Over the past few days i’ve been playing with adding animations to augmented reality applications. The code is in a rough shape, since i’m juggling the relevant libraries for the first time, and i’m going to publish it over the next week. Here is the first video that shows my current progress:

What is it using?

I’m not going to take credit for this demo. The vast majority of work is being done by JMF, NYArToolkit and Java3D (and they’re quite painful to set up, by the way). The actual visuals are amateur at best and can look much better. But at least it shows where you can take your Java today :)

Trident is an animation library for Java applications, and this week i’ve written about the concepts behind it and APIs available to interested applications:

What’s next? Version 1.0 (code-named Acumen) is right around the corner. Release candidate is scheduled for June 29, and the final release is scheduled for July 13.

Trident is a new library, and its APIs need to be tested in real-world scenarios. While i have a few small test applications that illustrate the specific API methods, as well as medium sized demos (Onyx and Amber), there is always room for improvement.

Going forward, i intend to evolve Trident, and i already have a couple of post-1.0 features in the pipeline. Trident has evolved from the internal animation layer of Substance look-and-feel, and the next major release of Substance will be rewritten to use Trident – further testing the published APIs for usage in real-world scenarios. In addition, the next major release of Flamingo ribbon will add Trident-based animations – where applicable.

Your input and feedback are always highly appreciated. Download the latest daily bits, and read the documentation. Subscribe to the mailing lists and let me know what is missing, and how the existing APIs can be improved. If you find a bug, report it in the issue tracker. If you want to take a look at the code, check out the SVN repository and subscribe to the “commits” mailing list.

Swing / SWT applications do not have to be boring. Trident aims to simplify the development of rich animation effects in Java based UI applications, addressing both simple and complex scenarios. But it can only be as good as the applications that are using it. So, read the documentation, download the sources / binaries, integrate it in your applications and let me know what you think.

Over the course of this week i’m talking about different concepts in the Trident animation library for Java applications. Part ten talks about the plugin layer allows interested applications to provide additional property interpolators for custom application classes and support additional Java-based UI toolkits.

Plugin overview

The core functionality of the Trident library can be extended to address custom needs of the specific applications. Out of the box Trident supports:

  • Interpolating float and integer fields of any Java object that provides the matching public setter methods.
  • Swing and SWT UI toolkits, respecting the threading rules and providing interpolators for the custom graphic classes

The extensibility (plugin) layer allows interested applications to:

  • Provide additional property interpolators for custom application classes
  • Support additional Java-based UI toolkits

Creating a Trident plugin

A Trident plugin is specified by the META-INF/trident-plugin.properties file that should be placed in the runtime classpath. Note that you can have multiple plugins in the same runtime environment – if each one is coming from a different classpath jar, for example.

The format of trident-plugin.properties is simple. Each line in this file should be of the following format:

Key=FullyQualifiedClassName

There are two supported keys:

Sample plugin specification

The core Trident library contains a plugin that supports Swing and SWT UI toolkits, as well as property interpolators for a few core classes. Here are the contents of the plugin descriptor:

UIToolkitHandler=org.pushingpixels.trident.swing.SwingToolkitHandler
PropertyInterpolatorSource=org.pushingpixels.trident.swing.AWTPropertyInterpolators

UIToolkitHandler=org.pushingpixels.trident.swt.SWTToolkitHandler
PropertyInterpolatorSource=org.pushingpixels.trident.swt.SWTPropertyInterpolators

PropertyInterpolatorSource=org.pushingpixels.trident.interpolator.CorePropertyInterpolators

Property interpolator plugins

The PropertyInterpolatorSource entries in the plugin descriptor files allow application code to provide property interpolators for custom application classes. The value associated with this key must be the fully qualified class name of an application class that implements the org.pushingpixels.trident.interpolator.PropertyInterpolatorSource interface.

This interface has one method – public Set<PropertyInterpolator> getPropertyInterpolators() – which returns a set of custom property interpolators. Custom property interpolators can be used in two ways:

  • The Timeline.addPropertyToInterpolate(String, Object, Object, PropertyInterpolator) API to explicitly specify the property interpolator to be used
  • The Timeline.addPropertyToInterpolate(String, Object, Object) API that will choose the property interpolator that matches the types of the from and to values

Property interpolator specification

The org.pushingpixels.trident.interpolator.PropertyInterpolator interface has two methods.

The public Class getBasePropertyClass() is used to choose the property interpolator in the Timeline.addPropertyToInterpolate(String, Object, Object). Internally, all registered property interpolators are queried to check whether they support the specified from and to values using the Class.isAssignableFrom(Class). The first property interpolator that has a match for both values will be used.

For example, the PointInterpolator in the core AWT property interpolator source (AWTPropertyInterpolators class) has the following implementation of this method:

	@Override
	public Class getBasePropertyClass() {
		return Point.class;
	}

The public T interpolate(T from, T to, float timelinePosition) is used to compute the interpolated value during the current timeline pulse. For example, the PointInterpolator in the core AWT property interpolator source (AWTPropertyInterpolators class) has the following implementation of this method:

	public Point interpolate(Point from, Point to, float timelinePosition) {
		int x = from.x + (int) (timelinePosition * (to.x - from.x));
		int y = from.y + (int) (timelinePosition * (to.y - from.y));
		return new Point(x, y);
	}

Bringing it together

Let’s look at the following Swing snippet that has a class with a Point field and a timeline that interpolates the value of that field:

import java.awt.*;

public static class MyRectangle {
	private Point corner = new Point(0, 0);

	public void setCorner(Point corner) {
		this.corner = corner;
	}

	...
}

Timeline move = new Timeline(rectangle);
move.addPropertyToInterpolate("corner", new Point(0, 0),
	new Point(100, 80));
move.playLoop(RepeatBehavior.REVERSE);

What happens when move.addPropertyToInterpolate is called? Internally, the Trident core looks at all available property interpolators and finds that the AWTPropertyInterpolators.PointInterpolator is the best match for the passed values (which are both java.awt.Points). Then, at every pulse of the move timeline, the MyRectangle.setCorner(Point) is called.

Note that the application code did not explicitly specify which property interpolator should be used.

UI toolkit plugins

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.

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. 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 UIToolkitHandler are used to determine the relevance of these threading rules.

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,
			Color.red);

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:

	@Override
	public void runOnUIThread(Runnable runnable) {
		if (SwingUtilities.isEventDispatchThread())
			runnable.run();
		else
			SwingUtilities.invokeLater(runnable);
	}

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.

SWT implementation of the UI toolkit handler

The trident-plugin.properties descriptor bundled with the core Trident library has the following line:

UIToolkitHandler=org.pushingpixels.trident.swt.SWTToolkitHandler

And the SWTToolkitHandler is:

public class SWTToolkitHandler implements UIToolkitHandler {
	@Override
	public boolean isHandlerFor(Object mainTimelineObject) {
		return (mainTimelineObject instanceof Widget);
	}

	@Override
	public boolean isInReadyState(Object mainTimelineObject) {
		return !((Widget) mainTimelineObject).isDisposed();
	}

	@Override
	public void runOnUIThread(Runnable runnable) {
		Display.getDefault().asyncExec(runnable);
	}
}

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

  • The first method implements the logic that associates this handler with all SWT widgets
  • The second method marks disposed widgets to skip the property interpolation / callback invocations
  • The third method runs the UI related logic on the SWT thread

Over the course of this week i’m talking about different concepts in the Trident animation library for Java applications. Part nine talks about key frames that allow interpolating properties between more than two values.

Key frames

The sample application in multiple timelines example has the following basic timeline:

this.rolloverTimeline = new Timeline(this);
this.rolloverTimeline.addPropertyToInterpolate("backgroundColor",
		Color.yellow, Color.black);

This creates a timeline that animates the backgroundColor field from yellow to black when the timeline is played. What if you want to animate this field from yellow to green, and then to black? One option is to create two timelines, one to animate from yellow to green, and another to animate from green to black. Since the second timeline needs to wait until the first one ends, you will have to either use the timeline callbacks, or create a parallel timeline scenario.

However, there is a simpler solution for interpolating the value of the specific field between more than two values – key frames. A key frame defines the value of the field at the particular timeline duration fraction (not timeline position).

Key times and key values

In our example, the timeline to interpolate the background color will have three key frames:

  • The beginning key frame at key time 0.0f with associated key value of Color.yellow
  • The intermediate key frame at the matching key time (any value between 0.0f and 1.0f based on the application requirement) with associated key value of Color.green
  • The ending key frame at key time 1.0f with associated key value of Color.black

Each key frame has two mandatory associated properties: key time and key value. Key times must form a strictly increasing sequence that starts at 0.0f and ends at 1.0f. Key values must have either an explicit or an implicit property interpolator. This interpolator must be able to compute interpolated value for any successive pair of key values; this value should be of a class that can be passed to the public setter of the relevant property.

Simple example

To put it all together, here is the definition of a key frame-driven timeline for the example above:

this.rolloverTimeline = new Timeline(this);

KeyValues alphaValues = KeyValues.create(Color.yellow,
		Color.green, Color.black);
KeyTimes alphaTimes = new KeyTimes(0.0f, 0.5f, 1.0f);
this.rolloverTimeline.addPropertyToInterpolate("backgroundColor",
		new KeyFrames(alphaValues, alphaTimes));

Here, we specify that the backgroundColor starts at yellow, goes to green at half the duration of the timeline, and then goes to black at the end.

Simple key frame example

The SWT application discussed below implements a simple infinite progress indication illustrated in this screenshot:

The bluish highlighter moves from left to right, fading in as it appears on the left edge, and fading out as it disappears on the right edge. There are two properties that control the appearance of the highlighter:

  • xPosition – an integer property that is linearly interpolated between the left X and the right X.
  • alpha – a float property that starts at 0.0f, goes to 1.0f at 30% of the timeline duration, stays at 1.0f until the timeline reaches its 70% mark and then goes back to 0.0f

The alpha property in this example is interpolated using key frames.

The progress indication panel is an SWT Canvas with two fields and matching public setters:

public static class ProgressPanel extends Canvas {
	private int xPosition;

	private float alpha;

	public void setXPosition(int position) {
		xPosition = position;
	}

	public void setAlpha(float alpha) {
		this.alpha = alpha;
	}
}

The constructor of this panel wires a mouse listener that starts the indefinite progress animation. The boolean started field tracks whether this animation has been started:

private boolean started;

public ProgressPanel(Composite parent) {
	super(parent, SWT.DOUBLE_BUFFERED);

	this.xPosition = 0;
	this.alpha = 0;

	this.addMouseListener(new MouseAdapter() {
		@Override
		public void mouseUp(MouseEvent e) {
			if (started)
				return;
			start();
			started = true;
		}
	});
}

The start() method creates a timeline that interpolates the X position and alpha of the progress highlight. The X position is a simple interpolation between two values (taking into account that the highlight should not be painted outside the track). The alpha interpolation uses key frames:

public void start() {
	Timeline progressTimeline = new Timeline(this);

	int startX = (this.getBounds().width - INNER_WIDTH) / 2 + 18
			+ HIGHLIGHTER_WIDTH / 2;
	int endX = (this.getBounds().width + INNER_WIDTH) / 2 - 18
			- HIGHLIGHTER_WIDTH / 2;
	progressTimeline
			.addPropertyToInterpolate("xPosition", startX, endX);

	KeyValues alphaValues = KeyValues.create(0.0f, 1.0f, 1.0f, 0.0f);
	KeyTimes alphaTimes = new KeyTimes(0.0f, 0.3f, 0.7f, 1.0f);
	progressTimeline.addPropertyToInterpolate("alpha", new KeyFrames(
			alphaValues, alphaTimes));

	progressTimeline.setDuration(1500);
	progressTimeline.playLoop(RepeatBehavior.LOOP);
}

The panel constructor also creates a repaint timeline so that the progress animation is properly reflected on the screen:

new SWTRepaintTimeline(this).playLoop(RepeatBehavior.LOOP);

The actual painting is done in a custom PaintListener added in the panel constructor. The full code can be found in the test.swt.ProgressIndication class. It uses the matching SWT graphics operations to paint the overall background, the inner gradient background and contour, the track and the track highlight. The track highlight painting uses the current values of both xPosition and alpha fields to display the correct visuals.

Click below for the WebStart demo of the Swing version

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