Listen to display focus state in Launcher

As part of the Connected Display project, some UI features in
Launcher such as Alt-Tab need to be shown only on the focused
display.

This change adds FocusState, which registers to WMShell, and
provides a listener to other components in Launcher so they can
easily consume the display focus state (following the same pattern
of HomeVisibilityState).

Bug: 356109871
Flag: com.android.window.flags.enable_display_focus_in_shell_transitions
Test: Switching focus between displays and confirm signals are sent to Launcher.
Change-Id: I51d51ac0ce3e24afa363607cdfb41728748a4a5f

quickstep/src/com/android/quickstep/FocusState.kt

@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.quickstep
+
+import android.os.RemoteException
+import android.util.Log
+import android.view.Display.DEFAULT_DISPLAY
+import com.android.launcher3.util.Executors
+import com.android.wm.shell.shared.IFocusTransitionListener.Stub
+import com.android.wm.shell.shared.IShellTransitions
+
+/** Class to track focus state of displays and windows */
+class FocusState {
+
+    var focusedDisplayId = DEFAULT_DISPLAY
+        private set
+
+    private var listeners = mutableSetOf<FocusChangeListener>()
+
+    fun addListener(l: FocusChangeListener) = listeners.add(l)
+
+    fun removeListener(l: FocusChangeListener) = listeners.remove(l)
+
+    fun init(transitions: IShellTransitions?) {
+        try {
+            transitions?.setFocusTransitionListener(
+                object : Stub() {
+                    override fun onFocusedDisplayChanged(displayId: Int) {
+                        Executors.MAIN_EXECUTOR.execute {
+                            listeners.forEach { it.onFocusedDisplayChanged(displayId) }
+                        }
+                    }
+                }
+            )
+        } catch (e: RemoteException) {
+            Log.w(TAG, "Failed call setFocusTransitionListener", e)
+        }
+    }
+
+    interface FocusChangeListener {
+        fun onFocusedDisplayChanged(displayId: Int)
+    }
+
+    override fun toString() = "{FocusState focusedDisplayId=$focusedDisplayId}"
+
+    companion object {
+        private const val TAG = "FocusState"
+    }
+}

quickstep/src/com/android/quickstep/SystemUiProxy.java

@@ -161,6 +161,7 @@ public class SystemUiProxy implements ISystemUiProxy, NavHandle, SafeCloseable {
     private IRemoteAnimationRunner mBackToLauncherRunner;
     private IDragAndDrop mDragAndDrop;
     private final HomeVisibilityState mHomeVisibilityState = new HomeVisibilityState();
+    private final FocusState mFocusState = new FocusState();
 
     // Used to dedupe calls to SystemUI
     private int mLastShelfHeight;
@@ -300,6 +301,7 @@ public class SystemUiProxy implements ISystemUiProxy, NavHandle, SafeCloseable {
         registerSplitScreenListener(mSplitScreenListener);
         registerSplitSelectListener(mSplitSelectListener);
         mHomeVisibilityState.init(mShellTransitions);
+        mFocusState.init(mShellTransitions);
         setStartingWindowListener(mStartingWindowListener);
         setLauncherUnlockAnimationController(
                 mLauncherActivityClass, mLauncherUnlockAnimationController);
@@ -1144,6 +1146,10 @@ public class SystemUiProxy implements ISystemUiProxy, NavHandle, SafeCloseable {
         return mHomeVisibilityState;
     }
 
+    public FocusState getFocusState() {
+        return mFocusState;
+    }
+
     /**
      * Returns a surface which can be used to attach overlays to home task or null if
      * the task doesn't exist or sysui is not connected
@@ -1592,6 +1598,7 @@ public class SystemUiProxy implements ISystemUiProxy, NavHandle, SafeCloseable {
         pw.println("\tmOneHanded=" + mOneHanded);
         pw.println("\tmShellTransitions=" + mShellTransitions);
         pw.println("\tmHomeVisibilityState=" + mHomeVisibilityState);
+        pw.println("\tmFocusState=" + mFocusState);
         pw.println("\tmStartingWindow=" + mStartingWindow);
         pw.println("\tmStartingWindowListener=" + mStartingWindowListener);
         pw.println("\tmSysuiUnlockAnimationController=" + mSysuiUnlockAnimationController);