Android View的工作流程- draw过程

View的draw过程主要是将View绘制到屏幕上面，View的draw过程遵循如下几步：

• 绘制背景：drawBackground
• 绘制自己：onDraw
• 绘制子元素：dispatchDraw
• 绘制滑动边缘渐变和滑动条，绘制前景：onDrawForeground

这一点通过draw方法的源码可以明显看出来，如下：

public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
    int saveCount;

    //绘制背景
    drawBackground(canvas);

    // skip step 2 & 5 if possible (common case)
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    if (!verticalEdges && !horizontalEdges) {
        // Step 3, draw the content
        //绘制自己
        onDraw(canvas);

        // Step 4, draw the children
        //绘制子元素
        dispatchDraw(canvas);

        drawAutofilledHighlight(canvas);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        //绘制滑动边缘渐变和滑动条，绘制前景
        onDrawForeground(canvas);

        // Step 7, draw the default focus highlight
        drawDefaultFocusHighlight(canvas);

        if (isShowingLayoutBounds()) {
            debugDrawFocus(canvas);
        }

        // we're done...
        return;
    }
    ...
}

View绘制过程的传递是dispatchDraw来实现的，dispatchDraw会遍历所有的子元素，如此draw事件就一层层地传递了下去。

View有一个特殊的方法setWillNotDraw，先看下它的源码，如下：

/**
 * 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);
}

从setWillNotDraw这个方法的注释中可以看出，如果一个View不需要绘制任何内容，那么设置这个标记位为true以后，系统会进行相应的优化。默认情况下，View没有启用这个优化标记位，但是ViewGroup会默认启用这个优化标记位。这个标记位对实际开发的意义是：当我们自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。当然，当明确知道一个ViewGroup需要通过onDraw来绘制内容时，我们需要显式的关闭WILL_NOT_DRAW这个标记位。