自定义View——Draw

• draw流程前必须完成layout流程
• 所有的视图最终都是调用View.draw来完成绘制(ViewGroup中没有重写此方法)
• 在自定义View中，不应重写draw方法，而应重写onDraw方法
• 如果一定要重写draw，必须加上super.draw(canvas)，再进行自定义绘制

android/view/View.java
public void draw(Canvas canvas) {
    //第一步：绘制背景
    drawBackground(canvas);
    //检查是否有图层状态保存
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    //如果没有，则跳过图层保存(第二步)恢复操作(第五步)，直接执行绘制，
    //所以，如果没必要，不要使用save-restore，可以提高绘制效率
    if (!verticalEdges && !horizontalEdges) {
        //第二步：绘制主体
        onDraw(canvas);
        //第三步：绘制子View
        dispatchDraw(canvas);
        //第四步：绘制前景(滑动条 前景等)
        onDrawForeground(canvas);
        //第五步：绘制焦点高亮处
        drawDefaultFocusHighlight(canvas);
        return;
    }
    //如果有图层状态，则执行下列步骤
    //第二步：保存图层状态
    ... ...
    if (drawTop) {
       topSaveCount = canvas.saveUnclippedLayer(left, top, right, top + length);
   }
   if (drawBottom) {
       bottomSaveCount = canvas.saveUnclippedLayer(left, bottom - length, right, bottom);
   }
    if (drawLeft) {
       leftSaveCount = canvas.saveUnclippedLayer(left, top, left + length, bottom);
   }
    if (drawRight) {
       rightSaveCount = canvas.saveUnclippedLayer(right - length, top, right, bottom);
   }
    //第三步：绘制主体
    onDraw(canvas);
    //第四步：绘制子view
    dispatchDraw(canvas);
    //第五步：恢复图层状态
    canvas.restoreToCount(saveCount);
    //第六步：绘制装饰(前景 滑动条等)
    onDrawForeground(canvas);
}

android/view/View.java
private void drawBackground(Canvas canvas) {
    final Drawable background = mBackground;
    //根据layout流程后获取的位置信息来设置背景边界
    setBackgroundBounds();
    //根据x y是否有偏移量，决定是否需要移动画布
    final int scrollX = mScrollX;
   final int scrollY = mScrollY;
   if ((scrollX | scrollY) == 0) {
       background.draw(canvas);
   } else {
       canvas.translate(scrollX, scrollY);
        //调用Drawable.draw绘制背景
       background.draw(canvas);
       canvas.translate(-scrollX, -scrollY);
   }
}

android/view/View.java
protected void onDraw(Canvas canvas) {}

android/view/View.java
protected void dispatchDraw(Canvas canvas) {}

android/view/ViewGroup.java
protected void dispatchDraw(Canvas canvas) {
    ... ...
    //绘制子view
    more |= drawChild(canvas, transientChild, drawingTime);
    ... ...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    //调用子View.draw
    return child.draw(canvas, this, drawingTime);
}

public void onDrawForeground(Canvas canvas) {
    //绘制滚动条及指示器
    onDrawScrollIndicators(canvas);
   onDrawScrollBars(canvas);
   final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
   if (foreground != null) {
        //绘制前景
        foreground.draw(canvas);
   }
}