深入理解Android中View绘制的三大流程

WindowManagerGlobal.java

root = new ViewRootImpl(view.getContext(), display); 
root.setView(view, wparams, panelParentView);

MeasureSpec.java

public static class MeasureSpec { 
 private static final int MODE_SHIFT = 30;
 private static final int MODE_MASK = 0x3 << MODE_SHIFT;

 /**
  * UNSPECIFIED 模式：
  * 父View不对子View有任何限制，子View需要多大就多大
  */ 
 public static final int UNSPECIFIED = 0 << MODE_SHIFT;

 /**
  * EXACTYLY 模式：
  * 父View已经测量出子Viwe所需要的精确大小，这时候View的最终大小
  * 就是SpecSize所指定的值。对应于match_parent和精确数值这两种模式
  */ 
 public static final int EXACTLY = 1 << MODE_SHIFT;

 /**
  * AT_MOST 模式：
  * 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值，
  * 即对应wrap_content这种模式
  */ 
 public static final int AT_MOST = 2 << MODE_SHIFT;

 //将size和mode打包成一个32位的int型数值
 //高2位表示SpecMode，测量模式，低30位表示SpecSize，某种测量模式下的规格大小
 public static int makeMeasureSpec(int size, int mode) {
  if (sUseBrokenMakeMeasureSpec) {
  return size + mode;
  } else {
  return (size & ~MODE_MASK) | (mode & MODE_MASK);
  }
 }

 //将32位的MeasureSpec解包，返回SpecMode,测量模式
 public static int getMode(int measureSpec) {
  return (measureSpec & MODE_MASK);
 }

 //将32位的MeasureSpec解包，返回SpecSize，某种测量模式下的规格大小
 public static int getSize(int measureSpec) {
  return (measureSpec & ~MODE_MASK);
 }
 //...
 }

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

 private static int getRootMeasureSpec(int windowSize, int rootDimension) {
 int measureSpec;
 switch (rootDimension) {

 case ViewGroup.LayoutParams.MATCH_PARENT:
  // Window can't resize. Force root view to be windowSize.
  measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
  break;
 case ViewGroup.LayoutParams.WRAP_CONTENT:
  // Window can resize. Set max size for root view.
  measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
  break;
 default:
  // Window wants to be an exact size. Force root view to be that size.
  measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
  break;
 }
 return measureSpec;
 }

 protected void measureChildWithMargins(View child,
  int parentWidthMeasureSpec, int widthUsed,
  int parentHeightMeasureSpec, int heightUsed) {
 final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

 final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
  mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
   + widthUsed, lp.width);
 final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
  mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
   + heightUsed, lp.height);

 child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
 }

ViewGroup.java

public static int getChildMeasureSpec(int spec, int padding, int childDimension) { 
 int specMode = MeasureSpec.getMode(spec);
 int specSize = MeasureSpec.getSize(spec);

 int size = Math.max(0, specSize - padding);

 int resultSize = 0;
 int resultMode = 0;

 switch (specMode) {
 // Parent has imposed an exact size on us
 case MeasureSpec.EXACTLY:
 if (childDimension >= 0) {
  resultSize = childDimension;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.MATCH_PARENT) {
  // Child wants to be our size. So be it.
  resultSize = size;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
  // Child wants to determine its own size. It can't be
  // bigger than us.
  resultSize = size;
  resultMode = MeasureSpec.AT_MOST;
 }
 break;

 // Parent has imposed a maximum size on us
 case MeasureSpec.AT_MOST:
 if (childDimension >= 0) {
  // Child wants a specific size... so be it
  resultSize = childDimension;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.MATCH_PARENT) {
  // Child wants to be our size, but our size is not fixed.
  // Constrain child to not be bigger than us.
  resultSize = size;
  resultMode = MeasureSpec.AT_MOST;
 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
  // Child wants to determine its own size. It can't be
  // bigger than us.
  resultSize = size;
  resultMode = MeasureSpec.AT_MOST;
 }
 break;

 // Parent asked to see how big we want to be
 case MeasureSpec.UNSPECIFIED:
 if (childDimension >= 0) {
  // Child wants a specific size... let him have it
  resultSize = childDimension;
  resultMode = MeasureSpec.EXACTLY;
 } else if (childDimension == LayoutParams.MATCH_PARENT) {
  // Child wants to be our size... find out how big it should
  // be
  resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
  resultMode = MeasureSpec.UNSPECIFIED;
 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
  // Child wants to determine its own size.... find out how
  // big it should be
  resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
  resultMode = MeasureSpec.UNSPECIFIED;
 }
 break;
 }
 //noinspection ResourceType
 return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),heightSizeAndState); 

 public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
 final int specMode = MeasureSpec.getMode(measureSpec);
 final int specSize = MeasureSpec.getSize(measureSpec);
 final int result;
 switch (specMode) {
  case MeasureSpec.AT_MOST:
  if (specSize < size) {
   result = specSize | MEASURED_STATE_TOO_SMALL;
  } else {
   result = size;
  }
  break;
  case MeasureSpec.EXACTLY:
  result = specSize;
  break;
  case MeasureSpec.UNSPECIFIED:
  default:
  result = size;
 }
 return result | (childMeasuredState & MEASURED_STATE_MASK);
 }

View.java

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

View.java

 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;
 }

 public void layout(int l, int t, int r, int b) {
  if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
   onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
   mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
  }

  int oldL = mLeft;
  int oldT = mTop;
  int oldB = mBottom;
  int oldR = mRight;

  boolean changed = isLayoutModeOptical(mParent) ?
    setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

  if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
   onLayout(changed, l, t, r, b);

   if (shouldDrawRoundScrollbar()) {
    if(mRoundScrollbarRenderer == null) {
     mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
    }
   } else {
    mRoundScrollbarRenderer = null;
   }

   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;
  mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
 }

boolean changed = isLayoutModeOptical(mParent) ?setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); 

 protected boolean setFrame(int left, int top, int right, int bottom) {
  boolean changed = false;

  if (DBG) {
   Log.d("View", this + " View.setFrame(" + left + "," + top + ","
  + right + "," + bottom + ")");
  }
  //1、如果有一个值发生了改变，那么就需要重新调用onLayout方法了，后面会分析到
  if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
   changed = true;

   // Remember our drawn bit
   int drawn = mPrivateFlags & PFLAG_DRAWN;

   //2、保存旧的宽和高
   int oldWidth = mRight - mLeft;
   int oldHeight = mBottom - mTop;
   //计算新的宽和高
   int newWidth = right - left;
   int newHeight = bottom - top;
   //3、判断宽高是否有分生变化
   boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);

   //Invalidate our old position
   //4、如果大小变化了，在已绘制了的情况下就请求重新绘制
   invalidate(sizeChanged);

   //5、存储新的值
   mLeft = left;
   mTop = top;
   mRight = right;
   mBottom = bottom;
   mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

   mPrivateFlags |= PFLAG_HAS_BOUNDS;

   if (sizeChanged) {
   //6、大小变化时进行处理
   sizeChange(newWidth, newHeight, oldWidth, oldHeight);
   }

   if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
   //7、如果此时View是可见状态下，立即执行绘制操作
   invalidate(sizeChanged);

   }

   mPrivateFlags |= drawn;

   mBackgroundSizeChanged = true;
   if (mForegroundInfo != null) {
   mForegroundInfo.mBoundsChanged = true;
   }

   notifySubtreeAccessibilityStateChangedIfNeeded();
  }
  return changed;
 }

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

 @Override
 protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
  layoutChildren(left, top, right, bottom, false /* no force left gravity */);
 }

 void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
  final int count = getChildCount();

  final int parentLeft = getPaddingLeftWithForeground();
  final int parentRight = right - left - getPaddingRightWithForeground();

  final int parentTop = getPaddingTopWithForeground();
  final int parentBottom = bottom - top - getPaddingBottomWithForeground();

  for (int i = 0; i < count; i++) {
   final View child = getChildAt(i);
   if (child.getVisibility() != GONE) {
    final LayoutParams lp = (LayoutParams) child.getLayoutParams();

    final int width = child.getMeasuredWidth();
    final int height = child.getMeasuredHeight();

    int childLeft;
    int childTop;

    int gravity = lp.gravity;
    if (gravity == -1) {
     gravity = DEFAULT_CHILD_GRAVITY;
    }

    final int layoutDirection = getLayoutDirection();
    final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
    final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

    switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
     case Gravity.CENTER_HORIZONTAL:
      childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
      lp.leftMargin - lp.rightMargin;
      break;
     case Gravity.RIGHT:
      if (!forceLeftGravity) {
       childLeft = parentRight - width - lp.rightMargin;
       break;
      }
     case Gravity.LEFT:
     default:
      childLeft = parentLeft + lp.leftMargin;
    }

    switch (verticalGravity) {
     case Gravity.TOP:
      childTop = parentTop + lp.topMargin;
      break;
     case Gravity.CENTER_VERTICAL:
      childTop = parentTop + (parentBottom - parentTop - height) / 2 +
      lp.topMargin - lp.bottomMargin;
      break;
     case Gravity.BOTTOM:
      childTop = parentBottom - height - lp.bottomMargin;
      break;
     default:
      childTop = parentTop + lp.topMargin;
    }

    child.layout(childLeft, childTop, childLeft + width, childTop + height);
   }
  }
 }

 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);

   // Overlay is part of the content and draws beneath Foreground
   if (mOverlay != null && !mOverlay.isEmpty()) {
    mOverlay.getOverlayView().dispatchDraw(canvas);
   }

   // Step 6, draw decorations (foreground, scrollbars)
   onDrawForeground(canvas);

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

   ... ...

 }