//SystemServer.java
public static void main(String[] args) {
  new SystemServer().run();
}

private void run() {
  ...
  try {
    startBootstrapServices();
    startCoreServices();
    startOtherServices();
  } catch (Throwable ex) {
    throw ex;
  }
  ...
}

private void startOtherServices() {
  ...
  InputMethodManagerService imm = null;

  imm = new InputMethodManagerService(context, wm);
  ServiceManager.addService(Context.INPUT_METHOD_SERVICE, imm);
  ...
  final InputMethodManagerService immF = imm;

  if (immF != null) immF.systemRunning(statusBarF);
  ...
}

InputMethodManager imm = (InputMethodManager)Context.getSystemService(Context.INPUT_METHOD_SERVICE)

//ContextImpl.java
private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP
          = new HashMap<String, ServiceFetcher>();

@Override
public Object getSystemService(String name) {
    ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
    return fetcher == null ? null : fetcher.getService(this);
}

static {
  ...
  registerService(INPUT_METHOD_SERVICE, new StaticServiceFetcher() {
          public Object createStaticService() {
              return InputMethodManager.getInstance();
          }
  });
  ...
}  
private static void registerService(String serviceName, ServiceFetcher fetcher) {
      if (!(fetcher instanceof StaticServiceFetcher)) {
          fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
      }
      SYSTEM_SERVICE_MAP.put(serviceName, fetcher);
}

abstract static class StaticServiceFetcher extends ServiceFetcher {
    private Object mCachedInstance;

    @Override
    public final Object getService(ContextImpl unused) {
        synchronized (StaticServiceFetcher.this) {
            Object service = mCachedInstance;
            if (service != null) {
                return service;
            }
            return mCachedInstance = createStaticService();
        }
    }

    public abstract Object createStaticService();
}

registerService(INPUT_METHOD_SERVICE, new StaticServiceFetcher() {
        public Object createStaticService() {
          return InputMethodManager.getInstance();
       }
  });

//InputMethodManager
public static InputMethodManager getInstance() {
  synchronized (InputMethodManager.class) {
    if (sInstance == null) {
      // b实际上是一个 IMMS 对象
      IBinder b = ServiceManager.getService(Context.INPUT_METHOD_SERVICE);
      //service 就是一个 IMMS 的代理
      IInputMethodManager service = IInputMethodManager.Stub.asInterface(b);
      sInstance = new InputMethodManager(service, Looper.getMainLooper());
    }
    return sInstance;
  }
}

InputMethodManager(IInputMethodManager service, Looper looper) {
     mService = service;
     mMainLooper = looper;
     mH = new H(looper);
     mIInputContext = new ControlledInputConnectionWrapper(looper, mDummyInputConnection , this);
}

private void startOtherServices() {
    ...
    InputMethodManagerService imm = null;

    imm = new InputMethodManagerService(context, wm);
    ServiceManager.addService(Context.INPUT_METHOD_SERVICE, imm);
    ...
    final InputMethodManagerService immF = imm;
    ...
    if (immF != null) immF.systemRunning(statusBarF);
    ...
}

//IMMS
public void systemRunning(StatusBarManagerService statusBar) {
  synchronized (mMethodMap) {
    if (!mSystemReady) {//系统启动，执行该方法， mSystemReady 默认为 false ,所以会执行，
      mSystemReady = true;//标志设置为 true ,所以该方法只执行一次
      ...
      try {
        startInputInnerLocked();
      } catch (RuntimeException e) {
        Slog.w(TAG, "Unexpected exception", e);
      }
    }
  }
}

//InputMethodManagerService.java
InputBindResult startInputInnerLocked() {
 ...
 InputMethodInfo info = mMethodMap.get(mCurMethodId);
 //启动输入法service
 mCurIntent = new Intent(InputMethod.SERVICE_INTERFACE);
 mCurIntent.setComponent(info.getComponent());
 mCurIntent.putExtra(Intent.EXTRA_CLIENT_LABEL, com.android.internal.R.string.input_method_binding_label);
 mCurIntent.putExtra(Intent.EXTRA_CLIENT_INTENT, PendingIntent.getActivity(mContext, 0 , new Intent(Settings.ACTION_INPUT_METHOD_SETTINGS), 0));

 if (bindCurrentInputMethodService(mCurIntent, this , Context.BIND_AUTO_CREATE | Context.BIND_NOT_VISIBLE | Context.BIND_NOT_FOREGROUND | Context.BIND_SHOWING_UI)) {
    mLastBindTime = SystemClock.uptimeMillis();
    mHaveConnection = true;
    mCurId = info.getId();
  //这个 token 是给输入法 service 用来绑定输入法的 window 的，通过这个token
  //InputMethodManagerService可以很方便的直接管理输入法的window
   mCurToken = new Binder();
  try {
    mIWindowManager.addWindowToken(mCurToken, WindowManager.LayoutParams.TYPE_INPUT_METHOD);
  } catch (RemoteException e) {
  }
  return new InputBindResult(null, null , mCurId , mCurSeq , mCurUserActionNotificationSequenceNumber);
} else {
  mCurIntent = null;
}
return null;
}

public ComponentName getComponent() {
     return new ComponentName(mService.serviceInfo.packageName,mService.serviceInfo.name);
}

private boolean bindCurrentInputMethodService(Intent service, ServiceConnection conn , int flags) {
  if (service == null || conn == null) {
    return false;
  }
  return mContext.bindServiceAsUser(service, conn , flags , newUserHandle(mSettings.getCurrentUserId()));
}

//InputMethodManagerService.java
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
  synchronized (mMethodMap) {
    if (mCurIntent != null && name.equals(mCurIntent.getComponent())) {
      //保存输入法 service 传递过来的通信接口IInputMethod
      mCurMethod = IInputMethod.Stub.asInterface(service);
      if (mCurToken == null) {
        unbindCurrentMethodLocked(false, false);
        return;
      }
      //将刚刚创建的 window token 传递给输入法 service ,然后输入用这个 token 创建 window ,
      // 这样 IMMS 可以用根据这个 token 找到输入法在 IMMS 里的数据及输入法 window 在 WindowManagerService 里的数据
      executeOrSendMessage(mCurMethod, mCaller.obtainMessageOO(MSG_ATTACH_TOKEN, mCurMethod , mCurToken));
      if (mCurClient != null) {
        clearClientSessionLocked(mCurClient);
        //请求为程序和输入法建立一个连接会话，这样 client 就可以直接和输入法通信了
        requestClientSessionLocked(mCurClient);
      }
    }
  }
}

case MSG_ATTACH_TOKEN:
  args = (SomeArgs) msg.obj;
  try {
    //args.arg1其实就是 InputMethodImpl 对象，
    ((IInputMethod) args.arg1).attachToken((IBinder) args.arg2);
  } catch (RemoteException e) {
  }
  args.recycle();
  return true;

//IInputMethodWrapper.java
public void attachToken(IBinder token) {
  mCaller.executeOrSendMessage(mCaller.obtainMessageO(DO_ATTACH_TOKEN, token));
}
//处理 DO_ATTACH_TOKEN 这个消息。
@Override
public void executeMessage(Message msg) {
  InputMethod inputMethod = mInputMethod.get();
  ...
  switch (msg.what) {
    ...
    case DO_ATTACH_TOKEN: {
      inputMethod.attachToken((IBinder) msg.obj);
      return;
    }  
  }
}

//IInputMethodWrapper.java
public IInputMethodWrapper(AbstractInputMethodService context, InputMethod inputMethod) {
  ...
  mInputMethod = new WeakReference<InputMethod>(inputMethod);
  ...
}

// AbstractInputMethodService.java
@Override
final public IBinder onBind(Intent intent) {
  if (mInputMethod == null) {
    mInputMethod = onCreateInputMethodInterface();
  }
  return new IInputMethodWrapper(this, mInputMethod);
}
//IMS.java
@Override
public AbstractInputMethodImpl onCreateInputMethodInterface() {
  return new InputMethodImpl();
}

public void attachToken(IBinder token) {
  if (mToken == null) {
    //保存token
    mToken = token;
    //这样输入法的window就绑定这个window token
    mWindow.setToken(token);
  }
}

//InputMethodService.java
public void onCreate() {
    mWindow = new SoftInputWindow(this, "InputMethod", mTheme , null , null , mDispatcherState , WindowManager.LayoutParams.TYPE_INPUT_METHOD, Gravity.BOTTOM, false);
    ...
    initViews();
    mWindow.getWindow().setLayout(MATCH_PARENT, WRAP_CONTENT);
}
void initViews() {
  ...
  mRootView = mInflater.inflate(com.android.internal.R.layout.input_method, null);
  ...
  mWindow.setContentView(mRootView);
  ...
  mFullscreenArea = (ViewGroup) mRootView.findViewById(com.android.internal.R.id.fullscreenArea);
  ...
  mExtractFrame = (FrameLayout) mRootView.findViewById(android.R.id.extractArea);
  ...
  mCandidatesFrame = (FrameLayout) mRootView.findViewById(android.R.id.candidatesArea);
  mInputFrame = (FrameLayout) mRootView.findViewById(android.R.id.inputArea);
}