一、简介
WMS全称WindowManagerService，它作为一个系统服务负责管理着window，对于应用层开发来讲，平时更多的接触是Activity、Dialog等组件，其实它们都对应着相关的window对象，可以说Activity和Dialog最终能显示到屏幕上都是由于window的存在。关于window的定义文章就不具体描述了，想要从头到尾分析的比较彻底还是比较复杂的，文章将从WMS的启动和WMS的应用这两方面来分析WMS在Android体系中所发挥的作用。
二、WMS的启动
WMS与AMS一样也是在SystemServer进程中启动的，核心流程的代码都在SystemServer的startOtherServices方法中。
SystemServer#startOtherServices
private void startOtherServices() {
WindowManagerService wm = null;
//创建WMS对象
wm = WindowManagerService.main(context, inputManager,
mFactoryTestMode != FactoryTest.FACTORY_TEST_LOW_LEVEL,
!mFirstBoot, mOnlyCore, new PhoneWindowManager());
//将WMS注册到ServiceManager中，key为“window”
ServiceManager.addService(Context.WINDOW_SERVICE, wm, /* allowIsolated= */ false,
DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PROTO);
//WMS传给AMS
mActivityManagerService.setWindowManager(wm);
//WMS初始化完成
wm.onInitReady();
//WMS的Monitor传给InputManagerService
inputManager.setWindowManagerCallbacks(wm.getInputMonitor());
wm.displayReady();
从以上代码可以发现，在SystemServer中首先调用WindowManagerService.main()方法创建WindowManagerService对象，然后将WindowManagerService注册到ServiceManager中，随后为 ActivityManagerService设置WindowManagerService引用，调用onInitReady初始化完成方法，再将WindowManagerService的InputMonitor设置给InputManagerService，最后调用displayReady方法完成WindowManagerService的核心流程。
WindowManagerService#main
public static WindowManagerService main(final Context context, final InputManagerService im,
final boolean haveInputMethods, final boolean showBootMsgs, final boolean onlyCore,
WindowManagerPolicy policy) {
DisplayThread.getHandler().runWithScissors(() ->
//在DisplayThread中创建WindowManagerService对象
sInstance = new WindowManagerService(context, im, haveInputMethods, showBootMsgs,
onlyCore, policy), 0);
return sInstance;
}
WindowManagerService构造方法
private WindowManagerService(Context context, InputManagerService inputManager,
boolean haveInputMethods, boolean showBootMsgs, boolean onlyCore,
WindowManagerPolicy policy) {
installLock(this, INDEX_WINDOW);
...省略一些resource获取
//设置InputManagerService
mInputManager = inputManager;
//获取DisplayManager
mDisplayManagerInternal = LocalServices.getService(DisplayManagerInternal.class);
//初始化Display相关设置
mDisplaySettings = new DisplaySettings();
mDisplaySettings.readSettingsLocked();
//WindowManagerPolicy
mPolicy = policy;
//管理Window对象的动画
mAnimator = new WindowAnimator(this);
....
//WindowManagerPolicy注册到LocalServices
LocalServices.addService(WindowManagerPolicy.class, mPolicy);
if(mInputManager != null) {
//获取InputManagerService的InputChannel用于分发点击事件
final InputChannel inputChannel = mInputManager.monitorInput(TAG_WM);
mPointerEventDispatcher = inputChannel != null
? new PointerEventDispatcher(inputChannel) : null;
} else {
mPointerEventDispatcher = null;
}
//获取DisplayManager
mDisplayManager = (DisplayManager)context.getSystemService(Context.DISPLAY_SERVICE);
//获取PowerManager
mPowerManager = (PowerManager)context.getSystemService(Context.POWER_SERVICE);
mPowerManagerInternal = LocalServices.getService(PowerManagerInternal.class);
....
//获取ActivityManagerService
mActivityManager = ActivityManager.getService();
...
}
WindowManagerService的构造方法中主要涉及Display相关的设置、Window动画对象的设置、InputChannel的获取和一些其他服务的获取和注册。
三、WMS的应用
分析WindowManagerService对window的管理就需要来到多次提到的ViewRootImpl这个类中了。
ViewRootImpl构造方法
public ViewRootImpl(Context context, Display display) {
....
//获取IWindowSession
mWindowSession = WindowManagerGlobal.getWindowSession();
....
//创建W对象
mWindow = new W(this);
....
}
ViewRootImpl的构造方法和window相关的主要流程是获取IWindowSession和创建W对象，这两个对象都是用于ViewRootImpl和WMS通信使用的。
WindowManagerGlobal#getWindowSession
public static IWindowSession getWindowSession() {
synchronized (WindowManagerGlobal.class) {
if (sWindowSession == null) {
try {
//获取InputMethodManager
InputMethodManager imm = InputMethodManager.getInstance();
//获取WindowManagerService
IWindowManager windowManager = getWindowManagerService();
//调用openSession方法获取WindowSession
sWindowSession = windowManager.openSession(
new IWindowSessionCallback.Stub() {
@Override
public void onAnimatorScaleChanged(float scale) {
ValueAnimator.setDurationScale(scale);
}
},
imm.getClient(), imm.getInputContext());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
return sWindowSession;
}
}
WindowManagerService#openSession
public IWindowSession openSession(IWindowSessionCallback callback, IInputMethodClient client,
IInputContext inputContext) {
//创建Session
Session session = new Session(this, callback, client, inputContext);
return session;
}
class Session extends IWindowSession.Stub implements IBinder.DeathRecipient
从以上代码可以看出session对象是通过WindowManagerGlobal的getWindowSession方法，经过WindowManagerService的openSession方法创建的。Session这个类实现了IWindowSession接口，IWindowSession是一个AIDL接口，可以用于进程间通信。
static class W extends IWindow.Stub {
private final WeakReference<ViewRootImpl> mViewAncestor;
private final IWindowSession mWindowSession;
W(ViewRootImpl viewAncestor) {
//软引用方式获取ViewRootImpl
mViewAncestor = new WeakReference<ViewRootImpl>(viewAncestor);
mWindowSession = viewAncestor.mWindowSession;
}
....省略一些方法，这些方法都是最终调用ViewRootImpl的方法
}
W这个类是ViewRootImpl的静态内部类，实现了IWindow接口，IWindow也是一个AIDL接口，可以用于进程间通信，WindowManagerService通过W对象将逻辑调用到ViewRootImpl中。
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
....
//调用addToDisplay方法，传入mWindow对象
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(), mWinFrame,
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel);
....
}
Session#addToDisplay
@Override
public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
int viewVisibility, int displayId, Rect outFrame, Rect outContentInsets,
Rect outStableInsets, Rect outOutsets,
DisplayCutout.ParcelableWrapper outDisplayCutout, InputChannel outInputChannel) {
//调用到了WindowManagerService的addWindow方法
return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId, outFrame,
outContentInsets, outStableInsets, outOutsets, outDisplayCutout, outInputChannel);
}
在ViewRootImpl的setView方法中mWindowSession调用addToDisplay方法最后就调用到了WindowManagerService的addWindow方法。
注：以上分析基于API Level 28
