浅谈Android View绘制三大流程探索及常见问题

 private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
  Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
  try {
   mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
  } finally {
   Trace.traceEnd(Trace.TRACE_TAG_VIEW);
  }
 }

 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
  // ......

  if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
    widthMeasureSpec != mOldWidthMeasureSpec ||
    heightMeasureSpec != mOldHeightMeasureSpec) {
   // ......
   if (cacheIndex < 0 || sIgnoreMeasureCache) {
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
   }
   // .....
 }

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

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;
 }

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);
   }
  }
 }

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);
}

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);
   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;
}

 private boolean setOpticalFrame(int left, int top, int right, int bottom) {
  Insets parentInsets = mParent instanceof View ?
    ((View) mParent).getOpticalInsets() : Insets.NONE;
  Insets childInsets = getOpticalInsets();
  return setFrame(
    left + parentInsets.left - childInsets.left,
    top + parentInsets.top - childInsets.top,
    right + parentInsets.left + childInsets.right,
    bottom + parentInsets.top + childInsets.bottom);
 }

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

 private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
   boolean scalingRequired, Rect dirty) {

  // Draw with software renderer.
  final Canvas canvas;
  try {
   ......
   canvas = mSurface.lockCanvas(dirty);
  } 

  try {
   ......
   try {
    mView.draw(canvas);
   } 
  } 
 }

  if (!dirtyOpaque) {
   drawBackground(canvas);
  }

 /**
  * Implement this to do your drawing.
  *
  * @param canvas the canvas on which the background will be drawn
  */
 protected void onDraw(Canvas canvas) {
 }

 /**
  * Called by draw to draw the child views. This may be overridden
  * by derived classes to gain control just before its children are drawn
  * (but after its own view has been drawn).
  * @param canvas the canvas on which to draw the view
  */
 protected void dispatchDraw(Canvas canvas) {

 }

 @Override
 protected void dispatchDraw(Canvas canvas) {
  ......
  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;
    }
   }
   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);
   }
  }
  ......
 }

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

  mView.post(new Runnable() {
   @Override
   public void run() {
    width = mView.getMeasuredWidth();
    height = mView.getMeasuredHeight();
   }
  });

 ViewTreeObserver observer = mView.getViewTreeObserver();
  observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
   @Override
   public void onGlobalLayout() {
    mView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
    width = mScanIv.getMeasuredWidth();
    height = mScanIv.getMeasuredHeight();
   }
  });

 @Override
 public void onWindowFocusChanged(boolean hasFocus) {
  super.onWindowFocusChanged(hasFocus);
  if (hasFocus) {
   width = mView.getMeasuredWidth();
   height = mView.getMeasuredHeight();
  }
 }