Adding transitions and playing videos in Android

Animation
The Android framework provides two animation systems: property animation (introduced in Android 3.0) and view animation. Both animation systems are viable options, but the property animation system, in general, is the preferred method to use, because it is more flexible and offers more features. In addition to these two systems, you can utilize Drawable animation, which allows you to load drawable resources and display them one frame after another.

Property Animation
    Introduced in Android 3.0 (API level 11), the property animation system lets you animate properties of any object, including ones that are not rendered to the screen. The system is extensible and lets you animate properties of custom types as well.

View Animation
    View Animation is the older system and can only be used for Views. It is relatively easy to setup and offers enough capabilities to meet many application's needs.

Drawable Animation
    Drawable animation involves displaying Drawable resources one after another, like a roll of film. This method of animation is useful if you want to animate things that are easier to represent with Drawable resources, such as a progression of bitmaps.

The property animation system is a robust framework that allows you to animate almost anything. You can define an animation to change any object property over time, regardless of whether it draws to the screen or not. A property animation changes a property's (a field in an object) value over a specified length of time. To animate something, you specify the object property that you want to animate, such as an object's position on the screen, how long you want to animate it for, and what values you want to animate between.

The property animation system lets you define the following characteristics of an animation:

• Duration: You can specify the duration of an animation. The default length is 300 ms.
• Time interpolation: You can specify how the values for the property are calculated as a function of the animation's current elapsed time.
• Repeat count and behavior: You can specify whether or not to have an animation repeat when it reaches the end of a duration and how many times to repeat the animation. You can also specify whether you want the animation to play back in reverse. Setting it to reverse plays the animation forwards then backwards repeatedly, until the number of repeats is reached.
• Animator sets: You can group animations into logical sets that play together or sequentially or after specified delays.
• Frame refresh delay: You can specify how often to refresh frames of your animation. The default is set to refresh every 10 ms, but the speed in which your application can refresh frames is ultimately dependent on how busy the system is overall and how fast the system can service the underlying timer.

How Property Animation Works

First, let's go over how an animation works with a simple example. Figure 1 depicts a hypothetical object that is animated with its xproperty, which represents its horizontal location on a screen. The duration of the animation is set to 40 ms and the distance to travel is 40 pixels. Every 10 ms, which is the default frame refresh rate, the object moves horizontally by 10 pixels. At the end of 40ms, the animation stops, and the object ends at horizontal position 40. This is an example of an animation with linear interpolation, meaning the object moves at a constant speed.

Figure 1. Example of a linear animation

You can also specify animations to have a non-linear interpolation. Figure 2 illustrates a hypothetical object that accelerates at the beginning of the animation, and decelerates at the end of the animation. The object still moves 40 pixels in 40 ms, but non-linearly. In the beginning, this animation accelerates up to the halfway point then decelerates from the halfway point until the end of the animation. As Figure 2 shows, the distance traveled at the beginning and end of the animation is less than in the middle.

Figure 2. Example of a non-linear animation

Let's take a detailed look at how the important components of the property animation system would calculate animations like the ones illustrated above. Figure 3 depicts how the main classes work with one another.

Figure 3. How animations are calculated

The ValueAnimatorobject keeps track of your animation's timing, such as how long the animation has been running, and the current value of the property that it is animating.

The ValueAnimatorencapsulates a TimeInterpolator, which defines animation interpolation, and a TypeEvaluator, which defines how to calculate values for the property being animated. For example, in Figure 2, the TimeInterpolatorused would be AccelerateDecelerateInterpolatorand the TypeEvaluatorwould be IntEvaluator.

To start an animation, create a ValueAnimatorand give it the starting and ending values for the property that you want to animate, along with the duration of the animation. When you call start()the animation begins. During the whole animation, the ValueAnimatorcalculates an elapsed fraction between 0 and 1, based on the duration of the animation and how much time has elapsed. The elapsed fraction represents the percentage of time that the animation has completed, 0 meaning 0% and 1 meaning 100%. For example, in Figure 1, the elapsed fraction at t = 10 ms would be .25 because the total duration is t = 40 ms.

When the ValueAnimatoris done calculating an elapsed fraction, it calls the TimeInterpolatorthat is currently set, to calculate an interpolated fraction. An interpolated fraction maps the elapsed fraction to a new fraction that takes into account the time interpolation that is set. For example, in Figure 2, because the animation slowly accelerates, the interpolated fraction, about .15, is less than the elapsed fraction, .25, at t = 10 ms. In Figure 1, the interpolated fraction is always the same as the elapsed fraction.

When the interpolated fraction is calculated, ValueAnimatorcalls the appropriate TypeEvaluator, to calculate the value of the property that you are animating, based on the interpolated fraction, the starting value, and the ending value of the animation. For example, in Figure 2, the interpolated fraction was .15 at t = 10 ms, so the value for the property at that time would be .15 X (40 - 0), or 6.

The com.example.android.apis.animationpackage in the API Demos sample project provides many examples on how to use the property animation system.

How Property Animation Differs from View Animation

The view animation system provides the capability to only animate Viewobjects, so if you wanted to animate non-Viewobjects, you have to implement your own code to do so. The view animation system is also constrained in the fact that it only exposes a few aspects of a Viewobject to animate, such as the scaling and rotation of a View but not the background color, for instance.

Another disadvantage of the view animation system is that it only modified where the View was drawn, and not the actual View itself. For instance, if you animated a button to move across the screen, the button draws correctly, but the actual location where you can click the button does not change, so you have to implement your own logic to handle this.

With the property animation system, these constraints are completely removed, and you can animate any property of any object (Views and non-Views) and the object itself is actually modified. The property animation system is also more robust in the way it carries out animation. At a high level, you assign animators to the properties that you want to animate, such as color, position, or size and can define aspects of the animation such as interpolation and synchronization of multiple animators.

The view animation system, however, takes less time to setup and requires less code to write. If view animation accomplishes everything that you need to do, or if your existing code already works the way you want, there is no need to use the property animation system. It also might make sense to use both animation systems for different situations if the use case arises.

API Overview

You can find most of the property animation system's APIs in android.animation. Because the view animation system already defines many interpolators in android.view.animation, you can use those interpolators in the property animation system as well. The following tables describe the main components of the property animation system.

The Animatorclass provides the basic structure for creating animations. You normally do not use this class directly as it only provides minimal functionality that must be extended to fully support animating values. The following subclasses extend Animator:

Table 1. Animators

Evaluators tell the property animation system how to calculate values for a given property. They take the timing data that is provided by an Animatorclass, the animation's start and end value, and calculate the animated values of the property based on this data. The property animation system provides the following evaluators:

Table 2. Evaluators

A time interpolator defines how specific values in an animation are calculated as a function of time. For example, you can specify animations to happen linearly across the whole animation, meaning the animation moves evenly the entire time, or you can specify animations to use non-linear time, for example, accelerating at the beginning and decelerating at the end of the animation. Table 3 describes the interpolators that are contained in android.view.animation. If none of the provided interpolators suits your needs, implement the TimeInterpolatorinterface and create your own. See Using interpolators for more information on how to write a custom interpolator.

Table 3. Interpolators

Animating with ValueAnimator

The ValueAnimatorclass lets you animate values of some type for the duration of an animation by specifying a set of int, float, or color values to animate through. You obtain a ValueAnimatorby calling one of its factory methods: ofInt(), ofFloat(), or ofObject(). For example:

ValueAnimator animation = ValueAnimator.ofFloat(0f, 1f);
animation.setDuration(1000);
animation.start();

In this code, the ValueAnimatorstarts calculating the values of the animation, between 0 and 1, for a duration of 1000 ms, when the start()method runs.

You can also specify a custom type to animate by doing the following:

ValueAnimator animation = ValueAnimator.ofObject(new MyTypeEvaluator(), startPropertyValue, endPropertyValue);
animation.setDuration(1000);
animation.start();

In this code, the ValueAnimatorstarts calculating the values of the animation, between startPropertyValueand endPropertyValueusing the logic supplied by MyTypeEvaluatorfor a duration of 1000 ms, when the start()method runs.

The previous code snippets, however, has no real effect on an object, because the ValueAnimatordoes not operate on objects or properties directly. The most likely thing that you want to do is modify the objects that you want to animate with these calculated values. You do this by defining listeners in the ValueAnimatorto appropriately handle important events during the animation's lifespan, such as frame updates. When implementing the listeners, you can obtain the calculated value for that specific frame refresh by calling getAnimatedValue(). For more information on listeners, see the section about Animation Listeners.

Animating with ObjectAnimator

The ObjectAnimatoris a subclass of the ValueAnimator(discussed in the previous section) and combines the timing engine and value computation of ValueAnimatorwith the ability to animate a named property of a target object. This makes animating any object much easier, as you no longer need to implement the ValueAnimator.AnimatorUpdateListener, because the animated property updates automatically.

Instantiating an ObjectAnimatoris similar to a ValueAnimator, but you also specify the object and the name of that object's property (as a String) along with the values to animate between:

ObjectAnimator anim = ObjectAnimator.ofFloat(foo, "alpha", 0f, 1f);
anim.setDuration(1000);
anim.start();

To have the ObjectAnimatorupdate properties correctly, you must do the following:

• The object property that you are animating must have a setter function (in camel case) in the form of set(). Because the ObjectAnimatorautomatically updates the property during animation, it must be able to access the property with this setter method. For example, if the property name is foo, you need to have a setFoo()method. If this setter method does not exist, you have three options:
  • Add the setter method to the class if you have the rights to do so.
  • Use a wrapper class that you have rights to change and have that wrapper receive the value with a valid setter method and forward it to the original object.
  • Use ValueAnimatorinstead.
• If you specify only one value for the values...parameter in one of the ObjectAnimatorfactory methods, it is assumed to be the ending value of the animation. Therefore, the object property that you are animating must have a getter function that is used to obtain the starting value of the animation. The getter function must be in the form of get(). For example, if the property name is foo, you need to have a getFoo()method.
• The getter (if needed) and setter methods of the property that you are animating must operate on the same type as the starting and ending values that you specify to ObjectAnimator. For example, you must have targetObject.setPropName(float)and targetObject.getPropName(float)if you construct the following ObjectAnimator:

  ObjectAnimator.ofFloat(targetObject, "propName", 1f)

• Depending on what property or object you are animating, you might need to call the invalidate()method on a View force the screen to redraw itself with the updated animated values. You do this in the onAnimationUpdate()callback. For example, animating the color property of a Drawable object only cause updates to the screen when that object redraws itself. All of the property setters on View, such as setAlpha()and setTranslationX()invalidate the View properly, so you do not need to invalidate the View when calling these methods with new values. For more information on listeners, see the section about Animation Listeners.

Choreographing Multiple Animations with AnimatorSet

In many cases, you want to play an animation that depends on when another animation starts or finishes. The Android system lets you bundle animations together into an AnimatorSet, so that you can specify whether to start animations simultaneously, sequentially, or after a specified delay. You can also nest AnimatorSetobjects within each other.

The following sample code taken from the Bouncing Balls sample (modified for simplicity) plays the following Animatorobjects in the following manner:

1. Plays bounceAnim.
2. Plays squashAnim1, squashAnim2, stretchAnim1, and stretchAnim2at the same time.
3. Plays bounceBackAnim.
4. Plays fadeAnim.

AnimatorSet bouncer = new AnimatorSet();
bouncer.play(bounceAnim).before(squashAnim1);
bouncer.play(squashAnim1).with(squashAnim2);
bouncer.play(squashAnim1).with(stretchAnim1);
bouncer.play(squashAnim1).with(stretchAnim2);
bouncer.play(bounceBackAnim).after(stretchAnim2);