#### 7.1.2 自定义View动画 除了系统提供的四种View动画外，我们还可以自定义View动画。自定义动画是一件既简单又复杂的事情，说它简单，是因为派生一种新动画只需要继承Animation这个抽象类，然后重写它的initialize和applyTransformation方法，在initialize方法中做一些初始化工作，在applyTransformation中进行相应的矩阵变换即可，很多时候需要采用Camera来简化矩阵变换的过程。说它复杂，是因为自定义View动画的过程主要是矩阵变换的过程，而矩阵变换是数学上的概念，如果对这方面的知识不熟悉的话，就会觉得这个过程比较复杂了。 本节不打算详细地讲解自定义View动画的细节，因为这都是数学中的矩阵变换的细节，读者只需要知道自定义View的方法并在需要的时候参考矩阵变换的细节即可写出特定的自定义View动画。一般来说，在实际开发中很少用到自定义View动画。这里提供一个自定义View动画的例子，这个例子来自于Android的ApiDemos中的一个自定义View动画Rotate3dAnimation。Rotate3dAnimation可以围绕y轴旋转并且同时沿着z轴平移从而实现一种类似于3D的效果，它的代码如下： ``` public class Rotate3dAnimation extends Animation { private final float mFromDegrees; private final float mToDegrees; private final float mCenterX; private final float mCenterY; private final float mDepthZ; private final boolean mReverse; private Camera mCamera; /＊＊ ＊ Creates a new 3D rotation on the Y axis. The rotation is defined by its ＊ start angle and its end angle. Both angles are in degrees. The rotation ＊ is performed around a center point on the 2D space, definied by a pair ＊ of X and Y coordinates, called centerX and centerY. When the animation ＊ starts, a translation on the Z axis (depth) is performed. The length ＊ of the translation can be specified, as well as whether the translation ＊ should be reversed in time. ＊ ＊ @param fromDegrees the start angle of the 3D rotation ＊ @param toDegrees the end angle of the 3D rotation ＊ @param centerX the X center of the 3D rotation ＊ @param centerY the Y center of the 3D rotation ＊ @param reverse true if the translation should be reversed, false otherwise ＊/ public Rotate3dAnimation(float fromDegrees, float toDegrees, float centerX, float centerY, float depthZ, boolean reverse) { mFromDegrees = fromDegrees; mToDegrees = toDegrees; mCenterX = centerX; mCenterY = centerY; mDepthZ = depthZ; mReverse = reverse; } @Override public void initialize(int width, int height, int parentWidth, int parentHeight) { super.initialize(width, height, parentWidth, parentHeight); mCamera = new Camera(); } @Override protected void applyTransformation(float interpolatedTime, Transfor- mation t) { final float fromDegrees = mFromDegrees; float degrees = fromDegrees + ((mToDegrees - fromDegrees) ＊ interpo- latedTime); final float centerX = mCenterX; final float centerY = mCenterY; final Camera camera = mCamera; final Matrix matrix = t.getMatrix(); camera.save(); if (mReverse) { camera.translate(0.0f, 0.0f, mDepthZ ＊ interpolatedTime); } else { camera.translate(0.0f, 0.0f, mDepthZ ＊ (1.0f - interpolated- Time)); } camera.rotateY(degrees); camera.getMatrix(matrix); camera.restore(); matrix.preTranslate(-centerX, -centerY); matrix.postTranslate(centerX, centerY); } } ```