而不是使用矩阵来操纵图像的可见区域(视图),为什么不在剪切方面定义这个可见区域？这是一个相当简单的问题需要解决：我们需要做的就是定义可见的矩形,并忽略任何超出其边界的内容.如果我们然后为这些边界设置动画,则视觉效果就像裁剪边界向上和向下缩放一样.

幸运的是,Canvas支持剪切各种clip *()方法来帮助我们在这里.动画剪辑边界很简单,可以使用与您自己的代码片段类似的方式完成.

如果你把所有东西放在一个简单的常规ImageView扩展中(为了封装),你会得到一些看起来像这样的东西：

public class ClippingImageView extends ImageView {

    private final Rect mClipRect = new Rect();

    public ClippingImageView(Context context,AttributeSet attrs,int defStyle) {
        super(context,attrs,defStyle);
        initClip();
    }

    public ClippingImageView(Context context,AttributeSet attrs) {
        super(context,attrs);
        initClip();
    }

    public ClippingImageView(Context context) {
        super(context);
        initClip();
    }

    private void initClip() {
        // post to message queue,so it gets run after measuring & layout
        // sets initial crop area to half of the view's width & height
        post(new Runnable() {
            @Override public void run() {
                setImageCrop(0.5f);
            }
        });
    }

    @Override protected void onDraw(Canvas canvas) {
        // clip if needed and let super take care of the drawing
        if (clip()) canvas.clipRect(mClipRect);
        super.onDraw(canvas);
    }

    private boolean clip() {
        // true if clip bounds have been set aren't equal to the view's bounds
        return !mClipRect.isEmpty() && !clipEqualsBounds();
    }

    private boolean clipEqualsBounds() {
        final int width = getWidth();
        final int height = getHeight();
        // whether the clip bounds are identical to this view's bounds (which effectively means no clip)
        return mClipRect.width() == width && mClipRect.height() == height;
    }

    public void toggle() {
        // toggle between [0...0.5] and [0.5...0]
        final float[] values = clipEqualsBounds() ? new float[] { 0f,0.5f } : new float[] { 0.5f,0f };
        ObjectAnimator.ofFloat(this,"imageCrop",values).start();
    }

    public void setImageCrop(float value) {
        // nothing to do if there's no drawable set
        final Drawable drawable = getDrawable();
        if (drawable == null) return;

        // nothing to do if no dimensions are known yet
        final int width = getWidth();
        final int height = getHeight();
        if (width <= 0 || height <= 0) return;

        // construct the clip bounds based on the supplied 'value' (which is assumed to be within the range [0...1])
        final int clipWidth = (int) (value * width);
        final int clipHeight = (int) (value * height);
        final int left = clipWidth / 2;
        final int top = clipHeight / 2;
        final int right = width - left;
        final int bottom = height - top;

        // set clipping bounds
        mClipRect.set(left,top,right,bottom);
        // schedule a draw pass for the new clipping bounds to take effect visually
        invalidate();
    }

}

真正的“魔法”是重写的onDraw()方法的附加行,其中给定的Canvas被剪切为由mClipRect定义的矩形区域.所有其他代码和方法主要用于帮助计算剪辑边界,确定裁剪是否合理以及动画.

从Activity中使用它现在减少到以下内容：

public class MainActivity extends Activity {

    @Override protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.image_activity);

        final ClippingImageView img = (ClippingImageView) findViewById(R.id.img);
        img.setOnClickListener(new OnClickListener() {
            @Override public void onClick(View v) {
                img.toggle();
            }
        });
    }
}

布局文件将指向我们的自定义ClippingImageView,如下所示：

<mh.so.img.ClippingImageView
        android:id="@+id/img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/dog" />

为了让您了解视觉过渡：