View绘制流程详解手机开发

绘制流程图

- View.draw(Canvas) 源码分析

 /** 
     * Manually render this view (and all of its children) to the given Canvas. 
     * The view must have already done a full layout before this function is 
     * called.  When implementing a view, implement 
     * [email protected] #onDraw(android.graphics.Canvas)} instead of overriding this method. 
     * If you do need to override this method, call the superclass version. 
     * 
     * @param canvas The Canvas to which the View is rendered.   
     * 
     * 根据给定的 Canvas 自动渲染 View（包括其所有子 View）。在调用该方法之前必须要完成 layout。当你自定义 view 的时候， 
     * 应该去是实现 onDraw(Canvas) 方法，而不是 draw(canvas) 方法。如果你确实需要复写该方法，请记得先调用父类的方法。 
     */ 
    public void draw(Canvas canvas) { 
 
        / * Draw traversal performs several drawing steps which must be executed 
         * in the appropriate order: 
         * 
         *      1. Draw the background if need 
         *      2. If necessary, save the canvas' layers to prepare for fading 
         *      3. Draw view's content 
         *      4. Draw children (dispatchDraw) 
         *      5. If necessary, draw the fading edges and restore layers 
         *      6. Draw decorations (scrollbars for instance) 
         */ 
 
     // Step 1, draw the background, if needed 
        if (!dirtyOpaque) { 
            drawBackground(canvas); 
        } 
 
         // skip step 2 & 5 if possible (common case) 
        final int viewFlags = mViewFlags; 
        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; 
        } 
 
        // Step 2, save the canvas' layers 
        ... 
 
        // Step 3, draw the content 
        if (!dirtyOpaque)  
            onDraw(canvas); 
 
        // Step 4, draw the children 
        dispatchDraw(canvas); 
 
        // Step 5, draw the fade effect and restore layers 
 
        // Step 6, draw decorations (scrollbars) 
        onDrawScrollBars(canvas); 
    } 

由上面的处理过程，我们也可以得出一些优化的小技巧：当不需要绘制 Layer 的时候第二步和第五步会跳过。因此在绘制的时候，能省的 layer 尽可省，可以提高绘制效率

ViewGroup.dispatchDraw() 源码分析

dispatchDraw(Canvas canvas){ 
 
... 
 
 if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {//处理 ChildView 的动画 
     final boolean buildCache = !isHardwareAccelerated(); 
            for (int i = 0; i < childrenCount; i++) { 
                final View child = children[i]; 
                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {//只绘制 Visible 状态的布局，因此可以通过延时加载来提高效率 
                    final LayoutParams params = child.getLayoutParams(); 
                    attachLayoutAnimationParameters(child, params, i, childrenCount);// 添加布局变化的动画 
                    bindLayoutAnimation(child);//为 Child 绑定动画 
                    if (cache) { 
                        child.setDrawingCacheEnabled(true); 
                        if (buildCache) { 
                            child.buildDrawingCache(true); 
                        } 
                    } 
                } 
            } 
 
     final LayoutAnimationController controller = mLayoutAnimationController; 
            if (controller.willOverlap()) { 
                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE; 
            } 
 
    controller.start();// 启动 View 的动画 
} 
 
 // 绘制 ChildView 
 for (int i = 0; i < childrenCount; i++) { 
            int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; 
            final View child = (preorderedList == null) 
                    ? children[childIndex] : preorderedList.get(childIndex); 
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { 
                more |= drawChild(canvas, child, drawingTime); 
            } 
        } 
 
... 
 
} 
 
protected boolean drawChild(Canvas canvas, View child, long drawingTime) { 
        return child.draw(canvas, this, drawingTime); 
} 
 
/** 
     * This method is called by ViewGroup.drawChild() to have each child view draw itself. 
     * This draw() method is an implementation detail and is not intended to be overridden or 
     * to be called from anywhere else other than ViewGroup.drawChild(). 
     */ 
    boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) { 
        ... 
    } 

• drawChild(canvas, this, drawingTime)
  直接调用了 View 的child.draw(canvas, this,drawingTime)方法，文档中也说明了，除了被ViewGroup.drawChild()方法外，你不应该在其它任何地方去复写或调用该方法，它属于 ViewGroup。而View.draw(Canvas)方法是我们自定义控件中可以复写的方法，具体可以参考上述对view.draw(Canvas)的说明。从参数中可以看到，child.draw(canvas, this, drawingTime) 肯定是处理了和父视图相关的逻辑，但 View 的最终绘制，还是 View.draw(Canvas)方法。

• invalidate()
  请求重绘 View 树，即 draw 过程，假如视图发生大小没有变化就不会调用layout()过程，并且只绘制那些调用了invalidate()方法的 View。

• requestLayout()
  当布局变化的时候，比如方向变化，尺寸的变化，会调用该方法，在自定义的视图中，如果某些情况下希望重新测量尺寸大小，应该手动去调用该方法，它会触发measure()和layout()过程，但不会进行 draw。