• draw流程前必須完成layout流程
• 所有的視圖最終都是調(diào)用View.draw來完成繪制(ViewGroup中沒有重寫此方法)
• 在自定義View中，不應(yīng)重寫draw方法，而應(yīng)重寫onDraw方法
• 如果一定要重寫draw，必須加上super.draw(canvas)，再進行自定義繪制

android/view/View.java
public void draw(Canvas canvas) {
    //第一步：繪制背景
    drawBackground(canvas);
    //檢查是否有圖層狀態(tài)保存
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    //如果沒有，則跳過圖層保存(第二步)恢復(fù)操作(第五步)，直接執(zhí)行繪制，
    //所以，如果沒必要，不要使用save-restore，可以提高繪制效率
    if (!verticalEdges && !horizontalEdges) {
        //第二步：繪制主體
        onDraw(canvas);
        //第三步：繪制子View
        dispatchDraw(canvas);
        //第四步：繪制前景(滑動條 前景等)
        onDrawForeground(canvas);
        //第五步：繪制焦點高亮處
        drawDefaultFocusHighlight(canvas);
        return;
    }
    //如果有圖層狀態(tài)，則執(zhí)行下列步驟
    //第二步：保存圖層狀態(tài)
    ... ...
    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：繪制主體
    onDraw(canvas);
    //第四步：繪制子view
    dispatchDraw(canvas);
    //第五步：恢復(fù)圖層狀態(tài)
    canvas.restoreToCount(saveCount);
    //第六步：繪制裝飾(前景 滑動條等)
    onDrawForeground(canvas);
}

android/view/View.java
private void drawBackground(Canvas canvas) {
    final Drawable background = mBackground;
    //根據(jù)layout流程后獲取的位置信息來設(shè)置背景邊界
    setBackgroundBounds();
    //根據(jù)x y是否有偏移量，決定是否需要移動畫布
    final int scrollX = mScrollX;
   final int scrollY = mScrollY;
   if ((scrollX | scrollY) == 0) {
       background.draw(canvas);
   } else {
       canvas.translate(scrollX, scrollY);
        //調(diào)用Drawable.draw繪制背景
       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) {
    ... ...
    //繪制子view
    more |= drawChild(canvas, transientChild, drawingTime);
    ... ...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    //調(diào)用子View.draw
    return child.draw(canvas, this, drawingTime);
}

public void onDrawForeground(Canvas canvas) {
    //繪制滾動條及指示器
    onDrawScrollIndicators(canvas);
   onDrawScrollBars(canvas);
   final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
   if (foreground != null) {
        //繪制前景
        foreground.draw(canvas);
   }
}