4.3.3 draw過程 · Android開發藝術探索

#### 4.3.3 draw過程 Draw過程就比較簡單了，它的作用是將View繪制到屏幕上面。View的繪制過程遵循如下幾步： * （1）繪制背景background.draw(canvas)。 * （2）繪制自己（onDraw）。 * （3）繪制children（dispatchDraw）。 * （4）繪制裝飾（onDrawScrollBars）。這一點通過draw方法的源碼可以明顯看出來，如下所示。 public void draw(Canvas canvas) { final int privateFlags = mPrivateFlags; final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE && (mAttachInfo == null || ! mAttachInfo.mIgnoreDirtyState); mPrivateFlags = (privateFlags & ～PFLAG_DIRTY_MASK) | PFLAG_DRAWN; /＊＊ Draw traversal performs several drawing steps which must be executed ＊ in the appropriate order: ＊ 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) ＊/ // Step 1, draw the background, if needed int saveCount; if (! dirtyOpaque) { 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 if (! dirtyOpaque) onDraw(canvas); // Step 4, draw the children dispatchDraw(canvas); // Step 6, draw decorations (scrollbars) onDrawScrollBars(canvas); if (mOverlay ! = null && ! mOverlay.isEmpty()) { mOverlay.getOverlayView().dispatchDraw(canvas); } // we're done... return; } ... } View繪制過程的傳遞是通過dispatchDraw來實現的，dispatchDraw會遍歷調用所有子元素的draw方法，如此draw事件就一層層地傳遞了下去。View有一個特殊的方法setWillNotDraw，先看一下它的源碼，如下所示。 /＊＊＊ 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); } 從setWillNotDraw這個方法的注釋中可以看出，如果一個View不需要繪制任何內容，那么設置這個標記位為true以后，系統會進行相應的優化。默認情況下，View沒有啟用這個優化標記位，但是ViewGroup會默認啟用這個優化標記位。這個標記位對實際開發的意義是：當我們的自定義控件繼承于ViewGroup并且本身不具備繪制功能時，就可以開啟這個標記位從而便于系統進行后續的優化。當然，當明確知道一個ViewGroup需要通過onDraw來繪制內容時，我們需要顯式地關閉WILL_NOT_DRAW這個標記位。