Time for action – running on the UI thread

To execute code on the UI thread, Runnables are posted to the Display class via two methods, syncExec() and asyncExec(). The syncExec() method runs the code synchronously (that is, the caller blocks until the code has been run), while the asyncExec() method runs the code asynchronously (that is, the caller continues while the code is run in the background).

The Display class is SWT's handle to a monitor (so a runtime may have more than one Display object, and each may have its own resolution). To get hold of an instance, call either Display.getCurrent() or Display.getDefault(). However, it's much better to get a Display class from an associated view or widget. In this case, the Canvas has an associated Display.

  1. Go to the TickTock thread inside the createPartControl() method of the ClockView class.
  2. Inside the run() method, replace the call to clock.redraw() with:
    clock.getDisplay().asyncExec(new Runnable() {
  public void run() {
    if(clock != null && !clock.isDisposed())
      clock.redraw();
  }
});
  3. Run the test Eclipse instance and show the Clock View. The second hand should now update automatically.

What just happened?

This time, the event will execute as expected. One thread, TickTock, is running in the background, and every second it posts a Runnable to the UI thread, which then runs asynchronously. This example could have used syncExec(), and the difference would not have been noticeable; but in general, using asyncExec() is to be preferred unless there is a specific reason to need the synchronous blocking behavior.

The thread is in a while loop and is guarded with a call to clock.isDisposed(). Each SWT widget is non-disposed when it is initially created, and then subsequently disposed with a call to dispose(). Once a widget is disposed, any native operating system resources are returned and any further operations will throw an exception. In this example, the Canvas is disposed when the view is closed, which in turn disposes the components contained therein. As a result, when the view is closed, the Thread automatically ceases its loop. (The Thread can also be aborted by interrupting it during its 1 second sleep pauses.)

Note that the test for whether the widget is not null or disposed is done in the Runnable as well. Although the widget isn't disposed when it is added to the event queue, it might be when the event is processed some time later.

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