1、简介
组件在显示到界面上要经过三个过程测量、布局、绘制。测量就是计算组件的大小，布局就是摆放组件的位置，而绘制就是把计算好的大小和位置绘制到画布上。下面就在说说组件测量的那些事。组件的onMeasure方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
2、组件测量的3种类型
widthMeasureSpec和heightMeasureSpec是什么呢？他是一个测量规则，一个32位int值，前两位表示测量的模式，低30位表示测量的大小。组件也提供对应的方法来获取他们的值。
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
final int height = MeasureSpec.getSize(heightMeasureSpec);
通过size和mode也可以生成对应的测量规则
MeasureSpec.makeMeasureSpec(resultSize, resultMode);
测量模式有3种
EXACTLY
精确模式，当组件的长宽是具体值，如：
android:layout_width="200dp"
或是指定为match_parent（占据父view的大小），系统使用这个模式。
AT_MOST
最大值模式，当控件的长宽属性为warp_content时，组件的大小随自身的大小变化而变化，并且控件的尺寸不能超过父view允许最大尺寸。
UNSPECIFIED
不指定组件的大小，组件想多大就多大，通常在绘制自定义view时才会使用。
3、从源码来看组件测量过程
先来看View源码里的测量方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
setMeasuredDimension方法接受测量后的长和宽，并赋值给组件，这样组件就有长宽。那如何计算长宽呢，看下面的代码：
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;
}
getDefaultSize方法接收两个参数，一个是大小（系统传入的是组件的最小宽度或高度getSuggestedMinimumWidth()），一个是测量的规则。
通过测量规则获得测量的模式和指定的大小，
如果测量模式为MeasureSpec.UNSPECIFIED，则返回传入的大小为组件大小，及最小宽高；
如果测量模式为MeasureSpec.AT_MOST或MeasureSpec.EXACTLY，则返回父视图指定的大小。
再来看下ViewGroup中的测量实现，要实现ViewGroup的测量必须实现其子类的测量，但是ViewGroup中并没有重写onMeasure方法，需要具体实现类自己重写，但是他提供一些方法来便于我们测量子视图。
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);
}
}
}
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);
}
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);
}
对子视图的测量，实际上就是遍历子视图，然后调用他们measure方法，measure里会再调用onMeasure方法执行视图的测量。这里的关键实际上就是计算出子视图的测量规则measureSpec，及getChildMeasureSpec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;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
这个方法接收3个参数
spec，父视图的测量规则，可以计算出父视图的测量模式和测量大小
padding，用于计算扣除padding后的视图大小
childDimension，视图布局中定义的宽高大小
通过不同的模式来计算出子视图的大小和模式从而转换为测量规则
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:
...
break;
case MeasureSpec.AT_MOST:
...
break;
case MeasureSpec.UNSPECIFIED:
...
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
父视图为精确模式
1、子视图有定义具体的大小，resultSize为定义的具体大小，模式为MeasureSpec.EXACTLY
2、子视图的childDimension为LayoutParams.MATCH_PARENT，resultSize为定义的具体大小，模式为MeasureSpec.EXACTLY
3、子视图的childDimension为LayoutParams.WRAP_CONTEN，resultSize为定义的具体大小，模式为MeasureSpec.AT_MOST
其他两种情况也是类似，源码中已经描述的很清楚了。最后调用MeasureSpec.makeMeasureSpec(resultSize, resultMode);方法就获得了子视图的测量规则。
再回顾前面说的View的测量，就能确定具体的View的测量大小。视图的整个测量过程就是这样，关键还是在要理解清楚MeasureSpec测量规则的使用。
4、测量在实际开发中的应用
具有最大高度的滚动视图
通过计算ScrollView子视图的高度，超过设置的最大高度，则设置Scrollview的高度为最大高度，如果没有超过，则按子视图的高度来显示scrollview。
具体的代码如下：
@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = MeasureSpec.getSize(widthMeasureSpec);
int height = MeasureSpec.getSize(heightMeasureSpec);
int mMaxHeight = height > maxHeight ? maxHeight : height;
View child = getChildAt(0);
int child_height = child.getHeight();
if (child_height == 0)
child_height = mMaxHeight;
else if (child_height > maxHeight)
child_height = maxHeight;
setMeasuredDimension(width, child_height);
}
5、注意事项
1、要区分getMeasuredHeight()与getHeight()的区别，getMeasuredHeight()是测量时的大小，组件有可能会测量多次，多次测量的大小有可能不一样，getHeight()是要绘制的大小
2、视图只有测量过后，getMeasuredHeight()方法才会有值，不然返回的是0
