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Trail: Creating a GUI with JFC/Swing
Lesson: Writing Event Listeners

## How to Write an Undoable Edit Listener

> Undoable edit events occur when an operation that can be undone occurs on a component. Currently, only text components fire undoable edit events, and then only indirectly. The text component's document fires the events. For text components, undoable operations include inserting characters, deleting characters, and modifying the style of text. Programs typically listen to undoable edit events to assist in the implementation of undo and redo commands.

> Here is the undoable edit event handling code from an application called `TextComponentDemo`.

> >     ...
> >     //where initialization occurs
> >     document.addUndoableEditListener(new MyUndoableEditListener());
> >     ...
> >     protected class MyUndoableEditListener implements UndoableEditListener {
> >         public void undoableEditHappened(UndoableEditEvent e) {
> >             //Remember the edit and update the menus
> >             undo.addEdit(e.getEdit());
> >             undoAction.updateUndoState();
> >             redoAction.updateRedoState();
> >         }
> >     }

> You can find the full source code for the program and instructions for compiling and running it in General Rules for Using Text Components. For a discussion about the undoable edit listener aspect of the program see Implementing Undo and Redo

### The Undoable Edit Event API

> The `UndoableEditListener` interface has just one method, so it has no corresponding adapter class. Here's the method:
> `void undoableEditHappened(UndoableEditEvent)`
>   Called when an undoable event occurs on the listened-to component.
> The `undoableEditHappened` method has a single parameter: a `UndoableEditEvent` object. To get the document that fired the event, use the `getSource` method which `UndoableEditEvent` inherits from `EventObject`.

> The `UndoableEditEvent` class defines one method which returns an object that contains detailed information about the edit that occurred.

> `UndoableEdit getEdit()`
>   Returns an `UndoableEdit` object that represents the edit that occurred and contains information about and commands for undoing or redoing the edit.

### Examples that Use Undoable Edit Listeners

> The following table lists the examples that use undoable edit listeners.

> Example | Where Described | Notes
> --- | --- | ---
> `TextComponentDemo` | Implementing Undo and Redo | Implements undo and redo on a text pane with help from an undoable edit listener.

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