IWindowSession客户端是应用，服务端是WMS。
官网介绍其作用是是“System private per-application interface to the window manager.”，
客户端通过IWindowSession向WMS发送请求，IWindowSession接口代码如下
/*** System private per-application interface to the window manager.** {@hide}*/
interface IWindowSession {int addToDisplay(IWindow window, in WindowManager.LayoutParams attrs,in int viewVisibility, in int layerStackId, int requestedVisibleTypes,out InputChannel outInputChannel, out InsetsState insetsState,out InsetsSourceControl.Array activeControls, out Rect attachedFrame,out float[] sizeCompatScale);int addToDisplayAsUser(IWindow window, in WindowManager.LayoutParams attrs,in int viewVisibility, in int layerStackId, in int userId, int requestedVisibleTypes,out InputChannel outInputChannel, out InsetsState insetsState,out InsetsSourceControl.Array activeControls, out Rect attachedFrame,out float[] sizeCompatScale);int addToDisplayWithoutInputChannel(IWindow window, in WindowManager.LayoutParams attrs,in int viewVisibility, in int layerStackId, out InsetsState insetsState,out Rect attachedFrame, out float[] sizeCompatScale);@UnsupportedAppUsagevoid remove(IWindow window);
...
frameworks/base/core/java/android/view/IWindowSession.aidl
IWindowSession的方法可以参考链接如下：
IWindowSession.aidl - OpenGrok cross reference for /frameworks/base/core/java/android/view/IWindowSession.aidl
这里我们来看一下客户端如何获取WindowSession
WindowManagerGlobal.java - OpenGrok cross reference for /frameworks/base/core/java/android/view/WindowManagerGlobal.java
@UnsupportedAppUsagepublic static IWindowSession getWindowSession() {synchronized (WindowManagerGlobal.class) {if (sWindowSession == null) {try {
// Emulate the legacy behavior. The global instance of InputMethodManager
// was instantiated here.
// TODO(b/116157766): Remove this hack after cleaning up @UnsupportedAppUsageInputMethodManager.ensureDefaultInstanceForDefaultDisplayIfNecessary();
//第一步IWindowManager windowManager = getWindowManagerService();
//第二步sWindowSession = windowManager.openSession(new IWindowSessionCallback.Stub() {@Overridepublic void onAnimatorScaleChanged(float scale) {ValueAnimator.setDurationScale(scale);}});} catch (RemoteException e) {throw e.rethrowFromSystemServer();}}return sWindowSession;}}上述代码中，在第一步中，通过getWindowManagerService方法获取了server端binder对象，即获取了WMS服务, 看代码是单例模式，可以看出来这个sWindowSession是一个单例，也就是一个应用的所有ViewRootImpl的WindowSession都是同一个，也就是一个应用只有一个WindowSession,用于与WMS进行通信。
@UnsupportedAppUsagepublic static IWindowManager getWindowManagerService() {synchronized (WindowManagerGlobal.class) {if (sWindowManagerService == null) {
//获取server端binder对象，即获取WMS服务sWindowManagerService = IWindowManager.Stub.asInterface(ServiceManager.getService("window"));
...return sWindowManagerService;}}在第二步中，则调用到了Server端服务WMS中的方法，这里也是单例模式的写法，sWindowManagerService 在Client端也只有一个对象。
WindowManagerService.java - OpenGrok cross reference for /frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java
@Override
public IWindowSession openSession(IWindowSessionCallback callback) {return new Session(this, callback);}
然后在WMS中new了一个Session，Session实例化相关如下
Session.java - OpenGrok cross reference for /frameworks/base/services/core/java/com/android/server/wm/Session.java
class Session extends IWindowSession.Stub implements IBinder.DeathRecipient {final WindowManagerService mService;final IWindowSessionCallback mCallback;final int mUid;final int mPid;... public Session(WindowManagerService service, IWindowSessionCallback callback) {mService = service;mCallback = callback;mUid = Binder.getCallingUid();mPid = Binder.getCallingPid(); ...
来看看WMS，其实也是继承了 IWindowSession.Stub
/** {@hide} */
public class WindowManagerService extends IWindowManager.Stubimplements Watchdog.Monitor, WindowManagerPolicy.WindowManagerFuncs {
从上我们总结出：
一个应用只有一个WindowSession, 用于与WMS进行通信。
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