Android UI绘制流程及原理详解

ActivityThread.handleResumeActivity() 
--> WindowManagerImpl.addView(decorView, layoutParams) 
--> WindowManagerGlobal.addView()

ViewRootImpl.setView(decorView, layoutParams, parentView)
-->ViewRootImpl.requestLayout()
-->scheduleTraversals()
-->TraversalRunnable.run()
-->doTraversal()
-->performTraversals()（performMeasure、performLayout、performDraw）

private static final int MODE_SHIFT = 30;
// 0x3转换为二进制即为：11
// 左移30位后：11000000 00000000 00000000 00000000
private static final int MODE_MASK = 0x3 << MODE_SHIFT;

public static int getMode(int measureSpec) {
 // 与MODE_MASK按位与运算后，即将低30位清零，结果为mode左移30位后的值
 return (measureSpec & MODE_MASK);
}

public static int makeMeasureSpec(
 @IntRange(from = 0, 
  to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size, 
 @MeasureSpecMode int mode) {
 if (sUseBrokenMakeMeasureSpec) {
  return size + mode;
 } else {
  return (size & ~MODE_MASK) | (mode & MODE_MASK);
 }
}

01000000 00000000 00000000 00000000 | 
00001000 00001011 11110101 10101101 =
01001000 00001011 11110101 10101101

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;

private static 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;
}

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);
}

// 加上padding
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

// 检查是否小于最小宽度、最小高度
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

// 检查Drawable的最小高度和宽度
final Drawable drawable = getForeground();
if (drawable != null) {
 maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
 maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}

setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
  resolveSizeAndState(maxHeight, heightMeasureSpec,
    childState << MEASURED_HEIGHT_STATE_SHIFT));

View.performMeasure()
-->onMeasure(int widthMeasureSpec, int heightMeasureSpec)
-->setMeasuredDimension(int measuredWidth, int measuredHeight)
-->setMeasuredDimensionRaw(int measuredWidth, int measuredHeight)

private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
 // 存储测量结果
 mMeasuredWidth = measuredWidth;
 mMeasuredHeight = measuredHeight;

 // 设置测量完成的标志位
 mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

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:
  // 最终测量的结果都是父容器的大小
  result = specSize;
  break;
 }
 return result;
}

// 实则为DecorView的left, top, right, bottom四个信息
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

mLeft = left;
mTop = top;
mRight = right;
mBottom = 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();

 // 省略

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

   // 省略

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

ViewRootImpl.performDraw()
-->ViewRootImpl.draw()
-->ViewRootImpl.drawSoftware()
-->View.draw()

/*
 * 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)
 */