带前景色的容器

    如果您用google play会发现当按下应用图标时，会出现一透明色覆盖在图标上面（如下图）。注意这个不是background效果而是foreground或是遮罩(overlay)。
    
    实现这种效果通常的思路是在容器(LinearLayout、RelativeLayout等)画玩子控件后，再画一透明色或透明图片在容器顶层达到覆盖遮罩的效果。不过这种思路还需重载onTouchEvent来监听手指按下、抬起等，而且实现将非常的繁琐。
    幸好还有更加便捷的实现方法：那就是google在View类中提供的getDrawableState、drawableStateChanged方法。getDrawableState返回的是视图状态，而drawableStateChanged即是状态发生变化时调用。ok！有了这2个api我们就可以挂接到Drawable的callback设置相应的状态(setState)，从而根据不同的状态绘制不同的drawable状态(selector).

    下面是一个简单的控件代码

package com.droidwolf.overlay;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.widget.LinearLayout;

public class OverlayContainer extends LinearLayout {
	private Drawable mForeground;
	private final Rect mBound = new Rect();

	public OverlayContainer(Context context) {
		super(context);
	}

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

	public Drawable getForegroundDrawable() {
		return mForeground;
	}

	public void setForegroundDrawable(Drawable draw) {
		if (draw == mForeground) {
			return;
		}
		if (mForeground != null) {
			unscheduleDrawable(mForeground);
			mForeground.setCallback(null);
		}
		mForeground = draw;
		if (draw != null) {
			if (draw.isStateful()) {
				draw.setState(getDrawableState());
			}
			draw.setCallback(this);
		}
	}

	@Override
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
		super.onMeasure(widthMeasureSpec, heightMeasureSpec);
		mBound.set(0, 0, getMeasuredWidth(), getMeasuredHeight());
	}

	@Override
	protected void drawableStateChanged() {
		super.drawableStateChanged();
		if (mForeground != null && mForeground.isStateful()) {
			mForeground.setState(getDrawableState());
		}
	}

	@Override
	protected boolean verifyDrawable(Drawable who) {
		return super.verifyDrawable(who) || (who == mForeground);
	}

	@Override
	public void dispatchDraw(Canvas canvas) {
		if (mForeground == null) {
			super.dispatchDraw(canvas);
		} else {
			final int sc = canvas.save();
			super.dispatchDraw(canvas);
			mForeground.setBounds(mBound);
			mForeground.draw(canvas);
			canvas.restoreToCount(sc);
		}
	}
}

    需要注意的是如果容器的子控件设置了OnClickListener事件，mForeground绘画状态就有可能失效，这时您需要添加addStatesFromChildren属性并设置为true。
   另:FrameLayout已经实现了foreground功能。