DO NOT MERGE: Move startInput for WINDOW_FOCUS_GAIN to background thread

For a typical window with Editor, IMM#startInputOrWindowGainedFocus
is called twice: first for WINDOW_FOCUS_GAIN (dummyInput), then
CHECK_FOCUS when actual editor receives focus and startsInput.
The first once can be moved to background thread and second startInput
will wait till its finished.

Bug: 139806621
Test: Manually launch activities with editors and confirm no change in
behavior.

Change-Id: I6aa4a664cfd0c86f75cee2457715317194bbe5e2
(cherry picked from commit e0172102b9)

core/java/android/view/inputmethod/InputMethodManager.java

@@ -92,7 +92,10 @@ import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -421,6 +424,13 @@ public final class InputMethodManager {
     int mCursorCandStart;
     int mCursorCandEnd;
 
+    /**
+     * Initial startInput with {@link StartInputReason.WINDOW_FOCUS_GAIN} is executed
+     * in a background thread. Later, if there is an actual startInput it will wait on
+     * main thread till the background thread completes.
+     */
+    private CompletableFuture<Void> mWindowFocusGainFuture;
+
     /**
      * The instance that has previously been sent to the input method.
      */
@@ -1598,6 +1608,18 @@ public final class InputMethodManager {
     boolean startInputInner(@StartInputReason int startInputReason,
             @Nullable IBinder windowGainingFocus, @StartInputFlags int startInputFlags,
             @SoftInputModeFlags int softInputMode, int windowFlags) {
+        if (startInputReason != StartInputReason.WINDOW_FOCUS_GAIN
+                && mWindowFocusGainFuture != null) {
+            try {
+                mWindowFocusGainFuture.get();
+            } catch (ExecutionException | InterruptedException e) {
+                // do nothing
+            } catch (CancellationException e) {
+                // window no longer has focus.
+                return true;
+            }
+        }
+
         final View view;
         synchronized (mH) {
             view = mServedView;
@@ -1951,13 +1973,19 @@ public final class InputMethodManager {
             startInputFlags |= StartInputFlags.FIRST_WINDOW_FOCUS_GAIN;
         }
 
-        if (checkFocusNoStartInput(forceNewFocus)) {
+        final boolean forceNewFocus1 = forceNewFocus;
+        final int startInputFlags1 = startInputFlags;
+        if (mWindowFocusGainFuture != null) {
+            mWindowFocusGainFuture.cancel(false/* mayInterruptIfRunning */);
+        }
+        mWindowFocusGainFuture = CompletableFuture.runAsync(() -> {
+            if (checkFocusNoStartInput(forceNewFocus1)) {
                 // We need to restart input on the current focus view.  This
                 // should be done in conjunction with telling the system service
                 // about the window gaining focus, to help make the transition
                 // smooth.
                 if (startInputInner(StartInputReason.WINDOW_FOCUS_GAIN, rootView.getWindowToken(),
-                    startInputFlags, softInputMode, windowFlags)) {
+                        startInputFlags1, softInputMode, windowFlags)) {
                     return;
                 }
             }
@@ -1969,13 +1997,14 @@ public final class InputMethodManager {
                     if (DEBUG) Log.v(TAG, "Reporting focus gain, without startInput");
                     mService.startInputOrWindowGainedFocus(
                             StartInputReason.WINDOW_FOCUS_GAIN_REPORT_ONLY, mClient,
-                        rootView.getWindowToken(), startInputFlags, softInputMode, windowFlags,
+                            rootView.getWindowToken(), startInputFlags1, softInputMode, windowFlags,
                             null, null, 0 /* missingMethodFlags */,
                             rootView.getContext().getApplicationInfo().targetSdkVersion);
                 } catch (RemoteException e) {
                     throw e.rethrowFromSystemServer();
                 }
             }
+        });
     }
 
     /** @hide */
@@ -1990,6 +2019,10 @@ public final class InputMethodManager {
                 // If the mCurRootView is losing window focus, release the strong reference to it
                 // so as not to prevent it from being garbage-collected.
                 mCurRootView = null;
+                if (mWindowFocusGainFuture != null) {
+                    mWindowFocusGainFuture.cancel(false /* mayInterruptIfRunning */);
+                    mWindowFocusGainFuture = null;
+                }
             } else {
                 if (DEBUG) {
                     Log.v(TAG, "Ignoring onPreWindowFocus()."