前言
说起Android的自定义view，各位老油条肯定都不陌生了，我也是在最近重温《安卓开发艺术探索》的时候才发现的这个问题，**onMeasure的参数 MeasureSpec 到底表示了自身的属性还是父view的属性？**我问了身边的小伙伴，他们的回答都不对或者很模棱两可，所以我特意记录一下，希望小伙伴们能有个准确地认识。
结论先行
先说下结论，onMeasure是测量view或viewGroup时系统调用的，它的参数widthMeasureSpec和heightMeasureSpec既不代表父ViewGroup的宽高也不代表子View的宽高，它表示了父ViewGroup对自己的期望，至于子View实际的宽高得看setMeasuredDimension传的是什么。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// ...
}
具体分析
那么什么是父view的期望值？我们从头说起，在android绘制的过程中，view树的最顶层是ViewRootImpl，它是一个实现了ViewParent接口的特殊的类，我们可以把它看成所有View的爷爷，ViewRootImpl中又存着DecorView的实例，我们知道DecorView才是我们View树最上层的ViewGroup，系统会调用ViewRootImpl的performTraversal方法开始View树的绘制，这个方法里面会调用DecorView的measure方法，到此，View的绘制正式传递到DecorView中
好，停一下，ViewRootImpl传递给DecorView的measure方法中的参数是怎么来的？我们看一下performTraversal方法内部这一小段：
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
mWidth，mHeight指的是屏幕的宽高，lp是一个LayoutParams类型的参数，他表示当前window的尺寸参数，那么也就是说这里的值都是当前屏幕的参数，关键还得看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;
}
这里面会根据当前屏幕尺寸和当前window的参数进行判断
- MATCH_PARENT： 精确模式，大小就是屏幕大小
- WRAP_CONTENT：最大模式，最大不能超过windowSize
- 指定数值的固定大小：精确模式，大小定死为rootDimension
那么这里的MeasureSpec确定好了就传递给DecorView的onMeasure里面了，接下来的measure过程就是一层一层遍历View树了，一般来讲，我们在ViewGroup的onMeasure中都会调用measureChild会子view进行measure，那么我们看下对应代码
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);
}
这里 lp 表示子view的具体宽高参数，也就是我们在XML设置的宽高值。那么根据父viewGroup的MeasureSpec和子view的宽高（先忽略padding，不影响我们分析），通过getChildMeasureSpec方法去得到子view的MeasureSpec
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);
}
这个方法就是核心方法，系统会根据父ViewGroup的MeasureSpec去计算子view的MeasureSpec，具体思想如下：
- 父ViewGroup是EXACTLY：
- 子View参数是具体值： 子View的MeasureSpec就是childSize+EXACTLY
- 子View参数是MATCH_PARENT： 子View的MeasureSpec就是parentSize+EXACTLY
- 子View参数是WRAP_PARENT： 子View的MeasureSpec就是parentSize+AT_MOST
- 父ViewGroup是AT_MOST：
- 子View参数是具体值： 子View的MeasureSpec就是childSize+EXACTLY
- 子View参数是MATCH_PARENT： 子View的MeasureSpec就是parentSize+AT_MOST
- 子View参数是WRAP_PARENT： 子View的MeasureSpec就是parentSize+AT_MOST
- 父ViewGroup是UNSPECIFIED： 这种不管了╭(╯^╰)╮
所以才有了我们之前的结论，onMeasure参数中的MeasureSpec不是子view的参数也不是父viewGroup的参数，而是结合两者计算出来的一种理想值，他表示系统结合父ViewGroup和子View的设定情况而推算出子View应该是这样的一种假象情况，具体你遵不遵守随意
那么回想一下，如果我们的继承自View或ViewGroup的自定义View不重写onMeasure，那么它就不会支持wrap_content属性，看看默认的onMeasure干了什么：
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:
result = specSize;
break;
}
return result;
}
如果自定义的子View设置wrap_content，那么根据上面总结的结论，自定义的子View的onMeasure中的MeasureSpec是parentSize + AT_MOST的组合，而上述代码并不区分AT_MOST和EXACTLY，全部把result设成parentSize，所以自然就不支持wrap_content啦
最后我给大家举个例子帮助理解，假设父ViewGroup宽高是WRAP_CONTENT，子View的宽是MATCH_PARENT，高是WRAP_CONTENT，那么子View的onMeasure中的MeasureSpec是什么呢？
答案就是：onMeasure中子View的宽为AT_MOST，高为AT_MOST，怎么样，整明白了吗
