[0] = "JLabel";
options[1] = "JTextField";
options[2] = "JCheckBox";
options[3] = "HistoryTextField";
options[4} = "-- other --";

result = JOptionPane.showInputDialog(view,
    "Choose component class",
    "Select class for input component",
    JOptionPane.QUESTION_MESSAGE,
    null, options, options[0]);</programlisting>
            </informalexample>

            <para>The return value <varname>result</varname> will contain either
            the <classname>String</classname> object representing the selected
            text item or <constant>null</constant> representing no selection.
            Any further use of this fragment would have to test the value of
            <varname>result</varname> and likely exit from the macro if the
            value equaled <constant>null</constant>.</para>

            <para>A set of options can be similarly placed in a
            <classname>JComboBox</classname> component created as part of a
            larger dialog or <function>showMessageDialog()</function> layout.
            Here are some code fragments showing this approach:</para>

            <informalexample>
                <programlisting>// fragments from Display_Abbreviations.bsh
// import statements and other code omitted

// from main routine, this method call returns an array
// of Strings representing the names of abbreviation sets

abbrevSets = getActiveSets();

...

// from showAbbrevs() method

combo = new JComboBox(abbrevSets);
// set width to uniform size regardless of combobox contents
Dimension dim = combo.getPreferredSize();
dim.width = Math.max(dim.width, 120);
combo.setPreferredSize(dim);
combo.setSelectedItem(STARTING_SET); // defined as "global"

// end fragments</programlisting>
            </informalexample>
        </section>

        <section id="macro-tips-single-char">
            <title>Using a Single Keypress as Input</title>

            <para>Some macros may choose to emulate the style of character-based
            text editors such as <application>emacs</application> or
            <application>vi</application>. They will require only a single
            keypress as input that would be handled by the macro but not
            displayed on the screen. If the keypress corresponds to a character
            value, jEdit can pass that value as a parameter to a BeanShell
            script.</para>

            <para>The jEdit class <ulink
            url="../api/org/gjt/sp/jedit/gui/InputHandler.html">InputHandler</ulink>
            is an abstract class that that manages associations between keyboard
            input and editing actions, along with the recording of macros.
            Keyboard input in jEdit is normally managed by the derived class
            <ulink
            url="../api/org/gjt/sp/jedit/gui/DefaultInputHandler.html">DefaultInputHandler</ulink>.
            One of the methods in the <ulink
            url="../api/org/gjt/sp/jedit/gui/InputHandler.html">InputHandler</ulink>
            class handles input from a single keypress:</para>

            <itemizedlist>
                <listitem>
                    <funcsynopsis>
                        <funcprototype>
                            <funcdef>public void
                            <function>readNextChar</function></funcdef>

                            <paramdef>String
                            <parameter>prompt</parameter></paramdef>

                            <paramdef>String
                            <parameter>code</parameter></paramdef>
                        </funcprototype>
                    </funcsynopsis>
                </listitem>
            </itemizedlist>

            <para>When this method is called, the contents of the
            <varname>prompt</varname> parameter is shown in the view's status
            bar. The method then waits for a key press, after which the contents
            of the <varname>code</varname> parameter will be run as a BeanShell
            script, with one important modification. Each time the string
            <varname>__char__</varname> appears in the parameter script, it will
            be substituted by the character pressed. The key press is
            <quote>consumed</quote> by <function>readNextChar()</function>. It
            will not be displayed on the screen or otherwise processed by
            jEdit.</para>

            <para>Using <function>readNextChar()</function> requires a macro
            within the macro, formatted as a single, potentially lengthy string
            literal. The following macro illustrates this technique. It selects
            a line of text from the current caret position to the first
            occurrence of the character next typed by the user. If the character
            does not appear on the line, no new selection occurs and the display
            remains unchanged.</para>

            <informalexample>
                <programlisting>// Next_Char.bsh

script = new StringBuffer(512);
script.append( "start = textArea.getCaretPosition();"         );
script.append( "line = textArea.getCaretLine();"              );
script.append( "end = textArea.getLineEndOffset(line) + 1;"   );
script.append( "text = buffer.getText(start, end - start);"   );
script.append( "match = text.indexOf(__char__, 1);"           );
script.append( "if(match != -1) {"                            );
script.append(   "if(__char__ != '\\n') ++match;"             );
script.append(   "textArea.select(start, start + match - 1);" );
script.append( "}"                                            );

view.getInputHandler().readNextChar("Enter a character",
    script.toString());

// end Next_Char.bsh</programlisting>
            </informalexample>

            <para>Once again, here are a few comments on the macro's
            design.</para>

            <itemizedlist>
                <listitem>
                    <para>A <classname>StringBuffer</classname> object is used
                    for efficiency; it obviates multiple creation of
                    fixed-length <classname>String</classname> objects. The
                    parameter to the constructor of <varname>script</varname>
                    specifies the initial size of the buffer that will receive
                    the contents of the child script.</para>
                </listitem>

                <listitem>
                    <para>Besides the quoting of the script code, the formatting
                    of the macro is entirely optional but (hopefully) makes it
                    easier to read.</para>
                </listitem>

                <listitem>
                    <para>It is important that the child script be
                    self-contained. It does not run in the same namespace as the
                    <quote>parent</quote> macro
                    <filename>Next_Char.bsh</filename> and therefore does not
                    share variables, methods, or scripted objects defined in the
                    parent macro.</para>
                </listitem>

                <listitem>
                    <para>Finally, access to the <ulink
                    url="../api/org/gjt/sp/jedit/gui/InputHandler.html">InputHandler</ulink>
                    object used by jEdit is available by calling
                    <function>getInputHandler()</function> on the current
                    view.</para>
                </listitem>
            </itemizedlist>
        </section>
    </section>

    <section id="startup-scripts">
        <title>Startup Scripts</title>

        <para>On startup, jEdit runs any BeanShell scripts located in the
        <filename>startup</filename> subdirectory of the jEdit installation and
        user settings directories (see <xref linkend="settings-directory" />).
        As with macros, the scripts must have a <filename>.bsh</filename> file
        name extension. Startup scripts are run near the end of the startup
        sequence, after plugins, properties and such have been initialized, but
        before the first view is opened.</para>

        <para>Startup scripts can perform initialization tasks that cannot be
        handled by command line options or ordinary configuration options, such
        as customizing jEdit's user interface by changing entries in the Java
        platform's UIManager class.

        <para>Startup scripts have an additional feature lacking in ordinary
        macros that can help you further customize jEdit. Variables and methods
        defined in a startup script are available in all instances of the
        BeanShell interpreter created in jEdit. This allows you to create a
        personal library of methods and objects that can be accessed at any time
        during the editing session in another macro, the BeanShell shell of the
        Console plugin, or menu items such as
        <guimenu>Utilities</guimenu>><guisubmenu>BeanShell</guisubmenu>><guimenuitem>Evaluate
        BeanShell Expression</guimenuitem>.</para>

        <para>The startup script routine will run script files in the
        installation directory first, followed by scripts in the user settings
        directory. In each case, scripts will be executed in alphabetical order,
        applied without regard to whether the file name contains upper or lower
        case characters.</para>

        <para>If a startup script throws an exception (because, for example, it
        attempts to call a method on a <constant>null</constant> object). jEdit
        will show an error dialog box and move on to the next startup script. If
        script bugs are causing jEdit to crash or hang on startup, you can use
        the <userinput>-nostartupscripts</userinput> command line option to
        disable them for that editing session.</para>

        <para>Another important difference between startup scripts and ordinary
        macros is that startup scripts cannot use the pre-defined variables
        <varname>view</varname>, <varname>textArea</varname>,
        <varname>editPane</varname> and <varname>buffer</varname>. This is
        because they are executed before the initial view is created.</para>

        <para>If you are writing a method in a startup script and wish to use
        one of the above variables, pass parameters of the appropriate type to
        the method, so that a macro calling them after startup can supply the
        appropriate values. For example, a startup script could include a
        method</para>

        <informalexample>
            <programlisting>void doSomethingWithView(View v, String s)  {
    ...
}</programlisting>
        </informalexample>

        <para>so that during the editing session another macro can call the
        method using</para>

        <informalexample>
            <programlisting>doSomethingWithView(view, "something");</programlisting>
        </informalexample>

        <sidebar>
            <title>Reloading startup scripts without restarting</title>

            <para>It is actually possible to reload startup scripts or load
            other scripts without restarting jEdit, using a BeanShell statement
            like the following:</para>

            <programlisting>BeanShell.runScript(view,<replaceable>path</replaceable>,null,false);</programlisting>

            <para>For <replaceable>path</replaceable>, you can substitute any
            string, or a method call such as
            <function>buffer.getPath()</function>.</para>
        </sidebar>
    </section>

    <section id="scripts-command-line">
        <title>Running Scripts from the Command Line</title>

        <para>The <userinput>-run</userinput> command line switch specifies a
        BeanShell script to run on startup:</para>

        <screen><prompt>$ </prompt><userinput>jedit -run=test.bsh</userinput></screen>

        <para>Note that just like with startup scripts, the
        <varname>view</varname>, <varname>textArea</varname>,
        <varname>editPane</varname> and <varname>buffer</varname> variables are
        not defined.</para>

        <para>If another instance is already running, the script will be run in
        that instance, and you will be able to use the
        <function>jEdit.getLastView()</function> method to obtain a view.
        However, if a new instance of jEdit is being started, the script will be
        run at the same time as all other startup scripts; that is, before the
        first view is opened.</para>

        <para>If your script needs a view instance to operate on, you can use
        the following code pattern to obtain one, no matter how or when the
        script is being run:</para>

        <programlisting>void doSomethingUseful()
{
    void run()
    {
        view = jEdit.getLastView();

        // put actual script body here
    }

    if(jEdit.getLastView() == null)
        VFSManager.runInAWTThread(this);
    else
        run();
}

doSomethingUseful();</programlisting>

        <para>If the script is being run in a loaded instance, it can be invoked
        to perform its work immediately. However, if the script is running at
        startup, before an initial view exists, its operation must be delayed to
        allow the view object first to be created and displayed. In order to
        queue the macro's operation, the scripted closure named
        <function>doSomethingUseful()</function> implements the
        <classname>Runnable</classname> interface of the Java platform. That
        interface contains only a single <function>run()</function> method that
        takes no parameters and has no return value. The macro's implementation
        of the <function>run()</function> method contains the
        <quote>working</quote> portion of the macro. Then the scripted object,
        represented by a reference to <varname>this</varname>, is passed to the
        <function>runInAWTThread()</function> method. This schedules the macro's
        operations for execution after the startup routine is complete.</para>

        <para>As this example illustrates, the
        <function>runInAWTThread()</function> method can be used to ensure that
        a macro will perform operations after other operations have completed.
        If it is invoked during startup, it schedules the specified
        <classname>Runnable</classname> object to run after startup is complete.
        If invoked when jEdit is fully loaded, the
        <classname>Runnable</classname> object will execute after all pending
        input/output is complete, or immediately if there are no pending I/O
        operations. This will delay operations on a new buffer, for example,
        until after the buffer is loaded and displayed.</para>
    </section>

    <section id="macro-tips-BeanShell">
        <title>Advanced BeanShell Techniques</title>

        <para>BeanShell has a few advanced features that we haven't mentioned
        yet. They will be discussed in this section.</para>

        <section id="macro-tips-BeanShell-convenience">
            <title>BeanShell's Convenience Syntax

            <para>We noted earlier that BeanShell syntax does not require that
            variables be declared or defined with their type, and that variables
            that are not typed when first used can have values of differing
            types assigned to them. In addition to this <quote>loose</quote>
            syntax, BeanShell allows a <quote>convenience</quote> syntax for
            dealing with the properties of JavaBeans. They may be accessed or
            set as if they were data members. They may also be accessed using
            the name of the property enclosed in quotation marks and curly
            brackets. For example, the following statement are all equivalent,
            assuming <varname>btn</varname> is a <classname>JButton</classname>
            instance:</para>

            <informalexample>
                <programlisting>b.setText("Choose");
b.text = "Choose";
b{"text"} = "Choose";
</programlisting>
            </informalexample>

            <para>The last form can also be used to access a key-value pair of a
            <classname>Hashtable</classname> object.</para>

            
        </section>

        <section id="macro-tips-BeanShell-keywords">
            <title>Special BeanShell Keywords</title>

            <para>BeanShell uses special keywords to refer to variables or
            methods defined in the current or an enclosing block's scope:

            <itemizedlist>
                <listitem>
                    <para>The keyword <function>this</function> refers to the
                    current scope.</para>
                </listitem>

                <listitem>
                    <para>The keyword <function>super</function> refers to the
                    immediately enclosing scope.</para>
                </listitem>

                <listitem>
                    <para>The keyword <function>global</function> refers to the
                    top-level scope of the macro script.</para>
                </listitem>
            </itemizedlist>

            <para>The following script illustrates the use of these
            keywords:</para>

            <informalexample>
                <programlisting>a = "top\n";
foo() {
    a = "middle\n";
    bar() {
        a = "bottom\n";
        textArea.setSelectedText(global.a);
        textArea.setSelectedText(super.a);
        // equivalent to textArea.setSelectedText(this.a):
        textArea.setSelectedText(a);
    }

    bar();
}
foo();</programlisting>
            </informalexample>

            <para>When the script is run, the following text is inserted in the
            current buffer:</para>

            <screen>top
middle
bottom</screen>
        </section>

        <section id="macro-tips-BeanShell-class">
            <title>Implementing Classes and Interfaces</title>

            <para>As discussed in the macro example in <xref
            linkend="dialog-macro" />, scripted objects can implicitly implement
            Java interfaces such as <classname>ActionListener</classname>. For
            example:</para>

            <programlisting>myRunnable() {
    run() {
        System.out.println("Hello world!");
    }

    return this;
}

Runnable r = myRunnable();
new Thread(r).start();</programlisting>

            <para>Frequently it will not be necessary to implement all of the
            methods of a particular interface in order to specify the behavior
            of a scripted object. To prevent BeanShell from throwing exceptions
            for missing interface methods, implement the
            <function>invoke()</function> method, which is called when an
            undefined method is invoked on a scripted object. Typically, the
            implementation of this method will do nothing, as in the following
            example:</para>

            <informalexample>
                <programlisting>invoke(method, args) {}</programlisting>
            </informalexample>

            <para>In addition to the implicit interface definitions described
            above, BeanShell permits full-blown classes to be defined. Indeed,
            almost any Java class definition should work in BeanShell:</para>

            <programlisting>class Cons {
    // Long-live LISP!
    Object car;
    Object cdr;

    rplaca(Object car) {
        this.car = car;
    }

    rplacd(Object cdr) {
        this.cdr = cdr;
    }
}</programlisting>
        </section>
    </section>

    <section id="macro-tips-debugging">
        <title>Debugging Macros</title>

        <para>Here are a few techniques that can prove helpful in debugging
        macros.</para>

        <section id="macro-tips-debugging-exceptions">
            <title>Identifying Exceptions</title>

            <para>An <glossterm>exception</glossterm> is a condition reflecting
            an error or other unusual result of program execution that requires
            interruption of normal program flow and some kind of special
            handling. Java has a rich (and extensible) collection of exception
            classes which represent such conditions.</para>

            <para>jEdit catches exceptions thrown by BeanShell scripts and
            displays them in a dialog box. In addition, the full traceback is
            written to the activity log (see <xref linkend="activity-log" /> for
            more information about the activity log).</para>

            <para>There are two broad categories of errors that will result in
            exceptions:</para>

            <itemizedlist>
                <listitem>
                    <para><emphasis>Interpreter errors</emphasis>, which may
                    arise from typing mistakes like mismatched brackets or
                    missing semicolons, or from BeanShell's failure to find a
                    class corresponding to a particular variable.</para>

                    <para>Interpreter errors are usually accompanied by the line
                    number in the script, along with the cause of the
                    error.</para>
                </listitem>

                <listitem>
                    <para><emphasis>Execution errors</emphasis>, which result
                    from runtime exceptions thrown by the Java platform when
                    macro code is executed.</para>

                    <para>Some exceptions thrown by the Java platform can often
                    seem cryptic. Nevertheless, examining the contents of the
                    activity log may reveals clues as to the cause of the
                    error.</para>
                </listitem>
            </itemizedlist>
        </section>

        <section id="macro-tips-debugging-log">
            <title>Using the Activity Log as a Tracing Tool</title>

            <para>Sometimes exception tracebacks will say what kind of error
            occurred but not where it arose in the script. In those cases, you
            can insert calls that log messages to the activity log in your
            macro. If the logged messages appear when the macro is run, it means
            that up to that point the macro is fine; but if an exception is
            logged first, it means the logging call is located after the cause
            of the error.</para>

            <para>To write a message to the activity log, use the following
            method of the <ulink
            url="../api/org/gjt/sp/util/Log.html">Log</ulink> class:</para>

            <itemizedlist>
                <listitem>
                    <funcsynopsis>
                        <funcprototype>
                            <funcdef>public static void
                            <function>log</function></funcdef>

                            <paramdef>int
                            <parameter>urgency</parameter></paramdef>

                            <paramdef>Object
                            <parameter>source</parameter></paramdef>

                            <paramdef>Object
                            <parameter>message</parameter></paramdef>
                        </funcprototype>
                    </funcsynopsis>
                </listitem>
            </itemizedlist>

            <para>See the documentation for the <ulink
            url="../api/org/gjt/sp/util/Log.html">Log</ulink> class for
            information about the method's parameters.

            <para>The following code sends a typical debugging message to the
            activity log:</para>

            <informalexample>
            <programlisting>
Log.log(Log.DEBUG, BeanShell.class, "counter = " + counter);
            </programlisting>
            </informalexample>

            <para>The corresponding activity log entry might read as
            follows:</para>

            <informalexample>
                <programlisting>[debug] BeanShell: counter = 15</programlisting>
            </informalexample>

            <sidebar>
                <title>Using message dialog boxes as a tracing tool</title>

                <para>If you would prefer not having to deal with the activity
                log, you can use the <function>Macros.message()</function>
                method as a tracing tool. Just insert calls like the following
                in the macro code:</para>

                <programlisting>Macros.message(view,"tracing");</programlisting>

                <para>Execution of the macro is halted until the message dialog
                box is closed. When you have finished debugging the macro, you
                should delete or comment out the debugging calls to
                <function>Macros.message()</function> in your final source
                code.</para>
            </sidebar>
        </section>
    </section>
</chapter>

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