View Animation

原文URL：http://developer.android.com/guide/topics/graphics/view-animation.html

You can use the view animation system to perform tweened animation(补间动画) on Views. Tween animation calculates the animation with information such as the start point, end point, size, rotation, and other common aspects of an animation.

A tween animation can perform a series of simple transformations (position, size, rotation, and transparency) on the contents of a View object. So, if you have a TextView object, you can move, rotate, grow, or shrink the text. If it has a background image, the background image will be transformed along with the text. Theanimation package provides all the classes used in a tween animation.

A sequence of animation instructions defines the tween animation,defined by either XML or Android code（可以使用XML文件或者Android 代码的方式指定动画）.As with defining a layout, an XML file is recommended（和Layout一样，推荐使用XML文件的方式） because it's more readable, reusable, and swappable than hard-coding the animation. In the example below, we use XML. (To learn more about defining an animation in your application code, instead of XML, refer to the AnimationSet class and other Animation subclasses.)

The animation instructions define the transformations that you want to occur, when they will occur, and how long they should take to apply.Transformations can be sequential or simultaneous（动画可以有序的发生，也可以同时发生，这个根据各个动作的起始时间而定） - for example, you can have the contents of a TextView move from left to right, and then rotate 180 degrees, or you can have the text move and rotate simultaneously. Each transformation takes a set of parameters specific for that transformation (starting size and ending size for size change, starting angle and ending angle for rotation, and so on), and also a set of common parameters (for instance, start time and duration). To make several transformations happen simultaneously, give them the same start time; to make them sequential, calculate the start time plus the duration of the preceding transformation.

（为了使几个动作能够同时发生，将它们的起始时间设为一样；为了使它们能够有序的发生，将后一个动作的起始时间设置为前一个动作的起始时间加上它的持续时间）

The animation XML file belongs in the res/anim/ directory of your Android project（定义动画的XML文件位于Android 项目的 res/anim 目录下）. The file must have a single root element: this will be either a single <alpha>, <scale>, <translate>, <rotate>, interpolator element, or<set> element that holds groups of these elements (which may include another <set>). （该动画XML文件的根节点，可以是以上节点之一）。By default, all animation instructions are applied simultaneously（默认，所有的动作是同时发生的）. To make them occur sequentially, you must specify the startOffset attribute, as shown in the example below（为了使它们有序的发生，必须设置 startOffset 属性，见下面示例）.

The following XML from one of the ApiDemos is used to stretch, then simultaneously spin and rotate a View object.

<set android:shareInterpolator="false">    <scale        android:interpolator="@android:anim/accelerate_decelerate_interpolator"        android:fromXScale="1.0"        android:toXScale="1.4"        android:fromYScale="1.0"        android:toYScale="0.6"        android:pivotX="50%"        android:pivotY="50%"        android:fillAfter="false"        android:duration="700" />    <set android:interpolator="@android:anim/decelerate_interpolator">        <scale           android:fromXScale="1.4"           android:toXScale="0.0"           android:fromYScale="0.6"           android:toYScale="0.0"           android:pivotX="50%"           android:pivotY="50%"           android:startOffset="700"           android:duration="400"           android:fillBefore="false" />        <rotate           android:fromDegrees="0"           android:toDegrees="-45"           android:toYScale="0.0"           android:pivotX="50%"           android:pivotY="50%"           android:startOffset="700"           android:duration="400" />    </set></set>

Screen coordinates (not used in this example) are (0,0) at the upper left hand corner, and increase as you go down and to the right.

Some values, such as pivotX, can be specified relative to the object itself or relative to the parent. Be sure to use the proper format for what you want ("50" for 50% relative to the parent, or "50%" for 50% relative to itself).

You can determine how a transformation is applied over time by assigning an Interpolator(可以指定动画转换的样式，如旋转的时候可以先快后慢，或者先慢后快等). Android includes several Interpolator subclasses that specify various speed curves: for instance, AccelerateInterpolatortells a transformation to start slow and speed up. Each one has an attribute value that can be applied in the XML.

With this XML saved as hyperspace_jump.xml in the res/anim/ directoryof the project, the following code will reference it and apply it to an ImageView object from the layout.

ImageView spaceshipImage = (ImageView) findViewById(R.id.spaceshipImage);Animation hyperspaceJumpAnimation = AnimationUtils.loadAnimation(this, R.anim.hyperspace_jump);spaceshipImage.startAnimation(hyperspaceJumpAnimation);

As an alternative to startAnimation(), you can define a starting time for the animation withAnimation.setStartTime(), then assign the animation to the View with View.setAnimation()（作为startAnimation()方法的替代方法，我们也可以使用Animation.setStartTime()设置动画的起始时间，然后使用View.setAnimation()将该动画适用于一个View）.

For more information on the XML syntax, available tags and attributes, see Animation Resources.

Note: Regardless of how your animation may move or resize, the bounds of the View that holds your animation will not automatically adjust to accommodate it(无论动画如何变换，使用动画的View的边框都不会自适应改变 。－－ps:也就是说，如果你在该view上设置了一个点击事件，当动画开进行的过程中，你点击当前的view，是不会触发上述点击事件的。只有点击动画开始之前View的边界内的区域，才会触发该点击事件。还有另一种情形就是，你想实时获取动画过程中当前View 的坐标，得到的也只是动画之前的坐标。).Even so, the animation will still be drawn beyond the bounds of its View and will not be clipped. However, clipping will occur if the animation exceeds the bounds of the parent View.