在自定义ViewGroup时，通常需要在onMeasure方法中调用measureChildWithMargins或measureChild方法来测量子视图的尺寸。其中需要参考当前ViewGroup的MeasureSpec，及其他条件用于生成子视图的MeasureSpec(测量规格)。我们以measureChildWithMargins为例。
/**
* 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);
}
以上的代码中，我们可以看到，在measureChildWithMargins中，又调用了getChildMeasureSpec方法。从方法名可以得知，此方法就是用于确认传递给子视图的MeasureSpec的方法。方法返回MeasureSpec后，最后调用子视图的measure方法，用于测量子视图的尺寸。getChildMeasureSpec的传参为其父控件，即当前ViewGroup的MeasureSpec,以及Padding和Margin和已使用的空间的和，最后是子控件的宽高。根据参数，我们可以推测，子控件的MeasureSpec是根据父控件的MeasureSpec、自己自身的尺寸(final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(),用户从xml或代码中设置的尺寸)，以及已经占用的空间来确定的。下面我们带着猜测看一下具体的源码实现。
/**
* 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);
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 them 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 = 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 = size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
方法开头部分获取了当前ViewGroup的MeasureSpec，并且int size = Math.max(0, specSize - padding)获取了留给子控件的剩余空间。
int resultSize = 0;
int resultMode = 0;
声明了变量，尺寸和模式，用于构建子视图的MeasureSpec.
接下来的大段switch语句中，根据当前ViewGroup的测量模式分别处理，区分MeasureSpec.EXACTLY,MeasureSpec.AT_MOST,MeasureSpec.UNSPECIFIED。可以看出父控件的MeasureSpec影响子控件的MeasureSpec的生成.
然后，我们分别看这几种情况是如何处理的。
以MeasureSpec.EXACTLY为例，
// 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;
childDimension是getChildMeasureSpec的最后一个参数，具体指为从measureChildWithMargins中获取的开发者设置的子视图的尺寸(xml或代码)，然后判断其是否大于等于0，如果为true，则设置resultSize = childDimension; resultMode = MeasureSpec.EXACTLY;,即父控件为精确模式，并且子控件也有精确尺寸的时候，那么子控件的测量模式也为精确模式。 如果childDimension == LayoutParams.MATCH_PARENT时，即父控件为精确模式，子控件尺寸为LayoutParams.MATCH_PARENT时，那么子控件的测量模式也为精确模式。 实际上，LayoutParams.MATCH_PARENT本身也可以看成一个精确的尺寸，尺寸大小等于其可用的最大尺寸，即方法开头的size。所以最后设置resultSize = size;resultMode = MeasureSpec.EXACTLY;。如果子控件尺寸为childDimension == LayoutParams.WRAP_CONTENT，即父控件为精确模式，子控件尺寸为childDimension == LayoutParams.WRAP_CONTENT时，子控件的测量模式为最大限制模式。 所以最后设置resultSize = size;resultMode = MeasureSpec.AT_MOST;。
同理分析父控件为MeasureSpec.AT_MOST和MeasureSpec.UNSPECIFIED,整理成一下表格
| 子控件MeasureSpec | 成立条件 |
|---|---|
| MeasureSpec.EXACTLY | 1. 父控件为MeasureSpec.EXACTLY，同时子控件使用精确尺寸 2. 父控件为MeasureSpec.EXACTLY，同时子控件尺寸为LayoutParams.MATCH_PARENT 3. 父控件为MeasureSpec.AT_MOST，同时子控件使用精确尺寸 4. 父控件为MeasureSpec.UNSPECIFIED，并且子控件使用精确尺寸 |
| MeasureSpec.AT_MOST | 1. 父控件为MeasureSpec.EXACTLY，同时子控件尺寸为LayoutParams.WRAP_CONTENT 2. 父控件为MeasureSpec.AT_MOST，同时子控件尺寸为LayoutParams.MATCH_PARENT 3. 父控件为LayoutParams.MATCH_PARENT，同时子控件尺寸为LayoutParams.WRAP_CONTENT |
| MeasureSpec.UNSPECIFIED | 1. 父控件为MeasureSpec.UNSPECIFIED， 并且同时子控件尺寸为LayoutParams.MATCH_PARENT 2. 父控件为MeasureSpec.UNSPECIFIED，并且同时子控件尺寸为LayoutParams.WRAP_CONTENT |
最后，调用MeasureSpec.makeMeasureSpec(resultSize, resultMode);方法最终生成子控件的MeasureSpec。
了解MeasureSpec的生成过程，有助于理解自定义控件的整个测量过程。但是需要注意的是，在实际开发中，我们并不太需要关心这一部分的代码实现，直接调用现成的方法即可，如在自定义ViewGroup时调用measureChild、measureChildren、measureChildWithMargins方法。
当我们自定义控件测量其尺寸时，最终都要通过调用setMeasuredDimension(width, height)完成，onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) 的入参中传递了正确的测量规格。如果测量规格为MeasureSpec.EXACTLY，我们直接获取其尺寸使用即可；而如果测量规格为MeasureSpec.AT_MOST,其传递的是当前视图所能占用的最大尺寸，所以其具体尺寸需要我们自己通过逻辑来确定；MeasureSpec.UNSPECIFIED则一般不会遇到。
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