【帧动画总结】AnimationDrawable Frame

Drawable Animation 开发者文档

位置：/sdk/docs/guide/topics/graphics/drawable-animation.html

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. The AnimationDrawable class is the basis for Drawable animations.

Drawable动画可以让您一个接一个地加载一系列的Drawable资源来创建动画。这是一种传统的动画，它的意义是创建一系列不同的图像，按顺序播放，像一卷电影。 AnimationDrawable类是可绘制动画的基础。

While you can define the frames of an animation in your code, using the AnimationDrawable class API, it's more simply accomplished with完成 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.

虽然您可以在代码中定义动画的框架，但使用AnimationDrawable类API，只需完成一个列出构成动画的框架的XML文件即可完成。这种动画的XML文件属于您的Android项目的res / drawable /目录。在这种情况下，(需要设定的)指令是动画每帧的顺序和持续时间。

The XML file consists of an <animation-list> element as the root node and a series of child <item> 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文件由作为根节点的<animation-list>元素和每个定义一个帧的一系列子<item>节点组成：每一帧的drawable资源和持续时间。以下是可绘制动画的XML文件示例：

<animation-list xmlns:android="http://schemas.android.com/apk/res/android"
    android:oneshot="true">
    <item android:drawable="@drawable/rocket_thrust1" android:duration="200" />
    <item android:drawable="@drawable/rocket_thrust2" android:duration="200" />
    <item android:drawable="@drawable/rocket_thrust3" android:duration="200" />
</animation-list>

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:

此动画只运行三帧。通过将列表的android:oneshot属性设置为true，它将循环一次，然后停止并按住最后一帧。如果设置为false，则动画将循环。将此XML保存为项目的res / drawable /目录中的rocket_thrust.xml，可以将其作为背景图像添加到View，然后调用播放。下面是一个示例，其中将动画添加到ImageView中，然后在触摸屏幕时进行动画：

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.

请注意，在Activity的onCreate()方法中，不能调用AnimationDrawable的start()方法，因为AnimationDrawable尚未完全附加到窗口。如果要立即播放动画，而不需要交互，那么您可能希望在Activity中的onWindowFocusChanged()方法中调用它，当Android将您的窗口置于焦点时，该动画将被调用。

AnimationDrawable 类

继承体系

public class AnimationDrawable extends DrawableContainer implements Runnable, Animatable

java.lang.Object

↳ android.graphics.drawable.Drawable

↳ android.graphics.drawable.DrawableContainer

↳ android.graphics.drawable.AnimationDrawable

可以看到 AnimationDrawable 是 Drawable 的简介子类。

公共方法

void addFrame(Drawable frame, int duration)：Adds a frame to the animation
int getDuration(int i)：Return the duration in milliseconds of the frame at the specified index
Drawable getFrame(int index)：Return the Drawable at the specified frame index
int getNumberOfFrames()：Return the number of frames in the animation
void inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Resources.Theme theme)：Inflate this Drawable from an XML resource optionally styled by a theme.
boolean isOneShot()：Return true of the animation will play once, false otherwise
boolean isRunning()：Indicates whether the animation is currently running or not.
Drawable mutate()：Make this drawable mutable易变的. This operation cannot be reversed反转.
void run()：This method exists for implementation purpose only and should not be called directly. Invoke start() instead.
void setOneShot(boolean oneShot)：Sets whether the animation should play once or repeat.
boolean setVisible(boolean visible, boolean restart)：Sets whether this AnimationDrawable is visible.
void start()：Starts the animation, looping if necessary. This method has no effect if the animation is running.
void stop()：Stops the animation. This method has no effect if the animation is not running.
void unscheduleSelf(Runnable what)：Use the current Drawable.Callback implementation to have this Drawable unscheduled.

打底色的三个方法是 Animatable 接口中定义的方法

帧动画简介

Drawable Animation 或者 Frame Animation，帧动画，就像GIF图片(或电影)一样，是通过依次显示一系列的 Drawable 来模拟动画的效果。

以<animation-list>为根元素，一个<item>表示一帧要轮换显示的图片

oneshot代表着是否只展示一遍，设置为false会不停的循环播放动画

duration属性表示此帧显示的时间

注意：帧动画是指背景动画，所以只能设置 background 属性为指定的帧动画，或在代码中通过 setBackgroundResource(R.drawable.amin_id) 指定帧动画，但是不能设置 src 属性为指定的帧动画。当然两者可以同时设置，此时src保持不变，背景是个动画会改变。

注意：开启或关闭帧动画前最好先判断获取的帧动画是否为空，因为帧动画是通过 getBackground() 强转过来的，可能不存在！

最重要的：不要在onCreate()中调用start方法开启帧动画，因为此时帧动画还没有完全跟Window相关联，如果想要在界面显示时就立即开始动画的话，可以在onWindowFoucsChanged()方法中调用start方法。

虽然定义三种动画的xml文件都可以放置在res/anim/文件夹中，但是建议：

帧动画放在res/drawable文件夹中
补间动画放在res/anim文件夹中
属性动画放在res/animator文件夹中

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