一起Talk Android吧（第一百五十三回：Android自定义View之Draw五）

各位看官们大家好，上一回中咱们说的是Android中自定义View之Draw的例子，这一回咱们继续说该例子。闲话休提，言归正转。让我们一起Talk Android吧！

看官们，我们在一回中想介绍ViewGroup的Draw流程，但是却因为没有找到draw()函数和onDraw()函数而不了了之，这一回中，我们将从其它地方着手，继续分析ViewGroup的Draw流程。

好了现在回到正题，我们从View类的draw()函数着手，从源代码中找到突破点，具体为下面的代码：

// Step 4, draw the children dispatchDraw(canvas);

接着查看该函数的源代码，却发现它是一个空函数，那么肯定在子类中重写了它，于是我们又回到ViewGroup类中，查找该函数，庆幸的是，我们不但找到了它，而且还发现它被重写了，看来我们的推测是正确的，下面是它的源代码:

@Override 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 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); } } final LayoutAnimationController controller = mLayoutAnimationController; if (controller.willOverlap()) { mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE; } controller.start(); mGroupFlags &= ~FLAG_RUN_ANIMATION; mGroupFlags &= ~FLAG_ANIMATION_DONE; 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.CLIP_SAVE_FLAG); 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(); final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size(); int transientIndex = transientCount != 0 ? 0 : -1; // 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++) { while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) { final View transientChild = mTransientViews.get(transientIndex); if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE || transientChild.getAnimation() != null) { more |= drawChild(canvas, transientChild, drawingTime); } transientIndex++; if (transientIndex >= transientCount) { transientIndex = -1; } } final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder); final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex); if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { more |= drawChild(canvas, child, drawingTime); } } while (transientIndex >= 0) { // there may be additional transient views after the normal views final View transientChild = mTransientViews.get(transientIndex); if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE || transientChild.getAnimation() != null) { more |= drawChild(canvas, transientChild, drawingTime); } transientIndex++; if (transientIndex >= transientCount) { break; } } if (preorderedList != null) preorderedList.clear(); // Draw any disappearing views that have animations if (mDisappearingChildren != null) { final ArrayList<View> disappearingChildren = mDisappearingChildren; final int disappearingCount = disappearingChildren.size() - 1; // Go backwards -- we may delete as animations finish for (int i = disappearingCount; i >= 0; i--) { final View child = disappearingChildren.get(i); more |= drawChild(canvas, child, drawingTime); } } if (usingRenderNodeProperties) canvas.insertInorderBarrier(); if (debugDraw()) { onDebugDraw(canvas); } if (clipToPadding) { canvas.restoreToCount(clipSaveCount); } // mGroupFlags might have been updated by drawChild() flags = mGroupFlags; if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) { invalidate(true); } if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 && mLayoutAnimationController.isDone() && !more) { // We want to erase the drawing cache and notify the listener after the // next frame is drawn because one extra invalidate() is caused by // drawChild() after the animation is over mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER; final Runnable end = new Runnable() { @Override public void run() { notifyAnimationListener(); } }; post(end); } }

源代码比较多，不过核心功能是通过drawChild()函数实现的，接下来我们看看该函数的源代码:

protected boolean drawChild(Canvas canvas, View child, long drawingTime) { return child.draw(canvas, this, drawingTime); }

从源代码中可以看出，它回调了View中的draw()函数，而且是View中专门用来绘制ViewGroup对象的函数，这个函数，我们前面章回中介绍过，相信大家还有印象。

绕了这么一圈，大家可能觉得有点乱，那么我们对ViewGroup的Draw流程做一个总结：

doTraversal()->performTraversals()->performDraw()->draw(1)->dispatchDraw()->draw(3)->draw(1)->onDraw().

该流程中前三个函数都在ViewRootImpl.java文件中，第四个函数draw(1)在View类中，这里的1表示它有1个参数，而第五个函数dispatchDraw()在View类中，不过它是个空函数，真正调用的是ViewGoup类中的dispatchDraw()函数。第六个函数draw(3)又回到了View类中，这里的3表示它有3个参数。第七个函数draw(1)也在View类中。最后一个函数onDraw()在哪里取决于它代表的类，如果它代表的类是某个具体的控件（LinearLayout），那么就在该控件所属的类中。

各位看官，关于Androd中自定义View之Draw的例子咱们就介绍到这里，欲知后面还有什么例子，且听下回分解！