View测量流程

View测量流程简介

ViewGroup继承自View，在View的测量方法measure方法中，调用了onMeasure，onMeasure是View具体实现测量过程的方法，也是自定义View时需要自己去实现的方法。
对于一个activity来说测量流程是子上而下的：
1.ViewRootImpl 调用performMeasure 测量Activity 顶层View DecorView，决定其测量模式。
2.ViewGroup在onMeasure中调用measureChildren或child.measure测量出子View的大小。
3.View在onMeasure中实现对自身大小的测量,并调用setMeasuredDimension设置具体宽高尺寸。

MeasureSpec

MeasureSpec是View测量中很重要的一个类，由mode和size两部分组成。mode表示父View对其子View大小的约束，size则是具体尺寸。mode+size对应最终MeasureSpec的值。

mode	对子View的约束
MeasureSpec.UNSPECIFIED	对子View大小无任何约束，子View可为任意大小
MeasureSpec.EXACTLY	确切的大小，如：100dp或者march_parent
MeasureSpec.AT_MOST	子View可以达到其想要达到的大小，如：wrap_content

MeasureSpec.makeMeasureSpec(size,mode)可以得到int类型的MeasureSpec的值，在测量过程中measure onMeasure都是通过此值来进行View的测量。

View测量起点ViewRoot

ViewRoot是View绘制的起点，实现类ViewRootImpl中会在performTraversals中调用performMeasure、performLayout、performDraw分别开始View的测量，布局，绘制。

 int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
 int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

在performTraversals调用performMeasure时，传入了两个参数，所测量View（此处为DecorView）的宽高MeasureSpec，而这两个参数都是有getRootMeasureSpec方法返回，在getRootMeasureSpec接受了两个参数，windowSize（一般为屏幕尺寸），rootDimension（DecorView的宽高属性）

 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;
    }

在getRootMeasureSpec方法中可以看出：
1.当DecorView的宽或者高为MATCH_PARENT的时候，DecorView的测量模式为EXACTLY,size为window的size。
2.当DecorView的宽或者高为WRAP_CONTENT的时候，DecorView的测量模式为AT_MOST,size为window的size。
3.其他情况，即宽高为指定大小的值，如100dp的时候，DecorView的测量模式为EXACTLY,size为具体的值。

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

在performMeasure方法中调用了mView.measure,此处的mView就是DecorView，DecorView继承自FrameLayout，则具体测量任务交给ViewGroup处理。

ViewGroup测量

ViewGroup继承自View，但是并没有具体onMeasure实现，因为ViewGroup中是可以包含View的，当ViewGroup的款或者高为WRAP_CONTENT的时候，对于不同的ViewGroup采用的方法不同，如RelativeLayout和LinearLayout。不过VIewGroup提供了默认实现measureChildren、measureChild、measureChildWithMargins来测量子VIew的大小。

 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);
            }
        }
    }

在measureChildren中遍历了ViewGroup中的View，并调用measureChild对非GONE的View进行测量。

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);
}

measureChild中则是先获取了子View的Width和heigth，然后调用了子View的measure方法。

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);
    }

在getChildMeasureSpec方法中会根据ViewGroup的测量模式和子view的具体宽高来决定子View的MeasureSpec。具体对应关系如下

View测量

View是测绘的最底层，一般自定义View的时候都需要重写onMeasure方法，当不去重写且View设置了WRAP_CONTENT的时候，在实际展示效果中却是填充了整个该View所在的ViewGroup。查看View onMeasure的默认实现方法可以知道原因。

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

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

View的onMeasure中直接根据ViewGroup传递过来的width和heigth获取了默认的宽高并设置给View。

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;
    }

在getDefaultSize中可以看到当View的测量模式为UNSPECIFIED的时候返回的是View设置的最小尺寸，而当测量模式为AT_MOST和EXACTLY的时候，返回的都是ViewGroup传过来的尺寸。根据源码和ViewGroup View测量模式对应关系可知道，出现上述问题的情况有3中情况
1.ViewGroup的测量模式为EXACTLY,View的宽或者高为wrap_content。
2.ViewGroup的测量模式为AT_MOST,View的宽或者高为match_parent。
3.ViewGroup的测量模式为AT_MOST,View的宽或者高为wrap_content。