一.MeasureSpec
MeasureSpec是View类的一个内部类，该类封装了一个View的规格尺寸，包括View的宽和高的信息。
MeasureSpec封装了父布局传递给子布局的布局要求，然后结合View的LayoutParams转换成View对应的MeasureSpec，然后调用View#measure继续传递这个MeasureSpec。
举个例子，LinearLayout中的子View测绘时候MeasureSpec获取：
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
//获取子view的LayoutParams
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//getChildMeasureSpec的三个参数是父容器的MeasureSpec，child相对父容器的内间距，child View所需要的宽度
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);
}
MeasureSpec代表一个32位的int值，高两位代表SpecMode测量模式，低30位代表SpecSize某种测量模式下的规格大小。将SpecMode和SpecSize打包成一个，避免过多内存分配，对应的MeasureSpec有getXXX()方法获取SpecMode和SpecSize。
MeasureSpec类部分源码：
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* UNSPECIFIED 模式：
* 父View不对子View有任何限制，子View需要多大就多大
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* EXACTYLY 模式：
* 父View已经测量出子Viwe所需要的精确大小，这时候View的最终大小
* 就是SpecSize所指定的值。对应于match_parent和精确数值这两种模式
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* AT_MOST 模式：
* 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值，
* 即对应wrap_content这种模式
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
//将size和mode打包成一个32位的int型数值
//高2位表示SpecMode，测量模式，低30位表示SpecSize，某种测量模式下的规格大小
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
//将32位的MeasureSpec解包，返回SpecMode,测量模式
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
//将32位的MeasureSpec解包，返回SpecSize，某种测量模式下的规格大小
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
//...
}
二.MeasureSpec和LayoutParams的对应关系
MeasureSpec不是由LayoutParams唯一决定的，LayoutParams需要和父容器一起才能决定View的MeasureSpec，从而进一步决定View的宽高。
对于顶级View（DecorView）其MeasureSpec由窗口的尺寸和其自身的LayoutParams来共同决定。
对于普通的View其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams来共同决定。
1.先来说说顶级View(DecorView)的MeasureSpec
页面的measure从ViewRootImpl的performMeasure开始：
ViewRootImpl#performMeasure
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
...
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
mView就是页面的根View，那么这个mView是谁？
从代码中可以发现，ViewRootImpl#setView设置了mView。
ViewRootImpl#setView:
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
...
mView = view;
...
}
所以我们要找到ViewRootImpl的初始化对象的地方，然后再看看这个对象调用setView(...)具体set了什么。
Activity组件都有一个关联的Window对象，用来描述一个应用程序窗口，每一个应用程序窗口内部又包含有一个View对象，用来描述应用程序窗口的视图。
页面绘制从根View开始，Activity的Window中也有一个视图View，那么他们是否是同一个View呢？
Activity创建的时候在ActivityThread中通过WindowManager具体实现类WindowManagerImpl将PhoneWindow中的DecorView展示到页面。
ActivityThread#handleResumeActivity
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {
//...
ActivityClientRecord r = performResumeActivity(token, clearHide); // 这里会调用到onResume()方法
if (r != null) {
final Activity a = r.activity;
//...
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow(); // 获得window对象
View decor = r.window.getDecorView(); // 获得DecorView对象
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager(); // 获得windowManager对象
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
wm.addView(decor, l); // 调用addView方法
}
//...
}
}
}
WindowManagerImpl是WindowManager的具体实现类，WindowManagerImpl的addView(...)调用WindowManagerGlobal的addView(...)。
WindowManagerGlobal#addView：
public final class WindowManagerGlobal {
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
//创建ViewRootImpl
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
//DecorView和ViewRootImpl建立联系
root.setView(view, wparams, panelParentView);
}
}
所以到这里就可以确认到这个顶级View就是DecorView了。
针对measure的流程我们看看measureHierarchy()，它展示了DecorView的MeasureSpec创建过程：
// desiredWindowWidth和desiredWindowHeight是当前屏幕显示窗口的大小
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//根据LayoutParams的width和height的值计算MeasureSpec
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
这样就得到DecorView的MeasureSpec。
2.普通View的MeasureSpec
继续分析DecorView中的子View的MeasureSpec计算流程。
ViewRootImpl执行measure的时候如下：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
调用了View的View#measure，在View的measure方法中调用了onMeasure方法。
DecorView中也实现了onMeasure()，所以实际调用的就是DecorView#onMeasure。
DecorView#onMeasure中重新计算了widthMeasureSpec，heightMeasureSpec，然后调用父类的onMeasure()。DecorView的父类是FramLayout，所以我们就看看FramLayout中的对子View的MeasureSpec计算。
FramLayout#onMeasure
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
...
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//宽度和高度的MeasureSpec的计算流程基本一致
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
//如果child View的宽度是match_parent那么宽度就是父布局宽度减去父布局的pad，减去view的margin。
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
//spec父容器的MeasureSpec，padding是child相对父容器的间距，childDimension是
child的LayoutParam的width属性值。
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;
//根据父容器的MeasureSpec，计算子容器的MeasureSpec
switch (specMode) {
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;
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;
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);
}
上述方法getChildMeasureSpec(...)主要是根据父容器的MeasureSpec同时结合子View的LayoutParams，来确定子元素的MeasureSpec。
参考：《Android 开发艺术探索》
http://blog.csdn.net/a553181867/article/details/51494058
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