先看一下View的draw()源码

/** * 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. */ 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: * * 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; } ... }

可以看出绘制分为6步，其中第二步和第五步通常都是跳过的。我们只看剩下的四步：

以上步骤不难理解，自己在Bitmap画过自定义图形的同学都知道，canvas画图，如果位置重叠的话，后绘制的内容会把先前的内容覆盖掉。这个canvas是由ViewRoot传过来的，这样保证界面上所有的子view都绘制在一张画布上。其实所有view的测量、布局、绘制过程都是由ViewRoot发起的。

注释里仍然说明了draw()方法不应该被重写，应该在onDraw()方法里处理本身的绘制，而在dispatchDraw()里绘制子view。如果一定要重写draw()方法，那么也一定要在开始调用super.draw(Canvas canvas)。

由于不同的view绘制方法不同，而且有的是layout有的是view，对于是否需要绘制子view的需求也不同，所以View类中的onDraw()和dispatchDraw()都是空实现。而由于ViewGroup是容器，自身需要绘制的东西比较少，主要在于子view的绘制，因此ViewGroup主要实现了dispatchDraw()。相反的，TextView、ImageView等这些非容器的控件则主要实现onDraw()方法来呈现更为复杂的内容。本来流程并不是很复杂，相比起流程，draw过程的细节却多得让人可怕。不过为了完全弄明白容器和控件的绘制过程，我们仍然节选一点源码简单了解一下。

节选一段ViewGroup类中的dispatchDraw()方法

protected void dispatchDraw(Canvas canvas) { boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode); final int childrenCount = mChildrenCount; final View[] children = mChildren; int flags = mGroupFlags; if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) { final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE; final boolean buildCache = !isHardwareAccelerated(); for (int i = 0; i < childrenCount; i++) { final View child = children[i]; if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) { final LayoutParams params = child.getLayoutParams(); attachLayoutAnimationParameters(child, params, i, childrenCount); bindLayoutAnimation(child); if (cache) { child.setDrawingCacheEnabled(true); if (buildCache) { child.buildDrawingCache(true); } } } } final LayoutAnimationController controller = mLayoutAnimationController; if (controller.willOverlap()) { mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE; } controller.start(); mGroupFlags &= ~FLAG_RUN_ANIMATION; mGroupFlags &= ~FLAG_ANIMATION_DONE; if (cache) { mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE; } if (mAnimationListener != null) { mAnimationListener.onAnimationStart(controller.getAnimation()); } } int clipSaveCount = 0; final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK; if (clipToPadding) { clipSaveCount = canvas.save(); canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop, mScrollX + mRight - mLeft - mPaddingRight, mScrollY + mBottom - mTop - mPaddingBottom); } // We will draw our child's animation, let's reset the flag mPrivateFlags &= ~PFLAG_DRAW_ANIMATION; mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED; boolean more = false; final long drawingTime = getDrawingTime(); if (usingRenderNodeProperties) canvas.insertReorderBarrier(); // Only use the preordered list if not HW accelerated, since the HW pipeline will do the // draw reordering internally final ArrayList<View> preorderedList = usingRenderNodeProperties ? null : buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); 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); } } if (preorderedList != null) preorderedList.clear(); ... }