October 3rd, 2009

Trident 1.1 – supporting Java-based UI toolkits

Trident animation library for Java applications is nearing release 1.1 (code-named Bogeyman), and it’s time to take a look at the new APIs added in this version. This entry is going to talk about supporting Java UI toolkits.

UI toolkit handlers

Graphical applications are a natural fit for animations, and Trident core has built-in support for Swing and SWT. This support covers threading rules, custom property interpolators and repaint timelines. Application code that needs to support additional Java-based UI toolkits should register a custom UI toolkit handler.

Most modern UI toolkits have threading rules that the applications must respect in order to prevent application freeze and visual artifacts. The threading rules for both Swing and SWT specify that the UI-related operations must be done on a special UI thread, and the methods in the org.pushingpixels.trident.UIToolkitHandler are used to determine the relevance of these threading rules. The UIToolkitHandler interface is illustrated by the core support for SWT:

public class SWTToolkitHandler implements UIToolkitHandler {
	public boolean isHandlerFor(Object mainTimelineObject) {
		return (mainTimelineObject instanceof Widget);
	public boolean isInReadyState(Object mainTimelineObject) {
		return !((Widget) mainTimelineObject).isDisposed();
	public void runOnUIThread(Runnable runnable) {

This is a very simple implementation of a UI toolkit handler that respects the relevant threading rules:

  • The isHandlerFor associates this handler with all SWT widgets
  • The isInReadyState marks disposed widgets to skip the property interpolation / callback invocations
  • The runOnUIThread runs the UI related logic on the SWT thread

Registering custom UI toolkit handlers

Trident provides two ways to register custom UI toolkit handlers – customization APIs and plugins.

The TridentConfig class has the following APIs to work with UI toolkit handlers:

  • addUIToolkitHandler(UIToolkitHandler) – registers the UI toolkit handler
  • removeUIToolkitHandler(UIToolkitHandler) – unregisters the UI toolkit handler
  • getUIToolkitHandlers() – retrieves an unmodifiable collection of all registered (core and custom) UI toolkit handlers

The UIToolkitHandler entries in the plugin descriptor files allow application code to support additional Java-based UI toolkits. The value associated with this key must be the fully qualified class name of an application class that implements the org.pushingpixels.trident.UIToolkitHandler interface.

Respecting the threading rules

The UIToolkitHandler.isHandlerFor(Object) is used to determine whether the main timeline object is a component / widget for the specific UI toolkit. At runtime, all fields registered with the Timeline.addPropertyToInterpolate methods will be changed on the UI thread using the UIToolkitHandler.runOnUIThread method.

In the simple Swing example that interpolates the foreground color of a button on mouse rollover, the timeline is configured as

Timeline rolloverTimeline = new Timeline(button);
rolloverTimeline.addPropertyToInterpolate("foreground", Color.blue,

If you put a breakpoint in the JComponent.setForeground(Color) – which is called on every timeline pulse – you will see that it is called on the Swing Event Dispatch Thread. Internally, this is what happens:

  • When the timeline is created, all registered UI toolkit handlers are asked whether they are handlers for the specified object
  • The org.pushingpixels.trident.swing.SwingToolkitHandler registered in the core library returns true for the button object in its isHandlerFor(Object)
  • On every timeline pulse, a Runnable object is created internally. The run() method calls the setters for all registered fields – using the PropertyInterpolator.interpolate method of the matching property interpolator
  • This Runnable is passed to the UIToolkitHandler.runOnUIThread method of the matching UI toolkit handler.

And this is how SwingToolkitHandler.runOnUIThread() is implemented:

public void runOnUIThread(Runnable runnable) {
	if (SwingUtilities.isEventDispatchThread())

Running custom application code on UI thread

The flow described above works for the fields registered with the Timeline.addPropertyToInterpolate methods. What about the custom application callbacks registered with the Timeline.addCallback()? If the callback methods need to respect the UI threading rules of the matching toolkit, the TimelineCallback implementation class needs to be tagged with the org.pushingpixels.trident.callback.RunOnUIThread annotation.

Callback implementations marked with this annotation will have both onTimelineStateChanged and onTimelinePulse invoked on the UI thread, making it safe to query and change the UI. The UIThreadTimelineCallbackAdapter is a core adapter class that is marked with this annotation.

Querying the readiness of the timeline object

The isInReadyState(Object) is the third and final method in the UIToolkitHandler interface. After the specific UI toolkit handler has declared that it will handle the main object of the specific timeline (by returning true from the isHandlerFor(Object) method), it will be used to interpolate the registered fields and run the registered callbacks. However, some UI toolkits may impose additional restrictions on when the UI object is ready to be queried / changed.

For example, once an SWT control is disposed, it will throw an SWTException in the setForeground method. So, if the application code is running a slow animation that changes the foreground color of a button, and the application window containing this button is disposed in the meantime, the call to setForeground should be skipped.