Home Page
>
Creating a GUI With JFC/Swing
>
Writing Event Listeners
How to Write a Caret Listener
Caret events occur when the
caret — the cursor
indicating the insertion
point — in a text component moves
or when the selection in a text component changes.
The text component's document can initiate caret events
when it inserts or removes text, for example.
You can attach a caret listener to an instance of any
JTextComponent subclass with the
addCaretListener method.
Note: An alternate way of detecting caret changes is to attach a
listener directly to the caret object itself rather than to
the text component that manages the caret.
A caret fires change events (
not
caret events), so you would need to write a
change listener
rather than a caret listener.
Here is the caret event handling code from an application
called TextComponentDemo:
...
//where initialization occurs
CaretListenerLabel caretListenerLabel =
new CaretListenerLabel("Caret Status");
...
textPane.addCaretListener(caretListenerLabel);
...
protected class CaretListenerLabel extends JLabel
implements CaretListener
{
...
//Might not be invoked from the event dispatching thread.
public void caretUpdate(CaretEvent e) {
displaySelectionInfo(e.getDot(), e.getMark());
}
//This method can be invoked from any thread. It
//invokes the setText and modelToView methods, which
//must run in the event dispatching thread. We use
//invokeLater to schedule the code for execution
//in the event dispatching thread.
protected void displaySelectionInfo(final int dot,
final int mark) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
if (dot == mark) { // no selection
...
}
});
}
}
}
Note: The caretUpdate method is not guaranteed
to be called in the event-dispatching thread.
To use any methods inside of caretUpdate
that update the GUI special
handling is required to ensure they are executed on
the event-dispatching thread. You can do this by wrapping
the code inside a Runnable and calling
SwingUtilities.invokeLater on that
Runnable.
You can find a link to the source file for
TextComponentDemo in the
example index for using Swing Components. For a discussion about the caret listener aspect
of the program see
Listening for Caret and Selection Changes in
Text Component Features.
The CaretListener
Interface
Because CaretListener has only one method,
it has no corresponding adapter class.
| Method |
Purpose |
|
caretUpdate(CaretEvent) |
Called when the caret in the listened-to component moves
or when the selection in the listened-to component changes.
|
The CaretEvent Class
| Method |
Purpose |
|
int getDot() |
Returns the current location of the caret.
If text is selected, the caret marks one end of the selection.
|
|
int getMark() |
Returns the other end of the selection.
If nothing is selected,
the value returned by this method is equal to the
value returned by getDot.
Note that the dot is not guaranteed to be less than the mark.
|
Object getSource()
(in java.util.EventObject)
|
Returns the object that fired the event.
|
Examples that Use Caret Listeners
The following table lists the
examples that use caret listeners.
