- 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 * {@link #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。