一、Window、DecorView、ViewRootImp的关系
1、Window和DecorView的创建
activity.setContentView()说起，查看这个方法的源码：
private Window mWindow;
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
public Window getWindow() {
return mWindow;
}Activity的setContentView()实际上调用的是Window的setContentView（）
mWindow什么时候被创建的呢？
Activity.java:
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window, ActivityConfigCallback activityConfigCallback) {
attachBaseContext(context);
mFragments.attachHost(null /*parent*/);
//创建PhoneWindow对象
mWindow = new PhoneWindow(this, window, activityConfigCallback);
mWindow.setWindowControllerCallback(this);
mWindow.setCallback(this);
mWindow.setOnWindowDismissedCallback(this);
mWindow.getLayoutInflater().setPrivateFactory(this);
///...
//通过context.getSystemService(Context.WINDOW_SERVICE)获取WindowManager的实例
//给window设置windowManager
mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
if (mParent != null) {
mWindow.setContainer(mParent.getWindow());
}
mWindowManager = mWindow.getWindowManager();
mCurrentConfig = config;
mWindow.setColorMode(info.colorMode);
setAutofillCompatibilityEnabled(application.isAutofillCompatibilityEnabled());
enableAutofillCompatibilityIfNeeded();
}mWindow是在Activity的attach()中被创建的，它实际是一个PhoneWindow对象，PhoneWindow是Android提供的Window唯一实现类。系统为每个Activity创建一个与之对应的Window。
创建Window后，通过Context.getSystemService(Context.WINDOW_SERVICE)的方式获取WindowManager的实例，并设置给Window。WindowManager是一个窗口管理类。
1、一个Activity对应一个Window，Window是在activity.attach()的时候创建的
2、在Activity中调用setContent()实际上就是调用了Window的setContentView()
3、调用setContentView()时，会去初始化DecorView以及内部的TitleView和mContentParent,可以通过设置Window.FEATURE_NO_TITLE使得DecorView内部只存在mContentParent
4、在xml中定义的layout_activity.xml实际上是mContentParent的一个子View
2、通过WindowManager管理Window
每个Activity都有一个与之关联的Window，那如何管理Window呢？需要借助WindowMananger类，WindowManager管理Window实际上就是在管理Window中的DecorView。
下面来看一下DecorView和WindowManager是如何关联起来的：
从Activity的启动的最后一步–activityThread.handleLaunchActivity()，即ActivityThread开始执行到DecorView被添加到WindowManager中的过程如下：
activityThread.handleLaunchActivity() —>activityThread.performLaunchActivity()—>activity.attach() —> activity.onCreate()—>
activityThread.handleResumeActivity() —>activityThread.performResumeActivity() —>activity.onResume() —> windowManager.addView()
具体如下：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
....
// 创建Activity，会调用Activity的onCreate方法
// 从而完成DecorView的创建
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
handleResumeActivity(r.tolen, false, r.isForward, !r.activity..mFinished && !r.startsNotResumed);
}
}
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume) {
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// 调用Activity的onResume方法
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
...
if (r.window == null &&& !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
// 得到DecorView
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
// 得到了WindowManager，WindowManager是一个接口
// 并且继承了接口ViewManager
ViewManager wm = a.getWindowManager();
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;
// WindowManager的实现类是WindowManagerImpl，
// 所以实际调用的是WindowManagerImpl的addView方法
wm.addView(decor, l);
}
}
}
}
public final class WindowManagerImpl implements WindowManager {
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
...
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);
}
...
}其中handleResumeActivity详细如下：
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
ActivityClientRecord r = mActivities.get(token);
if (!checkAndUpdateLifecycleSeq(seq, r, "resumeActivity")) {
return;
}
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// 这里会调用到activity.onResume()方法
r = performResumeActivity(token, clearHide, reason);
if (r != null) {
final Activity a = r.activity;
if (localLOGV) Slog.v(
TAG, "Resume " + r + " started activity: " +
a.mStartedActivity + ", hideForNow: " + r.hideForNow
+ ", finished: " + a.mFinished);
final int forwardBit = isForward ?
WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
// If the window hasn't yet been added to the window manager,
// and this guy didn't finish itself or start another activity,
// then go ahead and add the window.
boolean willBeVisible = !a.mStartedActivity;
if (!willBeVisible) {
try {
willBeVisible = ActivityManager.getService().willActivityBeVisible(
a.getActivityToken());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
if (r.window == null && !a.mFinished && willBeVisible) {
// 获得Window对象
r.window = r.activity.getWindow();
// 获得Window中的DecorView对象
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
// 获得WindowManager对象
//这个WindowManager就是在activity.attach()的时候和window一起创建的
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
// 调用windowManager.addView方法
wm.addView(decor, l);
} else {
// The activity will get a callback for this {@link LayoutParams} change
// earlier. However, at that time the decor will not be set (this is set
// in this method), so no action will be taken. This call ensures the
// callback occurs with the decor set.
a.onWindowAttributesChanged(l);
}
}
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// Get rid of anything left hanging around.
cleanUpPendingRemoveWindows(r, false /* force */);
// The window is now visible if it has been added, we are not
// simply finishing, and we are not starting another activity.
if (!r.activity.mFinished && willBeVisible
&& r.activity.mDecor != null && !r.hideForNow) {
if (r.newConfig != null) {
performConfigurationChangedForActivity(r, r.newConfig);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "
+ r.activityInfo.name + " with newConfig " + r.activity.mCurrentConfig);
r.newConfig = null;
}
if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="
+ isForward);
WindowManager.LayoutParams l = r.window.getAttributes();
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
l.softInputMode = (l.softInputMode
& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
| forwardBit;
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
r.activity.mVisibleFromServer = true;
mNumVisibleActivities++;
if (r.activity.mVisibleFromClient) {
//设置decorView可见
r.activity.makeVisible();
}
}
///...
}通过调用windowManager.addView(decor, lp)，这样一来，DecorView和WindowManager就建立了联系。WindowManager继承了ViewManger接口，但实际上其本身依然是一个接口，实现类是WindowManagerImpl，WindowManagerImpl并没有直接实现操作View的相关方法，而是全部交给WindowManagerGlobal。WindowManagerGlobal是一个单例类—即一个进程中最多仅有一个。创建WindowManagerGlobal对象的方式如下：
WindowManagerGlobal.java:
public static WindowManagerGlobal getInstance() {
synchronized (WindowManagerGlobal.class) {
if (sDefaultWindowManager == null) {
sDefaultWindowManager = new WindowManagerGlobal();
}
return sDefaultWindowManager;
}
}
ViewRootImpl实现了ViewParent接口，它是WindowManagerGlobal工作的实际实现者。绘制会从 ViewRootImp 的 performTraversals() 方法开始，从上到下遍历整个视图树进行View的绘制。ViewRoot对应于ViewRootImpl类，它是连接WindowManager和DecorView的纽带，View的三大流程均是通过ViewRoot来完成的。在ActivityThread中，当Activity对象被创建完毕后，会将DecorView添加到Window中，同时会创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView建立关联，相关源码如下所示：
// WindowManagerGlobal的addView方法
public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) {
...
ViewRootImpl root;
View pannelParentView = null;
synchronized (mLock) {
...
// 创建ViewRootImpl实例
root = new ViewRootImpl(view..getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
try {
// 把DecorView加载到Window中
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}二、Measure、Layout、Draw
绘制会从根视图ViewRoot的performTraversals()方法开始，从上到下遍历整个视图树，每个View控件负责绘制自己，而ViewGroup还需要负责通知自己的子View进行绘制操作。performTraversals()的源码非常的长，但是核心代码就是下面三个步骤。
private void performTraversals() {
///...
int childWidthMeasureSpec =getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec =getRootMeasureSpec(mHeight, lp.height);
...
//这里的mView就是DecorView
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
mView.layout(0, 0,mView.getMeasuredWidth(), mView.getMeasuredHeight());
...
mView.draw(canvas);
}measure：测量。系统会先根据xml布局文件和代码中对控件属性的设置，来获取或者计算出每个View和ViewGrop的尺寸，并将这些尺寸保存下来。
layout：布局。根据测量出的结果以及对应的参数，来确定每一个控件应该显示的位置。
draw：绘制。确定好位置后，就将这些控件绘制到屏幕上。
performTraversals的大致工作流程图如下所示：
1、preformLayout和performDraw的传递流程和performMeasure是类似的，唯一不同的是，performDraw的传递过程是在draw方法中通过dispatchDraw来实现的，不过这并没有本质区别。
2、获取content：
ViewGroup content = (ViewGroup)findViewById(android.R.id.content);
3、获取设置的View：
content.getChildAt(0);
三、理解MeasureSpec
MeasureSpec表示的是一个32位的整形值，它的高2位表示测量模式SpecMode，低30位表示某种测量模式下的规格大小SpecSize。MeasureSpec是View类的一个静态内部类，用来说明应该如何测量这个View。MeasureSpec的核心代码如下。
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0X3 << MODE_SHIFT;
// 不指定测量模式, 父视图没有限制子视图的大小，子视图可以是想要
// 的任何尺寸，通常用于系统内部，应用开发中很少用到。
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
// 精确测量模式，视图宽高指定为match_parent或具体数值时生效，
// 表示父视图已经决定了子视图的精确大小，这种模式下View的测量
// 值就是SpecSize的值。
public static final int EXACTLY = 1 << MODE_SHIFT;
// 最大值测量模式，当视图的宽高指定为wrap_content时生效，此时
// 子视图的尺寸可以是不超过父视图允许的最大尺寸的任何尺寸。
public static final int AT_MOST = 2 << MODE_SHIFT;
// 根据指定的大小和模式创建一个MeasureSpec
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
// 微调某个MeasureSpec的大小
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return make MeasureSpec(0, UNSPECIFIED);
}
int size = getSize(measureSpec) + delta;
if (size < 0) {
size = 0;
}
return makeMeasureSpec(size, mode);
}
}MeasureSpec通过将SpecMode和SpecSize打包成一个int值来避免过多的对象内存分配，为了方便操作，其提供了打包和解包的方法，打包方法为上述源码中的makeMeasureSpec，解包方法源码如下：
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}2.DecorView的MeasureSpec的创建过程：
//desiredWindowWidth和desiredWindowHeight是屏幕的尺寸
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
private static int getRootMeaureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATRCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT：
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}3.子元素的MeasureSpec的创建过程
// ViewGroup的measureChildWithMargins方法
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
// 子元素的MeasureSpec的创建与父容器的MeasureSpec和子元素本身
// 的LayoutParams有关，此外还和View的margin及padding有关
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);
}
public static int getChildMeasureSpec(int spec, int padding, int childDimesion) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
// padding是指父容器中已占用的空间大小，因此子元素可用的
// 大小为父容器的尺寸减去padding
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (sepcMode) {
// 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 (childDimesion == 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 = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size....
// find out how big it should be
resultSize = 0;
resultMode == MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}普通View的MeasureSpec的创建规则如下：
注意：UNSPECIFIED模式主要用于系统内部多次Measure的情形，一般不需关注。
结论：对于DecorView而言，它的MeasureSpec由窗口尺寸和其自身的LayoutParams共同决定；对于普通的View，它的MeasureSpec由父视图的MeasureSpec和其自身的LayoutParams共同决定。
四、View绘制流程之Measure
由前面的分析可知，页面的测量流程是从performMeasure方法开始的，相关的核心代码流程如下。
private void perormMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
...
// 具体的测量操作分发给ViewGroup
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
// 在ViewGroup中的measureChildren()方法中遍历测量ViewGroup中所有的View
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];
// 当View的可见性处于GONE状态时，不对其进行测量
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
// 测量某个指定的View
protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
// 根据父容器的MeasureSpec和子View的LayoutParams等信息计算
// 子View的MeasureSpec
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
// View的measure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
// ViewGroup没有定义测量的具体过程，因为ViewGroup是一个
// 抽象类，其测量过程的onMeasure方法需要各个子类去实现
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
// 不同的ViewGroup子类有不同的布局特性，这导致它们的测量细节各不相同，如果需要自定义测量过程，则子类可以重写这个方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// setMeasureDimension方法用于设置View的测量宽高
setMeasureDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
// 如果View没有重写onMeasure方法，则会默认调用getDefaultSize来获得View的宽高
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 = sepcSize;
break;
}
return result;
}2.对getSuggestMinimumWidth的分析
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinmumWidth());
}
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}如果View没有设置背景，那么返回android:minWidth这个属性所指定的值，这个值可以为0；如果View设置了背景，则返回android:minWidth和背景的最小宽度这两者中的最大值。
3.自定义View时手动处理wrap_content时的情形
直接继承View的控件需要重写onMeasure方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。解决方式如下：
protected void onMeasure(int widthMeasureSpec,
int height MeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int widtuhSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
// 在wrap_content的情况下指定内部宽/高(mWidth和mHeight)
int heightSpecSize = MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, mHeight);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
setMeasureDimension(mWidth, heightSpecSize);
} else if (heightSpecMode == MeasureSpec.AT_MOST) {
setMeasureDimension(widthSpecSize, mHeight);
}
}4.LinearLayout的onMeasure方法实现解析
protected void onMeasure(int widthMeasureSpec, int hegithMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
// measureVertical核心源码
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
...
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if need)
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
}系统会遍历子元素并对每个子元素执行measureChildBeforeLayout方法，这个方法内部会调用子元素的measure方法，这样各个子元素就开始依次进入measure过程，并且系统会通过mTotalLength这个变量来存储LinearLayout在竖直方向的初步高度。每测量一个子元素，mTotalLength就会增加，增加的部分主要包括了子元素的高度以及子元素在竖直方向上的margin等。
// LinearLayout测量自己大小的核心源码
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK；
...
setMeasuredDimension(resolveSizeAndSize(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
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:
// 高度不能超过父容器的剩余空间
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL；
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}5.在Activity中获取某个View的宽高
由于View的measure过程和Activity的生命周期方法不是同步执行的，如果View还没有测量完毕，那么获得的宽/高就是0。所以在onCreate、onStart、onResume中均无法正确得到某个View的宽高信息。解决方式如下：
1、Activity/View#onWindowFocusChanged
// 此时View已经初始化完毕
// 当Activity的窗口得到焦点和失去焦点时均会被调用一次
// 如果频繁地进行onResume和onPause，那么onWindowFocusChanged也会被频繁地调用
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if (hasFocus) {
int width = view.getMeasureWidth();
int height = view.getMeasuredHeight();
}
}2、view.post(runnable)
// 通过post可以将一个runnable投递到消息队列的尾部，// 然后等待Looper调用次runnable的时候，View也已经初
// 始化好了
protected void onStart() {
super.onStart();
view.post(new Runnable() {
@Override
public void run() {
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
}3、ViewTreeObserver
// 当View树的状态发生改变或者View树内部的View的可见// 性发生改变时，onGlobalLayout方法将被回调
protected void onStart() {
super.onStart();
ViewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
@SuppressWarnings("deprecation")
@Override
public void onGlobalLayout() {
view.getViewTreeObserver().removeGlobalOnLayoutListener(this);
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
}五、View的绘制流程之Layout
// ViewRootImpl.java
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) {
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
// View.java
public void layout(int l, int t, int r, int b) {
...
// 通过setFrame方法来设定View的四个顶点的位置，即View在父容器中的位置
boolean changed = isLayoutModeOptical(mParent) ?
set OpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
...
onLayout(changed, l, t, r, b);
...
}
// 空方法，子类如果是ViewGroup类型，则重写这个方法，实现ViewGroup
// 中所有View控件布局流程
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}2.LinearLayout的onLayout方法实现解析
protected void onlayout(boolean changed, int l, int t, int r, int b) {
if (mOrientation == VERTICAL) {
layoutVertical(l, t, r, b);
} else {
layoutHorizontal(l,)
}
}
// layoutVertical核心源码
void layoutVertical(int left, int top, int right, int bottom) {
...
final int count = getVirtualChildCount();
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasureWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
...
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
// 为子元素确定对应的位置
setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight);
// childTop会逐渐增大，意味着后面的子元素会被
// 放置在靠下的位置
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child,i)
}
}
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}注意：在View的默认实现中，View的测量宽/高和最终宽/高是相等的，只不过测量宽/高形成于View的measure过程，而最终宽/高形成于View的layout过程，即两者的赋值时机不同，测量宽/高的赋值时机稍微早一些。在一些特殊的情况下则两者不相等：
1、重写View的layout方法,使最终宽度总是比测量宽/高大100px
public void layout(int l, int t, int r, int b) {
super.layout(l, t, r + 100, b + 100);
}2、View需要多长measure才能确定自己的测量宽/高,在前几次测量的过程中，其得出的测量宽/高有可能和最终宽/高不一致，但最终来说，测量宽/高还是和最终宽/高相同
六、View的绘制流程之Draw
1.Draw的基本流程
private void performDraw() {
...
draw(fullRefrawNeeded);
...
}
private void draw(boolean fullRedrawNeeded) {
...
if (!drawSoftware(surface, mAttachInfo, xOffest, yOffset,
scalingRequired, dirty)) {
return;
}
...
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo,
int xoff, int yoff, boolean scallingRequired, Rect dirty) {
...
mView.draw(canvas);
...
}
// 绘制基本上可以分为六个步骤
public void draw(Canvas canvas) {
...
// 步骤一：绘制View的背景
drawBackground(canvas);
...
// 步骤二：如果需要的话，保持canvas的图层，为fading做准备
saveCount = canvas.getSaveCount();
...
canvas.saveLayer(left, top, right, top + length, null, flags);
...
// 步骤三：绘制View的内容
onDraw(canvas);
...
// 步骤四：绘制View的子View
dispatchDraw(canvas);
...
// 步骤五：如果需要的话，绘制View的fading边缘并恢复图层
canvas.drawRect(left, top, right, top + length, p);
...
canvas.restoreToCount(saveCount);
...
// 步骤六：绘制View的装饰(例如滚动条等等)
onDrawForeground(canvas)
}2.setWillNotDraw的作用
// 如果一个View不需要绘制任何内容，那么设置这个标记位为true以后，
// 系统会进行相应的优化。
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}1、默认情况下，View没有启用这个优化标记位，但是ViewGroup会默认启用这个优化标记位。
2、当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。
3、当明确知道一个ViewGroup需要通过onDraw来绘制内容时，我们需要显示地关闭WILL_NOT_DRAW这个标记位。
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