我们都知道系统要确定View的大小，首先得先获得MeasureSpec，再通过MeasureSpec来决定View的大小。
MeasureSpec（32为int值）由两部分组成：
SpecMode（高2位）:测量模式。
SpecSize（低30位）：某种测量模式下的规格大小。
SpecMode有3类：
UNSPECIFIED: 父容器不对view做大小限制，一般用于系统内部，表示一种测量状态。
EXACTLY:精确模式。对应于：LayoutPrams中的match_parent和具体数值。
AT_MOST:最大值模式。对应于LayoutParam中的wrap_content模式。
那么问题来了，这个MeasureSpec又是由什么决定的呢？我们从源代码里面入手。
ViewGroup里面有一个方法，叫做measureChildWithMargins，用来测量子view的大小的。
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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);
}
可以看出，在代码里，会先获取childWidthMeasureSpec和childHeightMeasureSpec，然后再测量子元素 child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
我们关键是要看这两个MeasureSpec是怎么获取到的，通过方法getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);我们可以看出MeasureSpec的获取不但与子元素本身的LayoutParam有关，还与父容器的MeasureSpec有关，当然，也与它的padding，margin有关。
我们进去看看，这部分代码有点长，不过里面的逻辑很简单，我们直接在源代码里面通过加注释来分析。
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec); //获取父容器的测量模式
int specSize = MeasureSpec.getSize(spec); //获取父容器在该测量模式下的大小 int size = Math.max(0, specSize - padding); int resultSize = 0; //子元素的specSize
int resultMode = 0; //子元素的specMode switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY: //如果父容器的测量模式为EXACTLY
if (childDimension >= 0) { //如果子元素的LayoutParam为具体数值>=0
resultSize = childDimension; //那么子元素的specSize就是childDimension
resultMode = MeasureSpec.EXACTLY; //那么子元素的specMode是EXACTLY
//以下的分析都是一样的，也就不写了
} 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
//这里View.sUseZeroUnspecifiedMeasureSpec一直都是false，因此resultSize==0；
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;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
由以上的分析可以看出子元素的MeasureSpec的获取不但与子元素本身的LayoutParam有关，还与父容器的MeasureSpec有关，当然，也与它的padding，margin有关。
其实我们可以得出下面的结论：
parentMeasureSpec childLayoutParam | EXACTLY | AT_MOST | UNSPECIFIED |
具体数值 | EXACTLLY childSize | EXACTLY childSize | EXACTLY childSize |
Match_parent | EXACTLY parentSize | AT_MOST parentSize | UNSPECIFIED 0 |
Wrap_content | AT_MOST parentSize | AT_MOST parentSize | UNSPECIFIED 0 |
parentSize是父容器的剩余空间。
从上面的表格也可以看出，当我们直接继承view来实现自定义控件的时候，需要重写onMeasure方法并设置wrap_content时候的自身大小，不然使用wrap_content得时候就相当于使用match_parent。我们可以用下面的模板：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode=MeasureSpec.getMode(widthMeasureSpec);
int heightSpecMode=MeasureSpec.getMode(heightMeasureSpec); int widthSpecSize=MeasureSpec.getSize(widthMeasureSpec);
int heightSpecSize=MeasureSpec.getSize(heightMeasureSpec); if (widthSpecMode==MeasureSpec.AT_MOST && heightSpecMode==MeasureSpec.AT_MOST)
{
setMeasuredDimension(100,100);
}
else if (widthSpecMode==MeasureSpec.AT_MOST)
{
setMeasuredDimension(100,heightSpecSize);
}
else if (heightSpecMode==MeasureSpec.AT_MOST)
{
setMeasuredDimension(widthSpecSize,100);
}
}
上面代码中的100是我们自己设置的数值，你也可以用其他数值，根据自己需要。
以上便是对MeasureSpec由何决定的分析。