ViewGroup 调用onDraw

ViewGroup默认情况下，出于性能考虑，会被设置成WILL_NOT_DROW,这样ondraw就不会被执行了，

如果我们想重写一个ViewGroup的onDraw()方法，有两种方法：

1、构造函数中，给ViewGroup设置一个颜色;

2、构造函数中，调用setWillNotDraw(false),去掉期WILL_NOT_DRAW flag,在ViewGroup初始化的时候，它调用了一个私有方法：initViewGroup，它里面会有一句setFlags(WILL_NOT_DRAW,DRAW_MASK);相当于调用 了setWillNOtDraw(true),所以说对于ViewGroup它就认为是透明的了，如果我们想要重写onDraw()，就要调用setWillNotDraw(false)。

源码:

/**
 * If this view doesn't do any drawing on its own, set this flag to
 * allow further optimizations. By default, this flag is not set on
 * View, but could be set on some View subclasses such as ViewGroup.
 *
 * Typically, if you override {@link #onDraw(android.graphics.Canvas)}
 * you should clear this flag.
 *
 * @param willNotDraw whether or not this View draw on its own
 */
public void setWillNotDraw(boolean willNotDraw) {
    setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}