Android开发艺术探索笔记
SpecMode
- UNSPECIFIED，表示一种测量状态，对View的大小不做限制，要多大给多大。一般用于系统内部。
- EXACTLY，父容器已经知道View的精确大小，即SpecSize。对应于LayoutParams的match_parent。
- AT_MOST，父容器指定了一个最大的SpecSize，View不能大于该值，View具体的大小要看View的具体实现。对应于LayoutParams的wrap_content。
整体Measure流程
主要是分为MeasureSpec和Measure两个过程
- MeasureSpec过程是根据View自身的LayoutParams和其父控件的MeasureSpec来确定View的MeasureSpec。顶级父控件DecorView的MeasureSpec由窗口的尺寸和自身的LayoutParams决定。
- Measure过程是根据View的MeasureSpec来测量的宽/高度（View的最终宽高和四个顶点的位置在layout结束后确定）。
MeasureSpec的过程
分为顶层View即DecorView，和普通View（包含ViewGroup）两种情况：
. DecorView，其MeasureSpec由窗口的尺寸和自身的LayoutParams决定。
. 普通View，其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams共同决定。
- DecorView的MeasureSpec的创建过程
DecorView对应的实现类是ViewRootImpl。MeasureSpec从measureHierarchy方法开始。
先码上measureHierarchy的所有代码，看看就好：
if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
// On large screens, we don't want to allow dialogs to just
// stretch to fill the entire width of the screen to display
// one line of text. First try doing the layout at a smaller
// size to see if it will fit.
final DisplayMetrics packageMetrics = res.getDisplayMetrics();
res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
int baseSize = 0;
if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
baseSize = (int)mTmpValue.getDimension(packageMetrics);
}
if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);
if (baseSize != 0 && desiredWindowWidth > baseSize) {
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
goodMeasure = true;
} else {
// Didn't fit in that size... try expanding a bit.
baseSize = (baseSize+desiredWindowWidth)/2;
if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
+ baseSize);
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
if (DEBUG_DIALOG) Log.v(TAG, "Good!");
goodMeasure = true;
}
}
}
}
measureHierarchy()中的参数：desiredWindowWidth窗口的宽 ；desiredWindowHeight窗口的高
measureHierarchy会处理WRAP_CONTENT时的大屏的特殊情况，见如下的注释
if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
// On large screens, we don't want to allow dialogs to just
// stretch to fill the entire width of the screen to display
// one line of text. First try doing the layout at a smaller
// size to see if it will fit.
measureHierarchy中大部分代码都是用来处理这个逻辑的。剩下关于MeasureSpec的代码只有短短三行。
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
MeasureSpec的逻辑重点自然在getRootMeasureSpec（）方法中。其注释就直白的说明了
/**
* Figures out the measure spec for the root view in a window based on it's
* layout params.
* 根据root view的 layout params确定其measure spec
* @param windowSize
* The available width or height of the window
*
* @param rootDimension
* The layout params for one dimension (width or height) of the
* window.
*
* @return The measure spec to use to measure the root view.
*/
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.
//对应MeasureSpec.EXACTLY的精确模式，大小为窗口大小。
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
//对应MeasureSpec.AT_MOST的最大模式，大小不能超过窗口大小。
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
//对应MeasureSpec.EXACTLY的精确模式，大小为布局中的指定大小
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
- 普通View的MeasureSpec的创建过程
View的measure的ViewGroup的measureChildWithMargins开始
/**
* 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);
}
又上面代码可以看出MeasureSpec的逻辑代码自然是在getChildMeasureSpec（）中。也可以看出，在确定好MeasureSpec后，就开始了child的measure流程
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
下面看getChildMeasureSpec的具体逻辑
/**
* 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);
//首先说下padding参数，以宽度为例。由measureChildWithMargins方法看出，该参数是将ViewGroup的
//左右padding，view自身的左右margin以及ViewGroup中已经被占用的宽度。由此可以看出，
//无论view怎样，其宽度都不能超过lp.width - padding
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY: //父控件为精确模式EXACTLY时
if (childDimension >= 0) { //1. 1View布局中为具体大小值
resultSize = childDimension; // view为EXACTLY，大小为布局中指定的值
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) { //1.2 View为MATCH_PARENT
// Child wants to be our size. So be it.
resultSize = size; //View可以精确到和父控件一样大，所以View为EXACTLY模式
resultMode = MeasureSpec.EXACTLY; //大小为父控件可用的大小（除掉了padding）
} else if (childDimension == LayoutParams.WRAP_CONTENT) { //1.3 View为WRAP_CONTENT
// Child wants to determine its own size. It can't be
// bigger than us. //此时View大小随内容变化，但不能超过父控件的可用空间
resultSize = size; //所以View为AT_MOST最大模式
resultMode = MeasureSpec.AT_MOST; //大小为父控件可用的大小（除掉了padding）
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST: //父控件为AT_MOST最大模式
if (childDimension >= 0) { //2.1 View布局中为具体大小值
// Child wants a specific size... so be it
resultSize = childDimension; // view为EXACTLY，大小为布局中指定的值
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {//2.2View为MATCH_PARENT
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size; //View大小和父控件一样，不能超过父控件的最大值
resultMode = MeasureSpec.AT_MOST; //View为最大模式，大小不能超过父控件的可用空间
} else if (childDimension == LayoutParams.WRAP_CONTENT) {//2.3 View为WRAP_CONTENT
// Child wants to determine its own size. It can't be
// bigger than us. //View大小随内容变化，打不能超过超过父控件的可用空间
resultSize = size; //所以View为AT_MOST最大模式
resultMode = MeasureSpec.AT_MOST; //大小为父控件可用的大小（除掉了padding）
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED: //父控件为UNSPECIFIED未指定模式
if (childDimension >= 0) { //3.1View布局中为具体大小值
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY; // view为EXACTLY，大小为布局中指定的值
} else if (childDimension == LayoutParams.MATCH_PARENT) {//3.2View为MATCH_PARENT
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED; //View也为UNSPECIFIED，大小为0
} else if (childDimension == LayoutParams.WRAP_CONTENT) {//3.3View为WRAP_CONTENT
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;//View也为UNSPECIFIED，大小为0
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
measure流程
分为View的measure过程和ViewGroup的measure过程。
- View的measure过程
ViewGroup在measureChildWithMargins中确定了View的MeasureSpec后调用
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
View的measure方法是个final方法，但其中会调用onMeasure（）；
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
...
}
View的onMeasure方法如下：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
setMeasuredDimension（）方法会设置View的宽高，如果我们在重写onMeasure时，也一定要该方法，是宽高生效。
默认的宽高逻辑代码在getDefaultSize（）中
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;
}
逻辑很简单MeasureSpec.AT_MOST和 MeasureSpec.EXACTLY模式时，View的大小即specSize。
MeasureSpec.UNSPECIFIED时View的大小为第一个参数（以宽为例），即onMeasure传过来的getSuggestedMinimumWidth。
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
这里的逻辑是，如果View没有设置背景，则去mMinWidth，对应的是android：minWidth设置的值；如果View设置了背景，怎取背景的getMinimumWidth()值和minWidth中大的那个.
背景的getMinimumWidth()也很简单，即有原始宽高时取原始宽高，没有取0.
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
总结，View的测量问题在于：View在WRAP_CONTENT时，其最终的模式时AT_MOST，且大小是父控件的可用大小。这和MATCH_PARENT时没有区别。因此在自定义View时，我们要处理View在WRAP_CONTENT时的这一问题。
- ViewGroup的measure过程
ViewGroup比较简单，它的measure流程和View一样，作为其父控件的子View处理，不同的是，ViewGroup没有重写onMeasure方法，而是交给其子类去按需实现。
以LinearLayout为例。LinearLayout会遍历子View，一次调用子View的measure方法，并纪录每个子View的测量值，最后再测量自己的大小。
获取View的宽高
因为Activity的生命周期和View的测量是异步的，所以并不能在Activity的生命周期函数中直接获取。通常可以通过以下几种方式获取：
- onWindowsFocusChanged
这个回调函数在View初始化完成后调用，所以在这里获取宽高可以确保是View测量后的宽高。需要注意的是，该函数在Activity失去和获取焦点的时候都会触发。
public void onWindowsFocusChanged(boolean hasFocus){
super.onWindowsFocusChanged(hasFocus);
if (hasFocus) {
int width = view.getMeasureWidth();
int herght = view.getMeasureHeight();
}
}
- view.post(runnable)
此方法通过post将一个runnable投递到消息队列的尾部，因为在尾部，所以此runnable被Looper处理时，View已经测量完成了。
protected void omStart(){
super.omStart();
view.post(new Runnable(){
@override
public void run(){
int width = view.getMeasureWidth();
int herght = view.getMeasureHeight();
}
});
}
- ViewTreeObserverd的OnGlobalLayoutListener接口
OnGlobalLayoutListener中的onGlobalLayout方法会在View树的状态发生变化时回调。注意，此方法回多次调用
protected void onStart(){
super.onStart();
ViewTreeOberver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new OnGlobalLayoutListener(){
@Override
public void onGlobalLayout() {
view.getViewTreeObserver().removeGlobalOnLayoutListener(this);
int width = view.getMeasureWidth();
int herght = view.getMeasureHeight();
}
}
)
}
4.view.measure(int widthMeasureSpec , int heightMeasureSpec)
手动触发测量