#### 7.1.2 自定義View動畫 除了系統提供的四種View動畫外，我們還可以自定義View動畫。自定義動畫是一件既簡單又復雜的事情，說它簡單，是因為派生一種新動畫只需要繼承Animation這個抽象類，然后重寫它的initialize和applyTransformation方法，在initialize方法中做一些初始化工作，在applyTransformation中進行相應的矩陣變換即可，很多時候需要采用Camera來簡化矩陣變換的過程。說它復雜，是因為自定義View動畫的過程主要是矩陣變換的過程，而矩陣變換是數學上的概念，如果對這方面的知識不熟悉的話，就會覺得這個過程比較復雜了。 本節不打算詳細地講解自定義View動畫的細節，因為這都是數學中的矩陣變換的細節，讀者只需要知道自定義View的方法并在需要的時候參考矩陣變換的細節即可寫出特定的自定義View動畫。一般來說，在實際開發中很少用到自定義View動畫。這里提供一個自定義View動畫的例子，這個例子來自于Android的ApiDemos中的一個自定義View動畫Rotate3dAnimation。Rotate3dAnimation可以圍繞y軸旋轉并且同時沿著z軸平移從而實現一種類似于3D的效果，它的代碼如下： ``` 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); } } ```