Android L 使用 ViewOutlineProvider 裁剪 View

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Android 5.0的 View 类中新增了 setOutlineProvider(ViewOutlineProvider provider) 方法，注释如下：

Sets the {@link ViewOutlineProvider} of the view, which generates the Outline that defines the shape of the shadow it casts, and enables outline clipping.

The default ViewOutlineProvider, {@link ViewOutlineProvider#BACKGROUND}, queries the Outline from the View’s background drawable, via {@link Drawable#getOutline(Outline)}. Changing the outline provider with this method allows this behavior to be overridden.

If the ViewOutlineProvider is null, if querying it for an outline returns false, or if the produced Outline is {@link Outline#isEmpty()}, shadows will not be cast.

Only outlines that return true from {@link Outline#canClip()} may be used for clipping.

@see #setClipToOutline(boolean)

@see #getClipToOutline()

@see #getOutlineProvider()

那么我们可以用它来把 View 裁剪成一些特定(圆形、矩形、圆角矩形)的形状：

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view.setOutlineProvider(new ViewOutlineProvider() {
    @Override
    public void getOutline(View view, Outline outline) {
        outline.setRoundRect(0, 0, view.getWidth(), view.getHeight(), 30);
    }
});
view.setClipToOutline(true);

也可以用来设置投影，但是投影的形状只能是凸多边形，为什么？看源码：

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/**
 * Sets the Constructs an Outline from a
 * {@link android.graphics.Path#isConvex() convex path}.
 */
public void setConvexPath(@NonNull Path convexPath) {
    if (convexPath.isEmpty()) {
        setEmpty();
        return;
    }
    //如果不是凸多边形，会抛异常
    if (!convexPath.isConvex()) {
        throw new IllegalArgumentException("path must be convex");
    }
    mMode = MODE_CONVEX_PATH;
    mPath.set(convexPath);
    mRect.setEmpty();
    mRadius = RADIUS_UNDEFINED;
}
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view.setElevation(5);
view.setOutlineProvider(new ViewOutlineProvider() {
    @Override
    public void getOutline(View view, Outline outline) {
      	//你可以用 Path 指定任何的形状，前提是凸多边形
        //这里设置投影的位置从右下角开始，投影形状是矩形
        Path path = new Path();
        path.moveTo(view.getWidth(), view.getHeight());
        path.lineTo(view.getWidth(), view.getHeight() * 2);
        path.lineTo(view.getWidth() * 2, view.getHeight() * 2);
        path.lineTo(view.getWidth() * 2, view.getHeight());
        path.close();
        outline.setConvexPath(path);
    }
});