Android View体系（七）从源码解析View的measure流程

前言

在上一篇我们了解了Activity的构成后，开始了解一下View的工作流程，就是measure、layout和draw。measure用来测量View的宽高，layout用来确定View的位置，draw则用来绘制View。这一讲我们来看看measure流程，measure流程分为View的measure流程和ViewGroup的measure流程，只不过ViewGroup的measure流程除了要完成自己的测量还要遍历去调用子元素的measure()方法。

1.View的measure流程

先来看看onMeasure()方法（View.java）：

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

看看setMeasuredDimension()方法：

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
       boolean optical = isLayoutModeOptical(this);
       if (optical != isLayoutModeOptical(mParent)) {
           Insets insets = getOpticalInsets();
           int opticalWidth  = insets.left + insets.right;
           int opticalHeight = insets.top  + insets.bottom;

           measuredWidth  += optical ? opticalWidth  : -opticalWidth;
           measuredHeight += optical ? opticalHeight : -opticalHeight;
       }
       setMeasuredDimensionRaw(measuredWidth, measuredHeight);
   }

很显然是用来设置View的宽高的，先来看看getDefaultSize()方法处理了什么：

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;
}

specMode是View的测量模式，而specSize是View的测量大小，看到这里我们有必要先看看MeasureSpec类：

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

        /**
         * Measure specification mode: The parent has not imposed any constraint
         * on the child. It can be whatever size it wants.
         */
        public static final int UNSPECIFIED = 0 << MODE_SHIFT;

        /**
         * Measure specification mode: The parent has determined an exact size
         * for the child. The child is going to be given those bounds regardless
         * of how big it wants to be.
         */
        public static final int EXACTLY     = 1 << MODE_SHIFT;

        /**
         * Measure specification mode: The child can be as large as it wants up
         * to the specified size.
         */
        public static final int AT_MOST     = 2 << MODE_SHIFT;

...

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

MeasureSpec类帮助我们来测量View，它是一个32位的int值，高两位为specMode （测量的模式），低30位为specSize （测量的大小），测量模式分为三种：

• UNSPECIFIED：未指定模式，View想多大就多大，父容器不做限制，一般用于系统内部的测量。
• AT_MOST：最大模式，对应于wrap_comtent属性，只要尺寸不超过父控件允许的最大尺寸就行。
• EXACTLY：精确模式，对应于match_parent属性和具体的数值，父容器测量出View所需要的大小，也就是specSize的值。

让我们回头看看getDefaultSize()方法，很显然在AT_MOST和EXACTLY模式下，都返回specSize这个值，也就是View测量后的大小，而在UNSPECIFIED模式返回的是getDefaultSize()方法的第一次个参数的值，这第一个参数从onMeasure()方法来看是getSuggestedMinimumWidth()方法和getSuggestedMinimumHeight()得到的，那我们来看看getSuggestedMinimumWidth()方法做了什么，我们只需要弄懂getSuggestedMinimumWidth()方法，因为这两个方法原理是一样的：

protected int getSuggestedMinimumWidth() {
      return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
  }

如果View没有设置背景则取值为mMinWidth，mMinWidth是可以设置的，它对应于android:minWidth这个属性设置的值或者View的setMinimumWidth的值，如果不指定的话则默认为0：

public void setMinimumWidth(int minWidth) {
    mMinWidth = minWidth;
    requestLayout();
}

如果View设置了背景在取值为max(mMinWidth, mBackground.getMinimumWidth())，取值mMinWidth和mBackground.getMinimumWidth()的最大值，上面我们说过了mMinWidth，那来看看mBackground.getMinimumWidth()，这个mBackground是Drawable类型的，看一下Drawable类的getMinimumWidth()方法（Drawable.java）：

public int getMinimumWidth() {
       final int intrinsicWidth = getIntrinsicWidth();
       return intrinsicWidth > 0 ? intrinsicWidth : 0;
}

intrinsicWidth得到的是这个Drawable的固有的宽度，如果固有宽度大于0则返回固有宽度，否则返回0。
总结一下getSuggestedMinimumWidth()方法就是：如果View没有设置背景则返回mMinWidth ，如果设置了背景就返回mMinWidth 和Drawable最小宽度两个值的最大值。

2.ViewGroup的measure流程

ViewGroup的measure原理

讲完了View的measure流程，接下来看看ViewGroup的measure流程，对于ViewGroup，它不只要measure自己本身，还要遍历的调用子元素的measure()方法，ViewGroup中没有定义onMeasure()方法，但他定义了measureChildren()方法（ViewGroup.java）：

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);
           }
       }
   }

就是遍历子元素并调用measureChild()方法：

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);
   }

调用child.getLayoutParams()方法来获得子元素的LayoutParams属性，并获取到子元素的MeasureSpec并调用子元素的measure()方法进行测量。getChildMeasureSpec()方法里写了什么呢？

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 = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = 0;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

很显然这是根据父容器的MeasureSpec的模式再结合子元素的LayoutParams属性来得出子元素的MeasureSpec属性，有一点需要注意的是如果父容器的MeasureSpec属性为AT_MOST，子元素的LayoutParams属性为WRAP_CONTENT，那根据代码我们会发现子元素的MeasureSpec属性也为AT_MOST，它的specSize值为父容器的specSize减去padding的值，也就是说跟这个子元素设置LayoutParams属性为MATCH_PARENT效果是一样的，为了解决这个问题需要在LayoutParams属性为WRAP_CONTENT时指定一下默认的宽和高。

LinearLayout的measure流程

ViewGroup并没有提供onMeasure()方法，而是让其子类来各自实现测量的方法，究其原因就是ViewGroup有不同的布局的需要很难统一，接下来我们来简单分析一下ViewGroup的子类LinearLayout的measure流程，先来看看它的onMeasure()方法（LinearLayout.java）：

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
       if (mOrientation == VERTICAL) {
           measureVertical(widthMeasureSpec, heightMeasureSpec);
       } else {
           measureHorizontal(widthMeasureSpec, heightMeasureSpec);
       }
   }

来看看垂直measureVertical()方法的部分源码：

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
        mTotalLength = 0;
     mTotalLength = 0;       
 ...
  for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);

            if (child == null) {
                mTotalLength += measureNullChild(i);
                continue;
            }

            if (child.getVisibility() == View.GONE) {
               i += getChildrenSkipCount(child, i);
               continue;
            }

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

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

            totalWeight += lp.weight;
            
            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
                // Optimization: don't bother measuring children who are going to use
                // leftover space. These views will get measured again down below if
                // there is any leftover space.
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
                skippedMeasure = true;
            } else {
                int oldHeight = Integer.MIN_VALUE;

                if (lp.height == 0 && lp.weight > 0) {
                    // heightMode is either UNSPECIFIED or AT_MOST, and this
                    // child wanted to stretch to fill available space.
                    // Translate that to WRAP_CONTENT so that it does not end up
                    // with a height of 0
                    oldHeight = 0;
                    lp.height = LayoutParams.WRAP_CONTENT;
                }

                // Determine how big this child would like to be. If this or
                // previous children have given a weight, then we allow it to
                // use all available space (and we will shrink things later
                // if needed).
                measureChildBeforeLayout(
                       child, i, widthMeasureSpec, 0, heightMeasureSpec,
                       totalWeight == 0 ? mTotalLength : 0);

                if (oldHeight != Integer.MIN_VALUE) {
                   lp.height = oldHeight;
                }

                final int childHeight = child.getMeasuredHeight();
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                       lp.bottomMargin + getNextLocationOffset(child));
...

        if (useLargestChild &&
                (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
            mTotalLength = 0;

            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);

                if (child == null) {
                    mTotalLength += measureNullChild(i);
                    continue;
                }

                if (child.getVisibility() == GONE) {
                    i += getChildrenSkipCount(child, i);
                    continue;
                }

                final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
                        child.getLayoutParams();
                // Account for negative margins
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
            }
        }

        // Add in our padding
        mTotalLength += mPaddingTop + mPaddingBottom;

        int heightSize = mTotalLength;

        // Check against our minimum height

定义了mTotalLength用来存储LinearLayout在垂直方向的高度，然后遍历子元素，根据子元素的MeasureSpec模式分别计算每个子元素的高度，如果是wrap_content则将每个子元素的高度和margin垂直高度等值相加并赋值给mTotalLength得出整个LinearLayout的高度。如果布局高度设置为match_parent者具体数值则和View的测量方法一样。