Android View绘制过程

Android的View绘制是从根节点（Activity是DecorView）开始，他是一个自上而下的过程。View的绘制经历三个过程：Measure、Layout、Draw。基本流程如下图：

 

performTraversals函数，具体的可以参考一下源代码：

 

private void performTraversals() {
    final View host = mView;
    ...
    host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
    ...
    draw(fullRedrawNeeded);
}

 

1、Measure过程

Measure过程是计算视图大小，View中视图measure过程相关的方法主要有三个：

 

public final void measure(int widthMeasureSpec, int heightMeasureSpec)
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight)
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)

 

 

    measure调用onMeasure，onMeasure调用setMeasureDimension，measure，setMeasureDimension是final类型，view的子类不需要重写，onMeasure在view的子类中重写。

 

measure函数：

 

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
            widthMeasureSpec != mOldWidthMeasureSpec ||
            heightMeasureSpec != mOldHeightMeasureSpec) {

        // first clears the measured dimension flag
        mPrivateFlags &= ~MEASURED_DIMENSION_SET;

        if (ViewDebug.TRACE_HIERARCHY) {
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
        }

        // measure ourselves, this should set the measured dimension flag back
        onMeasure(widthMeasureSpec, heightMeasureSpec);

        // flag not set, setMeasuredDimension() was not invoked, we raise
        // an exception to warn the developer
        if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
            throw new IllegalStateException("onMeasure() did not set the"
                    + " measured dimension by calling"
                    + " setMeasuredDimension()");
        }

        mPrivateFlags |= LAYOUT_REQUIRED;
    }

    mOldWidthMeasureSpec = widthMeasureSpec;
    mOldHeightMeasureSpec = heightMeasureSpec;
}

onMeasure函数：

 

 

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

 

 

    重写onMeasure时，要调用setMeasuredDimension或者super.onMeasure来设置自身的mMeasuredWidth和mMeasuredHeight，否则，就会抛出异常.

setMeasuredDimension函数，用来设置view的大小：

 

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    mMeasuredWidth = measuredWidth;
    mMeasuredHeight = measuredHeight;

    mPrivateFlags |= MEASURED_DIMENSION_SET;
}

 

 

再看一下onMeasure的getDefaultSize函数：

 

public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

这里用引入了MeasureSpec类：

 

 

public static class MeasureSpec {

    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
    public static final int UNSPECIFIED = 0 << MODE_SHIFT;
    public static final int EXACTLY     = 1 << MODE_SHIFT;
    public static final int AT_MOST     = 2 << MODE_SHIFT;

    public static int makeMeasureSpec(int size, int mode) {
        return size + mode;
    }

    public static int getMode(int measureSpec) {
        return (measureSpec & MODE_MASK);
    }

    public static int getSize(int measureSpec) {
        return (measureSpec & ~MODE_MASK);
    }
}

 

    MODE_MASK为30为长度的二进制数，前两位标示Mode，后面的标示Size。MeasureSpec有三种模式分别是UNSPECIFIED, EXACTLY和AT_MOST。

 EXACTLY表示父视图希望子视图的大小应该是由specSize的值来决定的，系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。

 AT_MOST表示子视图最多只能是specSize中指定的大小，开发人员应该尽可能小得去设置这个视图，并且保证不会超过specSize。系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。

 UNSPECIFIED表示开发人员可以将视图按照自己的意愿设置成任意的大小，没有任何限制。这种情况比较少见，不太会用到。

widthMeasureSpec和heightMeasureSpec决定了Mode和Size的值，widthMeasureSpec和heightMeasureSpec来自父视图，这两个值都是由父视图经过计算后传递给子视图的，说明父视图会在一定程度上决定子视图的大小。但是最外层的根视图，它的widthMeasureSpec和heightMeasureSpec又是从哪里得到的呢？这就需要去分析ViewRoot中的源码了，观察performTraversals()方法可以发现如下代码：

 

childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);

 

 可以看到，这里调用了getRootMeasureSpec()方法去获取widthMeasureSpec和heightMeasureSpec的值，注意方法中传入的参数，其中lp.width和lp.height在创建ViewGroup实例的时候就被赋值了，它们都等于MATCH_PARENT。然后看下getRootMeasureSpec()方法中的代码，如下所示：

 

private int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {
    case ViewGroup.LayoutParams.MATCH_PARENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

 

可以看到，这里使用了MeasureSpec.makeMeasureSpec()方法来组装一个MeasureSpec，当rootDimension参数等于MATCH_PARENT的时候，MeasureSpec的specMode就等于EXACTLY，当rootDimension等于WRAP_CONTENT的时候，MeasureSpec的specMode就等于AT_MOST。并且MATCH_PARENT和WRAP_CONTENT时的specSize都是等于windowSize的，也就意味着根视图总是会充满全屏的。

 

 Measure是一个复杂的过程，因为一个布局中一般都会包含多个子视图，每个视图都需要经历一次measure过程。ViewGroup中定义了一个measureChildren()方法来去测量子视图的大小，如下所示：

 

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
       final int size = mChildrenCount;
       final View[] children = mChildren;
       for (int i = 0; i < size; ++i) {
           final View child = children[i];
           if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
               measureChild(child, widthMeasureSpec, heightMeasureSpec);
           }
       }
   }

 

这里会去遍历当前布局下的所有子视图，然后逐个调用measureChild()方法来测量相应子视图的大小：

 

protected void measureChild(View child, int parentWidthMeasureSpec,
           int parentHeightMeasureSpec) {
       final LayoutParams lp = child.getLayoutParams();

       final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
               mPaddingLeft + mPaddingRight, lp.width);
       final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
               mPaddingTop + mPaddingBottom, lp.height);

       child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
   }

 

从这里我们可以看到视图的大小是由父视图和子视图共同决定的。子布局里面的android:layout_width和android:layout_height只是期望值，父View大小最终是由DecorView决定。父视图提供尺寸大小的一个能力，子视图最终尺寸与父视图能力、子视图期望的关系如下：

 

父视图能力尺寸	子视图期望尺寸	子视图最终允许尺寸
EXACTLY + Size1	EXACTLY + Size2	EXACTLY + Size2
EXACTLY + Size1	fill_parent/match_parent	EXACTLY+Size1
EXACTLY + Size1	wrap_content	AT_MOST+Size1
AT_MOST+Size1	EXACTLY + Size2	EXACTLY+Size2
AT_MOST+Size1	fill_parent/match_parent	AT_MOST+Size1
AT_MOST+Size1	wrap_content	AT_MOST+Size1
UNSPECIFIED+Size1	EXACTLY + Size2	EXACTLY + Size2
UNSPECIFIED+Size1	fill_parent/match_parent	UNSPECIFIED+0
UNSPECIFIED+Size1	wrap_content	UNSPECIFIED+0

 

 

 

 

关于视图的measure过程可以阅读以下LinearLayout源码，这样可以更清楚的了解过程。

 

2、Layout过程

 

measure过程确定视图的大小，而layout过程确定视图的位置。loyout是从view的layout方法开始的：

 

public void layout(int l, int t, int r, int b) {
       int oldL = mLeft;
       int oldT = mTop;
       int oldB = mBottom;
       int oldR = mRight;
       boolean changed = setFrame(l, t, r, b);
       if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
           onLayout(changed, l, t, r, b);
           mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

           ListenerInfo li = mListenerInfo;
           if (li != null && li.mOnLayoutChangeListeners != null) {
               ArrayList<OnLayoutChangeListener> listenersCopy =
                       (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
               int numListeners = listenersCopy.size();
               for (int i = 0; i < numListeners; ++i) {
                   listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
               }
           }
       }
       mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
   }

    函数中参数l、t、r、b是指view的左、上、右、底的位置，这几个参数是父视图传入的，而根视图中参数是由performTraversals()方法传入的。

 

 

host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);

 

 

layout中调用了onLayout方法，在view中onLayout方法是一个空函数，他需要其子类实现。

 

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
  }

我们关注一下LinearLayout：

 

 

@Override
  protected void onLayout(boolean changed, int l, int t, int r, int b) {
      if (mOrientation == VERTICAL) {
          layoutVertical();
      } else {
          layoutHorizontal();
      }
  }

void layoutVertical() {
      final int paddingLeft = mPaddingLeft;

      int childTop;
      int childLeft;

      // Where right end of child should go
      final int width = mRight - mLeft;
      int childRight = width - mPaddingRight;

      // Space available for child
      int childSpace = width - paddingLeft - mPaddingRight;

      final int count = getVirtualChildCount();

      final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
      final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

      switch (majorGravity) {
         case Gravity.BOTTOM:
             // mTotalLength contains the padding already
             childTop = mPaddingTop + mBottom - mTop - mTotalLength;
             break;

             // mTotalLength contains the padding already
         case Gravity.CENTER_VERTICAL:
             childTop = mPaddingTop + (mBottom - mTop - mTotalLength) / 2;
             break;

         case Gravity.TOP:
         default:
             childTop = mPaddingTop;
             break;
      }

      for (int i = 0; i < count; i++) {
          final View child = getVirtualChildAt(i);
          if (child == null) {
              childTop += measureNullChild(i);
          } else if (child.getVisibility() != GONE) {
              final int childWidth = child.getMeasuredWidth();
              final int childHeight = child.getMeasuredHeight();

              final LinearLayout.LayoutParams lp =
                      (LinearLayout.LayoutParams) child.getLayoutParams();

              int gravity = lp.gravity;
              if (gravity < 0) {
                  gravity = minorGravity;
              }
              final int layoutDirection = getLayoutDirection();
              final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
              switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                  case Gravity.CENTER_HORIZONTAL:
                      childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                              + lp.leftMargin - lp.rightMargin;
                      break;

                  case Gravity.RIGHT:
                      childLeft = childRight - childWidth - lp.rightMargin;
                      break;

                  case Gravity.LEFT:
                  default:
                      childLeft = paddingLeft + lp.leftMargin;
                      break;
              }

              if (hasDividerBeforeChildAt(i)) {
                  childTop += mDividerHeight;
              }

              childTop += lp.topMargin;
              setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                      childWidth, childHeight);
              childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

              i += getChildrenSkipCount(child, i);
          }
      }
  }

    layout设置了view的位置，还设置了子视图位置，layoutHorizontal()方法中调用了setChildFrame方法：

 

 

private void setChildFrame(View child, int left, int top, int width, int height) {
    child.layout(left, top, left + width, top + height);
}

 

    从上面看出，layout也是一个自上而下的过程，先设置父视图位置，在循环子视图，父视图位置一定程度上决定了子视图位置。

 

3、Draw过程

 

        draw过程调用顺序在measure()和layout()之后，同样的，performTraversals()发起的draw过程最终会调用到mView的draw()函数，这里的mView对于Activity来说就是PhoneWindow.DecorView。看一下view类的draw方法：

 

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) {
           final Drawable background = mBackground;
           if (background != null) {
               final int scrollX = mScrollX;
               final int scrollY = mScrollY;

               if (mBackgroundSizeChanged) {
                   background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
                   mBackgroundSizeChanged = false;
               }

               if ((scrollX | scrollY) == 0) {
                   background.draw(canvas);
               } else {
                   canvas.translate(scrollX, scrollY);
                   background.draw(canvas);
                   canvas.translate(-scrollX, -scrollY);
               }
           }
       }

       // 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);

           // we're done...
           return;
       }

       /*
        * Here we do the full fledged routine...
        * (this is an uncommon case where speed matters less,
        * this is why we repeat some of the tests that have been
        * done above)
        */

       boolean drawTop = false;
       boolean drawBottom = false;
       boolean drawLeft = false;
       boolean drawRight = false;

       float topFadeStrength = 0.0f;
       float bottomFadeStrength = 0.0f;
       float leftFadeStrength = 0.0f;
       float rightFadeStrength = 0.0f;

       // Step 2, save the canvas' layers
       int paddingLeft = mPaddingLeft;

       final boolean offsetRequired = isPaddingOffsetRequired();
       if (offsetRequired) {
           paddingLeft += getLeftPaddingOffset();
       }

       int left = mScrollX + paddingLeft;
       int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
       int top = mScrollY + getFadeTop(offsetRequired);
       int bottom = top + getFadeHeight(offsetRequired);

       if (offsetRequired) {
           right += getRightPaddingOffset();
           bottom += getBottomPaddingOffset();
       }

       final ScrollabilityCache scrollabilityCache = mScrollCache;
       final float fadeHeight = scrollabilityCache.fadingEdgeLength;
       int length = (int) fadeHeight;

       // clip the fade length if top and bottom fades overlap
       // overlapping fades produce odd-looking artifacts
       if (verticalEdges && (top + length > bottom - length)) {
           length = (bottom - top) / 2;
       }

       // also clip horizontal fades if necessary
       if (horizontalEdges && (left + length > right - length)) {
           length = (right - left) / 2;
       }

       if (verticalEdges) {
           topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
           drawTop = topFadeStrength * fadeHeight > 1.0f;
           bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
           drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
       }

       if (horizontalEdges) {
           leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
           drawLeft = leftFadeStrength * fadeHeight > 1.0f;
           rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
           drawRight = rightFadeStrength * fadeHeight > 1.0f;
       }

       saveCount = canvas.getSaveCount();

       int solidColor = getSolidColor();
       if (solidColor == 0) {
           final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

           if (drawTop) {
               canvas.saveLayer(left, top, right, top + length, null, flags);
           }

           if (drawBottom) {
               canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
           }

           if (drawLeft) {
               canvas.saveLayer(left, top, left + length, bottom, null, flags);
           }

           if (drawRight) {
               canvas.saveLayer(right - length, top, right, bottom, null, flags);
           }
       } else {
           scrollabilityCache.setFadeColor(solidColor);
       }

       // 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
       final Paint p = scrollabilityCache.paint;
       final Matrix matrix = scrollabilityCache.matrix;
       final Shader fade = scrollabilityCache.shader;

       if (drawTop) {
           matrix.setScale(1, fadeHeight * topFadeStrength);
           matrix.postTranslate(left, top);
           fade.setLocalMatrix(matrix);
           canvas.drawRect(left, top, right, top + length, p);
       }

       if (drawBottom) {
           matrix.setScale(1, fadeHeight * bottomFadeStrength);
           matrix.postRotate(180);
           matrix.postTranslate(left, bottom);
           fade.setLocalMatrix(matrix);
           canvas.drawRect(left, bottom - length, right, bottom, p);
       }

       if (drawLeft) {
           matrix.setScale(1, fadeHeight * leftFadeStrength);
           matrix.postRotate(-90);
           matrix.postTranslate(left, top);
           fade.setLocalMatrix(matrix);
           canvas.drawRect(left, top, left + length, bottom, p);
       }

       if (drawRight) {
           matrix.setScale(1, fadeHeight * rightFadeStrength);
           matrix.postRotate(90);
           matrix.postTranslate(right, top);
           fade.setLocalMatrix(matrix);
           canvas.drawRect(right - length, top, right, bottom, p);
       }

       canvas.restoreToCount(saveCount);

       // Step 6, draw decorations (scrollbars)
       onDrawScrollBars(canvas);
   }

draw方法分成了6个步骤：

 

 

/*
        * 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)
        */

 

 

第三部, Draw view's content步骤调用了onDraw方法，子类中实现onDraw方法。

第四步，Draw children步骤使用的dispatchDraw方法，这个方法在ViewGroup中有实现。

    View或ViewGroup的子类不用再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作。   

 

/**
    * {@inheritDoc}
    */
   @Override
   protected void dispatchDraw(Canvas canvas) {
       final int count = 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 < count; i++) {
               final View child = children[i];
               if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
                   final LayoutParams params = child.getLayoutParams();
                   attachLayoutAnimationParameters(child, params, i, count);
                   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 saveCount = 0;
       final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
       if (clipToPadding) {
           saveCount = 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 ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
           for (int i = 0; i < count; i++) {
               final View child = children[i];
               if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                   more |= drawChild(canvas, child, drawingTime);
               }
           }
       } else {
           for (int i = 0; i < count; i++) {
               final View child = children[getChildDrawingOrder(count, i)];
               if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                   more |= drawChild(canvas, child, drawingTime);
               }
           }
       }

       // 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 (debugDraw()) {
           onDebugDraw(canvas);
       }

       if (clipToPadding) {
           canvas.restoreToCount(saveCount);
       }

       // 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() {
              public void run() {
                  notifyAnimationListener();
              }
           };
           post(end);
       }
   }

 

    上面基本介绍完了View的绘制流程。更多的细节需要在日常学习中总结。