org.gnome.gtk
Interface Widget.ExposeEvent

Enclosing class:

Widget

public static interface Widget.ExposeEvent

The signal emitted when a portion or all of the Widget has been exposed and needs [re]drawing. This can happen when a Widget is first realized to the screen or when part of it has come back from being obscured (by another Window or because it was offscreen).

The event parameter to the callback contains information about the size of the region that was damaged or otherwise needs redrawing. For instance, if you just wanted to know what area was exposed, you could do:

 foo.connect(new Widget.ExposeEvent() {
     public boolean onExposeEvent(Widget source, EventExpose event) {
         Rectangle rect;
         int width, height, x, y;
         
         rect = event.getArea();
         
         width = rect.getWidth();
         height = rect.getHeight();
         x = rect.getX();
         y = rect.getY();
         
         System.out.println(width + "x" + height + " at " + x + "," + y);
     }
 }
 

The real purpose of Widget.ExposeEvent, however, is to be the the gateway to drawing. GNOME uses the excellent Cairo 2D graphics library to draw its user interfaces, which we make available in java-gnome in package org.freedesktop.cairo.

Code that does drawing needs to be written a little differently than code which just builds Widgets up into user interfaces. While we are accustomed to doing setting up various Widgets and packing them into Windows in our constructors, the one thing you cannot easily do there is graphics drawing. In order to be able to construct the Context used for drawing operations, Cairo needs the details of the [org.gnome.gdk] Display, Screen and Window it will be drawing to, and these are not available until the Widget has been realized and mapped. The Widget.ExposeEvent signal is emitted exactly at this point, and so that's when we have the environment we need to do our drawing.

To do drawing with Cairo you need a Context. You can instantiate one by passing the EventExpose object to the Context constructor:

 foo.connect(new Widget.ExposeEvent() {
     public boolean onExposeEvent(Widget source, EventExpose event) {
         Context cr;
         
         cr = new Context(event);
         
         // start drawing
     }
 }
 

Since:

4.0.7

Author:

Andrew Cowie

onExposeEvent

boolean onExposeEvent(Widget source,
                      EventExpose event)

As with other event signals, the boolean return value indicates whether or not you wish to block further emission of the signal. In general you want to leave the default handlers alone; let them run as well. Return false.

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