Android中View繪制過程（六） performDraw

performTraversals()中執行了performmeasure确定了每一個View的width，和height， 就會調用performLayout,确定這些View在螢幕

的位置，調用performDraw()在每一個View将圖像畫在螢幕上自己layout确定的位置上就可以了。

調用如下performDraw()->draw()->drawSoftware()->從DecorView開始遞歸調用繪制，代碼如下：

mView.draw(canvas);
           

ViewGroup,FrameLayout中都沒有重寫此方法，回到View中檢視，View中代碼也好長，關鍵是調用了一個diapatchDraw(),在ViewGroup中

重寫了這個方法，關鍵點在于

for (int i = 0; i < childrenCount; i++) {
            while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
                final View transientChild = mTransientViews.get(transientIndex);
                if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                        transientChild.getAnimation() != null) {
                    more |= drawChild(canvas, transientChild, drawingTime);
                }
                transientIndex++;
                if (transientIndex >= transientCount) {
                    transientIndex = -1;
                }
            }

            final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
            final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                more |= drawChild(canvas, child, drawingTime);
            }
        }
           

周遊自己的chlidView一次調用DrawChlid，這個方法如下

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        return child.draw(canvas, this, drawingTime);
    }
           

調用Chlid中的額draw方法，

draw方法會調用onDraw這個回調，在這個回調中可以自定義View的繪制。終于自上而下完成View的繪制。

其他說明draw方法的說明如下：

/*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */
           

第一步，Draw the background是發生在onDraw之前的，還有剩下的一些畫布的處理，應該注意相對回調onDraw的先後的順序。