View绘制流程

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   // ViewRootImpl#getRootMeasureSpec
   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;
       }

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// ViewGroup#measureChildWithMargins

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);
    }

// 在计算 margin, padding 后, 调用 getChildMeasureSpec 来获得子 view 大小和模式
// 然后进行子 view 测量循环

// ViewGroup#getChildMeasureSpec
// 该方法结果取值为上表

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

		// 去掉 margin 和 padding 之后的大小, 负值表示超出父 ViewGroup, 则取 0
        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);
    }

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// View#onMeasure

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  	// getSuggestMinimumXXX 主要是检测是否设置了背景, 如果设置了取
  	// mMinXXX 和 background.mMinXXX 的中的最大值. 没有设置则取 mMinXXX
  	// mMinXXX 由 xml 中的 android:minHeight, android:minWidth 来设置

	setMeasuredDimension(
      	getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
		getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

// View#getDefaultSize

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

      switch (specMode) {
        // 该值多用于系统控件的测量
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        // 一般自定义 view 时使用这两个选项
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
      }
      return result;
  }

// View#getSuggestMinimumWidth

// 如果 view 没有背景, 返回 android:minWidth, 默认为0
// 如果 view 有背景, 返回 背景的最小宽度 和 android:minWidth 中的最大值
// 高度下同
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth :
  			max(mMinWidth, mBackground.getMinimumWidth());
}

// View#getSuggestMinimumHeight

protected int getSuggestedMinimumHeight() {
	return (mBackground == null) ? mMinHeight :
  			max(mMinHeight, mBackground.getMinimumHeight());
}

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// 重写该 view 的 onMeasure, 测量内容大小
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 	super.onMeasure(widthMeasureSpec, heightMeasureSpec);

  	int widthMeasureMode = MeasureSpec.getMode(widthMeasureSpec);
  	int widthMeasureSize = MeasureSpec.getSize(widthMeasureSpec);

  	int heightMeasureMode = MeasureSpec.getMode(heightMeasureSpec);
  	int heightMeasureSize = MeasureSpec.getSize(heightMeasureSpec);

  	// mWidth, mHeight 是自己设定的一个默认的宽/高度
  	ViewGroup.LayoutParams lp = getLayoutParams();
  	int width = (widthSpecMode == MeasureSpec.AT_MOST &&
        lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) ? mWidth : widthMeasureSize;
	int height = (heightSpecMode == MeasureSpec.AT_MOST &&
		lp.height == ViewGroup.LayoutParams.WRAP_CONTENT) ? mHeight : heightMeasureSize;
	setMeasuredDimension(width, height)
}