Android中軸旋轉特效實現，制作別樣的圖片浏覽器

Android API Demos中有很多非常Nice的例子，這些例子的代碼都寫的很出色，如果大家把API Demos中的 每個例子研究透了，那麼恭喜你已經成為一個真正的Android高手了。這也算是給一些比較迷茫的Android開 發者一個指出了一個提升自我能力的方向吧。API Demos中的例子眾多，今天我們就來模仿其中一個3D變換的 特效，來實現一種別樣的圖片浏覽器。

既然是做中軸旋轉的特效，那麼肯定就要用到3D變換的功能。 在Android中如果想要實現3D效果一般有兩種選擇，一是使用Open GL ES，二是使用Camera。Open GL ES使用 起來太過復雜，一般是用於比較高級的3D特效或游戲，像比較簡單的一些3D效果，使用Camera就足夠了。

Camera中提供了三種旋轉方法，分別是rotateX()、rotateY()和rotateZ，調用這三個方法，並傳入相應 的角度，就可以讓視圖圍繞這三個軸進行旋轉，而今天我們要做的中軸旋轉效果其實就是讓視圖圍繞Y軸進行 旋轉。使用Camera讓視圖進行旋轉的示意圖，如下所示：

那我們就開始動手吧，首先創 建一個Android項目，起名叫做RotatePicBrowserDemo，然後我們准備了幾張圖片，用於稍後在圖片浏覽器中 進行浏覽。

而API Demos中已經給我們提供了一個非常好用的3D旋轉動畫的工具類Rotate3dAnimation ，這個工具類就是使用Camera來實現的，我們先將這個這個類復制到項目中來，代碼如下所示：

/** 
 * An animation that rotates the view on the Y axis between two specified angles. 
 * This animation also adds a translation on the Z axis (depth) to improve the effect. 
 */
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, Transformation t) {  
        final float fromDegrees = mFromDegrees;  
        float degrees = fromDegrees + ((mToDegrees - fromDegrees) * interpolatedTime);  
      
        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 - interpolatedTime));  
        }  
        camera.rotateY(degrees);  
        camera.getMatrix(matrix);  
        camera.restore();  
      
        matrix.preTranslate(-centerX, -centerY);  
        matrix.postTranslate(centerX, centerY);  
    }  
}