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。