Drawable Animation
Drawable animation lets you load a series of Drawable resources one after another to create an animation. This is a traditional animation in the sense that it is created with a sequence of different images, played in order, like a roll of film. TheAnimationDrawableclass is the basis for Drawable animations.
Drawable animation是一个一个的加载一些列的图片来创建动画的，他是一种传统的动画，事实上就是一个不同图片的序列AnimationDrawable是Drawable animations 的基类
While you can define the frames of an animation in your code, using theAnimationDrawableclass API, it's more simply accomplished with a single XML file that lists the frames that compose the animation. The XML file for this kind of animation belongs in the res/drawable/ directory of your Android project. In this case, the instructions are the order and duration for each frame of the animation.
虽然可以使用AnimationDrawable类在代码中定义帧动画，但最好是使用XML文件列出组成动画的所有帧。这个xml文件放在 res/drawable/目录下，事实上，所有的指令就是每一个帧动画的顺序和持续时间
The XML file consists of an element as the root node and a series of child nodes that each define a frame: a drawable resource for the frame and the frame duration. Here's an example XML file for a Drawable animation:
xml文件由根元素 和一系列子元素 组成，其中 定义了每个帧对应的图片和持续时间，如下：
This animation runs for just three frames. By setting the android:oneshot attribute of the list to true, it will cycle just once then stop and hold on the last frame. If it is set false then the animation will loop. With this XML saved as rocket_thrust. xml in the res/drawable/ directory of the project, it can be added as the background image to a View and then called to play. Here's an example Activity, in which the animation is added to an ImageView and then animated when the screen is touched:
上面的例子是一个3帧动画， 通过将android:oneshot设置为true，那他就只会循环一次，并停留在最后一个帧上，若为false的话，就会循环运行
AnimationDrawable rocketAnimation;public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); ImageView rocketImage = (ImageView) findViewById(R.id.rocket_image); rocketImage.setBackgroundResource(R.drawable.rocket_thrust); rocketAnimation = (AnimationDrawable) rocketImage.getBackground();}public boolean onTouchEvent(MotionEvent event) { if (event.getAction() == MotionEvent.ACTION_DOWN) { rocketAnimation.start(); return true; } return super.onTouchEvent(event);}
It's important to note that the start() method called on the AnimationDrawable cannot be called during the onCreate() method of your Activity, because the AnimationDrawable is not yet fully attached to the window. If you want to play the animation immediately, without requiring interaction, then you might want to call it from the onWindowFocusChanged() method in your Activity, which will get called when Android brings your window into focus
注意：start()方法不能在你的Activity的onCreate()方法中调用，因为那时候AnimationDrawable尚未完全附着于窗口。如果你必须立刻调用动画，切没有交互的话，可以在onWindowFocusChanged()中调用。因为这个方法是窗口获得焦点时调用的
For more information on the XML syntax, available tags and attributes, see Animation Resources.
