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！