Merge "Add new PRIORITY_SYSTEM_NAVIGATION_OBSERVER to OnBackInvokedDispatcher" into main

7 files changed, 281 insertions, 44 deletions

diff --git a/core/api/current.txt b/core/api/current.txt
index b740ef36debe..53da33835d31 100644
--- a/core/api/current.txt
+++ b/core/api/current.txt
@@ -61658,6 +61658,7 @@ package android.window {
     method public void unregisterOnBackInvokedCallback(@NonNull android.window.OnBackInvokedCallback);
     field public static final int PRIORITY_DEFAULT = 0; // 0x0
     field public static final int PRIORITY_OVERLAY = 1000000; // 0xf4240
+    field @FlaggedApi("com.android.window.flags.predictive_back_priority_system_navigation_observer") public static final int PRIORITY_SYSTEM_NAVIGATION_OBSERVER = -2; // 0xfffffffe
   }
 
   public interface SplashScreen {
diff --git a/core/java/android/view/ViewRootImpl.java b/core/java/android/view/ViewRootImpl.java
index 8fb17c72c462..0ca442d66e6f 100644
--- a/core/java/android/view/ViewRootImpl.java
+++ b/core/java/android/view/ViewRootImpl.java
@@ -7531,6 +7531,7 @@ public final class ViewRootImpl implements ViewParent,
                         if (keyEvent.isCanceled()) {
                             animationCallback.onBackCancelled();
                         } else {
+                            dispatcher.tryInvokeSystemNavigationObserverCallback();
                             topCallback.onBackInvoked();
                         }
                         break;
@@ -7538,6 +7539,7 @@ public final class ViewRootImpl implements ViewParent,
             } else if (topCallback != null) {
                 if (keyEvent.getAction() == KeyEvent.ACTION_UP) {
                     if (!keyEvent.isCanceled()) {
+                        dispatcher.tryInvokeSystemNavigationObserverCallback();
                         topCallback.onBackInvoked();
                     } else {
                         Log.d(mTag, "Skip onBackInvoked(), reason: keyEvent.isCanceled=true");
diff --git a/core/java/android/window/OnBackInvokedDispatcher.java b/core/java/android/window/OnBackInvokedDispatcher.java
index 0632a37a5dfb..02ed57d4be22 100644
--- a/core/java/android/window/OnBackInvokedDispatcher.java
+++ b/core/java/android/window/OnBackInvokedDispatcher.java
@@ -16,11 +16,14 @@
 
 package android.window;
 
+import android.annotation.FlaggedApi;
 import android.annotation.IntDef;
 import android.annotation.IntRange;
 import android.annotation.NonNull;
 import android.annotation.SuppressLint;
 
+import com.android.window.flags.Flags;
+
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 
@@ -43,6 +46,7 @@ public interface OnBackInvokedDispatcher {
     @IntDef({
             PRIORITY_DEFAULT,
             PRIORITY_OVERLAY,
+            PRIORITY_SYSTEM_NAVIGATION_OBSERVER,
     })
     @Retention(RetentionPolicy.SOURCE)
     @interface Priority{}
@@ -67,6 +71,20 @@ public interface OnBackInvokedDispatcher {
     int PRIORITY_SYSTEM = -1;
 
     /**
+     * Priority level of {@link OnBackInvokedCallback}s designed to observe system-level back
+     * handling.
+     *
+     * <p>Callbacks registered with this priority do not consume back events. They receive back
+     * events whenever the system handles a back navigation and have no impact on the normal back
+     * navigation flow. Useful for logging or analytics.
+     *
+     * <p>Only one callback with {@link #PRIORITY_SYSTEM_NAVIGATION_OBSERVER} can be registered at a
+     * time.
+     */
+    @FlaggedApi(Flags.FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER)
+    int PRIORITY_SYSTEM_NAVIGATION_OBSERVER = -2;
+
+    /**
      * Registers a {@link OnBackInvokedCallback}.
      *
      * Within the same priority level, callbacks are invoked in the reverse order in which
diff --git a/core/java/android/window/ProxyOnBackInvokedDispatcher.java b/core/java/android/window/ProxyOnBackInvokedDispatcher.java
index 56c05b2766ec..dfc4a589f92e 100644
--- a/core/java/android/window/ProxyOnBackInvokedDispatcher.java
+++ b/core/java/android/window/ProxyOnBackInvokedDispatcher.java
@@ -16,6 +16,8 @@
 
 package android.window;
 
+import static com.android.window.flags.Flags.predictiveBackPrioritySystemNavigationObserver;
+
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.content.Context;
@@ -95,7 +97,7 @@ public class ProxyOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
         synchronized (mLock) {
             mCallbacks.add(Pair.create(callback, priority));
             if (mActualDispatcher != null) {
-                if (priority <= PRIORITY_SYSTEM) {
+                if (priority == PRIORITY_SYSTEM) {
                     mActualDispatcher.registerSystemOnBackInvokedCallback(callback);
                 } else {
                     mActualDispatcher.registerOnBackInvokedCallback(priority, callback);
@@ -123,10 +125,19 @@ public class ProxyOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
         }
         for (Pair<OnBackInvokedCallback, Integer> callbackPair : mCallbacks) {
             int priority = callbackPair.second;
-            if (priority >= PRIORITY_DEFAULT) {
-                mActualDispatcher.registerOnBackInvokedCallback(priority, callbackPair.first);
+            if (predictiveBackPrioritySystemNavigationObserver()) {
+                if (priority >= PRIORITY_DEFAULT
+                        || priority == PRIORITY_SYSTEM_NAVIGATION_OBSERVER) {
+                    mActualDispatcher.registerOnBackInvokedCallback(priority, callbackPair.first);
+                } else {
+                    mActualDispatcher.registerSystemOnBackInvokedCallback(callbackPair.first);
+                }
             } else {
-                mActualDispatcher.registerSystemOnBackInvokedCallback(callbackPair.first);
+                if (priority >= PRIORITY_DEFAULT) {
+                    mActualDispatcher.registerOnBackInvokedCallback(priority, callbackPair.first);
+                } else {
+                    mActualDispatcher.registerSystemOnBackInvokedCallback(callbackPair.first);
+                }
             }
         }
         mCallbacks.clear();
diff --git a/core/java/android/window/WindowOnBackInvokedDispatcher.java b/core/java/android/window/WindowOnBackInvokedDispatcher.java
index bb89a2499838..51bc7d5c571b 100644
--- a/core/java/android/window/WindowOnBackInvokedDispatcher.java
+++ b/core/java/android/window/WindowOnBackInvokedDispatcher.java
@@ -16,6 +16,8 @@
 
 package android.window;
 
+import static com.android.window.flags.Flags.predictiveBackPrioritySystemNavigationObserver;
+
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.app.Activity;
@@ -103,6 +105,9 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
     public final TreeMap<Integer, ArrayList<OnBackInvokedCallback>>
             mOnBackInvokedCallbacks = new TreeMap<>();
 
+    @VisibleForTesting
+    public OnBackInvokedCallback mSystemNavigationObserverCallback = null;
+
     private Checker mChecker;
     private final Object mLock = new Object();
     // The threshold for back swipe full progress.
@@ -170,6 +175,20 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
         }
     }
 
+    private void registerSystemNavigationObserverCallback(@NonNull OnBackInvokedCallback callback) {
+        synchronized (mLock) {
+            // If callback has already been added as regular callback, remove it.
+            if (mAllCallbacks.containsKey(callback)) {
+                if (DEBUG) {
+                    Log.i(TAG, "Callback already added. Removing and re-adding it as "
+                            + "system-navigation-observer-callback.");
+                }
+                removeCallbackInternal(callback);
+            }
+            mSystemNavigationObserverCallback = callback;
+        }
+    }
+
     /**
      * Register a callback bypassing platform checks. This is used to register compatibility
      * callbacks.
@@ -181,6 +200,12 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 mImeDispatcher.registerOnBackInvokedCallback(priority, callback);
                 return;
             }
+            if (predictiveBackPrioritySystemNavigationObserver()) {
+                if (priority == PRIORITY_SYSTEM_NAVIGATION_OBSERVER) {
+                    registerSystemNavigationObserverCallback(callback);
+                    return;
+                }
+            }
             if (callback instanceof ImeOnBackInvokedDispatcher.ImeOnBackInvokedCallback) {
                 if (callback instanceof ImeOnBackInvokedDispatcher.DefaultImeOnBackAnimationCallback
                         && mImeBackAnimationController != null) {
@@ -202,6 +227,13 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 Integer prevPriority = mAllCallbacks.get(callback);
                 mOnBackInvokedCallbacks.get(prevPriority).remove(callback);
             }
+            if (mSystemNavigationObserverCallback == callback) {
+                mSystemNavigationObserverCallback = null;
+                if (DEBUG) {
+                    Log.i(TAG, "Callback already registered (as system-navigation-observer "
+                            + "callback). Removing and re-adding it.");
+                }
+            }
 
             OnBackInvokedCallback previousTopCallback = getTopCallback();
             callbacks.add(callback);
@@ -221,6 +253,10 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 mImeDispatcher.unregisterOnBackInvokedCallback(callback);
                 return;
             }
+            if (mSystemNavigationObserverCallback == callback) {
+                mSystemNavigationObserverCallback = null;
+                return;
+            }
             if (callback instanceof ImeOnBackInvokedDispatcher.DefaultImeOnBackAnimationCallback) {
                 callback = mImeBackAnimationController;
             }
@@ -230,25 +266,29 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 }
                 return;
             }
-            OnBackInvokedCallback previousTopCallback = getTopCallback();
-            Integer priority = mAllCallbacks.get(callback);
-            ArrayList<OnBackInvokedCallback> callbacks = mOnBackInvokedCallbacks.get(priority);
-            callbacks.remove(callback);
-            if (callbacks.isEmpty()) {
-                mOnBackInvokedCallbacks.remove(priority);
-            }
-            mAllCallbacks.remove(callback);
-            // Re-populate the top callback to WM if the removed callback was previously the top
-            // one.
-            if (previousTopCallback == callback) {
-                // We should call onBackCancelled() when an active callback is removed from
-                // dispatcher.
-                mProgressAnimator.removeOnBackCancelledFinishCallback();
-                mProgressAnimator.removeOnBackInvokedFinishCallback();
-                sendCancelledIfInProgress(callback);
-                mHandler.post(mProgressAnimator::reset);
-                setTopOnBackInvokedCallback(getTopCallback());
-            }
+            removeCallbackInternal(callback);
+        }
+    }
+
+    private void removeCallbackInternal(@NonNull OnBackInvokedCallback callback) {
+        OnBackInvokedCallback previousTopCallback = getTopCallback();
+        Integer priority = mAllCallbacks.get(callback);
+        ArrayList<OnBackInvokedCallback> callbacks = mOnBackInvokedCallbacks.get(priority);
+        callbacks.remove(callback);
+        if (callbacks.isEmpty()) {
+            mOnBackInvokedCallbacks.remove(priority);
+        }
+        mAllCallbacks.remove(callback);
+        // Re-populate the top callback to WM if the removed callback was previously the top
+        // one.
+        if (previousTopCallback == callback) {
+            // We should call onBackCancelled() when an active callback is removed from
+            // dispatcher.
+            mProgressAnimator.removeOnBackCancelledFinishCallback();
+            mProgressAnimator.removeOnBackInvokedFinishCallback();
+            sendCancelledIfInProgress(callback);
+            mHandler.post(mProgressAnimator::reset);
+            setTopOnBackInvokedCallback(getTopCallback());
         }
     }
 
@@ -304,6 +344,7 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
             mHandler.post(mProgressAnimator::reset);
             mAllCallbacks.clear();
             mOnBackInvokedCallbacks.clear();
+            mSystemNavigationObserverCallback = null;
         }
     }
 
@@ -315,6 +356,25 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
         }
     }
 
+    /**
+     * Tries to call {@link OnBackInvokedCallback#onBackInvoked} on the system navigation observer
+     * callback (if one is set and if the top-most regular callback has
+     * {@link OnBackInvokedDispatcher#PRIORITY_SYSTEM})
+     */
+    public void tryInvokeSystemNavigationObserverCallback() {
+        OnBackInvokedCallback topCallback = getTopCallback();
+        Integer callbackPriority = mAllCallbacks.getOrDefault(topCallback, null);
+        if (callbackPriority != null && callbackPriority == PRIORITY_SYSTEM) {
+            invokeSystemNavigationObserverCallback();
+        }
+    }
+
+    private void invokeSystemNavigationObserverCallback() {
+        if (mSystemNavigationObserverCallback != null) {
+            mSystemNavigationObserverCallback.onBackInvoked();
+        }
+    }
+
     private void setTopOnBackInvokedCallback(@Nullable OnBackInvokedCallback callback) {
         if (mWindowSession == null || mWindow == null) {
             return;
@@ -324,7 +384,9 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
             if (callback != null) {
                 int priority = mAllCallbacks.get(callback);
                 final IOnBackInvokedCallback iCallback = new OnBackInvokedCallbackWrapper(callback,
-                        mTouchTracker, mProgressAnimator, mHandler, this::callOnKeyPreIme);
+                        mTouchTracker, mProgressAnimator, mHandler, this::callOnKeyPreIme,
+                        this::invokeSystemNavigationObserverCallback,
+                        /*isSystemCallback*/ priority == PRIORITY_SYSTEM);
                 callbackInfo = new OnBackInvokedCallbackInfo(
                         iCallback,
                         priority,
@@ -416,18 +478,26 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
         private final Handler mHandler;
         @NonNull
         private final BooleanSupplier mOnKeyPreIme;
+        @NonNull
+        private final Runnable mSystemNavigationObserverCallbackRunnable;
+        private final boolean mIsSystemCallback;
 
         OnBackInvokedCallbackWrapper(
                 @NonNull OnBackInvokedCallback callback,
                 @NonNull BackTouchTracker touchTracker,
                 @NonNull BackProgressAnimator progressAnimator,
                 @NonNull Handler handler,
-                @NonNull BooleanSupplier onKeyPreIme) {
+                @NonNull BooleanSupplier onKeyPreIme,
+                @NonNull Runnable systemNavigationObserverCallbackRunnable,
+                boolean isSystemCallback
+        ) {
             mCallback = new WeakReference<>(callback);
             mTouchTracker = touchTracker;
             mProgressAnimator = progressAnimator;
             mHandler = handler;
             mOnKeyPreIme = onKeyPreIme;
+            mSystemNavigationObserverCallbackRunnable = systemNavigationObserverCallbackRunnable;
+            mIsSystemCallback = isSystemCallback;
         }
 
         @Override
@@ -494,9 +564,17 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                 OnBackAnimationCallback animationCallback = getBackAnimationCallback();
                 if (animationCallback != null
                         && !(callback instanceof ImeBackAnimationController)) {
-                    mProgressAnimator.onBackInvoked(callback::onBackInvoked);
+                    mProgressAnimator.onBackInvoked(() -> {
+                        if (mIsSystemCallback) {
+                            mSystemNavigationObserverCallbackRunnable.run();
+                        }
+                        callback.onBackInvoked();
+                    });
                 } else {
                     mProgressAnimator.reset();
+                    if (mIsSystemCallback) {
+                        mSystemNavigationObserverCallbackRunnable.run();
+                    }
                     callback.onBackInvoked();
                 }
             });
@@ -597,9 +675,18 @@ public class WindowOnBackInvokedDispatcher implements OnBackInvokedDispatcher {
                                 + " application manifest.");
                 return false;
             }
-            if (priority < 0) {
-                throw new IllegalArgumentException("Application registered OnBackInvokedCallback "
-                        + "cannot have negative priority. Priority: " + priority);
+            if (predictiveBackPrioritySystemNavigationObserver()) {
+                if (priority < 0 && priority != PRIORITY_SYSTEM_NAVIGATION_OBSERVER) {
+                    throw new IllegalArgumentException("Application registered "
+                            + "OnBackInvokedCallback cannot have negative priority. Priority: "
+                            + priority);
+                }
+            } else {
+                if (priority < 0) {
+                    throw new IllegalArgumentException("Application registered "
+                            + "OnBackInvokedCallback cannot have negative priority. Priority: "
+                            + priority);
+                }
             }
             Objects.requireNonNull(callback);
             return true;
diff --git a/core/java/android/window/flags/windowing_frontend.aconfig b/core/java/android/window/flags/windowing_frontend.aconfig
index 086063f3887c..c9b93c95e0c1 100644
--- a/core/java/android/window/flags/windowing_frontend.aconfig
+++ b/core/java/android/window/flags/windowing_frontend.aconfig
@@ -307,3 +307,11 @@ flag {
   bug: "364930619"
   is_fixed_read_only: true
 }
+
+flag {
+    name: "predictive_back_priority_system_navigation_observer"
+    namespace: "systemui"
+    description: "PRIORITY_SYSTEM_NAVIGATION_OBSERVER predictive back API extension"
+    is_fixed_read_only: true
+    bug: "362938401"
+}
diff --git a/core/tests/coretests/src/android/window/WindowOnBackInvokedDispatcherTest.java b/core/tests/coretests/src/android/window/WindowOnBackInvokedDispatcherTest.java
index 0bda0fff6819..0a4c5e6eb91a 100644
--- a/core/tests/coretests/src/android/window/WindowOnBackInvokedDispatcherTest.java
+++ b/core/tests/coretests/src/android/window/WindowOnBackInvokedDispatcherTest.java
@@ -18,6 +18,9 @@ package android.window;
 
 import static android.window.OnBackInvokedDispatcher.PRIORITY_DEFAULT;
 import static android.window.OnBackInvokedDispatcher.PRIORITY_OVERLAY;
+import static android.window.OnBackInvokedDispatcher.PRIORITY_SYSTEM_NAVIGATION_OBSERVER;
+
+import static com.android.window.flags.Flags.FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
@@ -39,6 +42,10 @@ import android.os.Handler;
 import android.os.Looper;
 import android.os.RemoteException;
 import android.platform.test.annotations.Presubmit;
+import android.platform.test.annotations.RequiresFlagsDisabled;
+import android.platform.test.annotations.RequiresFlagsEnabled;
+import android.platform.test.flag.junit.CheckFlagsRule;
+import android.platform.test.flag.junit.DeviceFlagsValueProvider;
 import android.view.IWindow;
 import android.view.IWindowSession;
 import android.view.ImeBackAnimationController;
@@ -80,6 +87,8 @@ public class WindowOnBackInvokedDispatcherTest {
 
     @Rule
     public final MockitoRule mockito = MockitoJUnit.rule();
+    @Rule
+    public final CheckFlagsRule mCheckFlagsRule = DeviceFlagsValueProvider.createCheckFlagsRule();
 
     @Mock
     private IWindowSession mWindowSession;
@@ -145,7 +154,8 @@ public class WindowOnBackInvokedDispatcherTest {
         assertEquals("No setOnBackInvokedCallbackInfo", mCallbackInfoCalls, actual);
     }
 
-    private void assertCallbacksSize(int expectedDefault, int expectedOverlay) {
+    private void assertCallbacksSize(int expectedDefault, int expectedOverlay,
+            int expectedObserver) {
         ArrayList<OnBackInvokedCallback> callbacksDefault = mDispatcher
                 .mOnBackInvokedCallbacks.get(PRIORITY_DEFAULT);
         int actualSizeDefault = callbacksDefault != null ? callbacksDefault.size() : 0;
@@ -155,6 +165,10 @@ public class WindowOnBackInvokedDispatcherTest {
                 .mOnBackInvokedCallbacks.get(PRIORITY_OVERLAY);
         int actualSizeOverlay = callbacksOverlay != null ? callbacksOverlay.size() : 0;
         assertEquals("mOnBackInvokedCallbacks OVERLAY size", expectedOverlay, actualSizeOverlay);
+
+        int actualSizeObserver = mDispatcher.mSystemNavigationObserverCallback == null ? 0 : 1;
+        assertEquals("mOnBackInvokedCallbacks SYSTEM_NAVIGATION_OBSERVER size", expectedObserver,
+                actualSizeObserver);
     }
 
     private void assertTopCallback(OnBackInvokedCallback expectedCallback) {
@@ -164,13 +178,13 @@ public class WindowOnBackInvokedDispatcherTest {
     @Test
     public void registerCallback_samePriority_sameCallback() throws RemoteException {
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         // The callback is removed and added again
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
@@ -182,13 +196,13 @@ public class WindowOnBackInvokedDispatcherTest {
     @Test
     public void registerCallback_samePriority_differentCallback() throws RemoteException {
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         // The new callback becomes the TopCallback
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback2);
-        assertCallbacksSize(/* default */ 2, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 2, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback2);
 
@@ -201,13 +215,13 @@ public class WindowOnBackInvokedDispatcherTest {
     @Test
     public void registerCallback_differentPriority_sameCallback() throws RemoteException {
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_OVERLAY, mCallback1);
-        assertCallbacksSize(/* default */ 0, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 1, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         // The callback is moved to the new priority list
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
@@ -220,13 +234,13 @@ public class WindowOnBackInvokedDispatcherTest {
     public void registerCallback_differentPriority_differentCallback() throws RemoteException {
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_OVERLAY, mCallback1);
         assertSetCallbackInfo();
-        assertCallbacksSize(/* default */ 0, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 1, /* observer */ 0);
         assertTopCallback(mCallback1);
 
         // The callback with higher priority is still the TopCallback
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback2);
         assertNoSetCallbackInfo();
-        assertCallbacksSize(/* default */ 1, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 1, /* observer */ 0);
         assertTopCallback(mCallback1);
 
         waitForIdle();
@@ -238,22 +252,22 @@ public class WindowOnBackInvokedDispatcherTest {
     @Test
     public void registerCallback_sameInstanceAddedTwice() throws RemoteException {
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_OVERLAY, mCallback1);
-        assertCallbacksSize(/* default */ 0, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 1, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback2);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 1, /* observer */ 0);
         assertNoSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
-        assertCallbacksSize(/* default */ 2, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 2, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback1);
 
         mDispatcher.registerOnBackInvokedCallback(PRIORITY_OVERLAY, mCallback2);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 1);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 1, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mCallback2);
 
@@ -570,6 +584,102 @@ public class WindowOnBackInvokedDispatcherTest {
         assertFalse(mDispatcher.mProgressAnimator.isBackAnimationInProgress());
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    @RequiresFlagsDisabled(FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER)
+    public void testNoUiCallback_registrationFailsWithoutFlaggedApiEnabled() {
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback2);
+    }
+
+    @Test
+    @RequiresFlagsEnabled(FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER)
+    public void testNoUiCallback_invokedWithSystemCallback() throws RemoteException {
+        mDispatcher.registerSystemOnBackInvokedCallback(mCallback1);
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback2);
+
+        assertCallbacksSize(/* default */ 0, /* overlay */ 0, /* observer */ 1);
+        OnBackInvokedCallbackInfo callbackInfo = assertSetCallbackInfo();
+        assertTopCallback(mCallback1);
+
+        callbackInfo.getCallback().onBackStarted(mBackEvent);
+        waitForIdle();
+        verify(mCallback1).onBackStarted(any());
+        verify(mCallback2, never()).onBackStarted(any());
+
+        callbackInfo.getCallback().onBackProgressed(mBackEvent);
+        waitForIdle();
+        verify(mCallback1).onBackProgressed(any());
+        verify(mCallback2, never()).onBackProgressed(any());
+
+        callbackInfo.getCallback().onBackCancelled();
+        waitForIdle();
+        verify(mCallback1, timeout(1000)).onBackCancelled();
+        verify(mCallback2, never()).onBackCancelled();
+
+        // start new gesture to test onBackInvoked case
+        callbackInfo.getCallback().onBackStarted(mBackEvent);
+        callbackInfo.getCallback().onBackInvoked();
+        waitForIdle();
+        verify(mCallback1, timeout(1000)).onBackInvoked();
+        verify(mCallback2).onBackInvoked();
+    }
+
+    @Test
+    @RequiresFlagsEnabled(FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER)
+    public void testNoUiCallback_notInvokedWithNonSystemCallback() throws RemoteException {
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback1);
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback2);
+
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 1);
+        OnBackInvokedCallbackInfo callbackInfo = assertSetCallbackInfo();
+        assertTopCallback(mCallback1);
+
+        callbackInfo.getCallback().onBackStarted(mBackEvent);
+        waitForIdle();
+        verify(mCallback1).onBackStarted(any());
+        verify(mCallback2, never()).onBackStarted(any());
+
+        callbackInfo.getCallback().onBackProgressed(mBackEvent);
+        waitForIdle();
+        verify(mCallback1).onBackProgressed(any());
+        verify(mCallback2, never()).onBackProgressed(any());
+
+        callbackInfo.getCallback().onBackCancelled();
+        waitForIdle();
+        verify(mCallback1, timeout(1000)).onBackCancelled();
+        verify(mCallback2, never()).onBackCancelled();
+
+        // start new gesture to test onBackInvoked case
+        callbackInfo.getCallback().onBackStarted(mBackEvent);
+        callbackInfo.getCallback().onBackInvoked();
+        waitForIdle();
+        verify(mCallback1, timeout(1000)).onBackInvoked();
+        verify(mCallback2, never()).onBackInvoked();
+    }
+
+    @Test
+    @RequiresFlagsEnabled(FLAG_PREDICTIVE_BACK_PRIORITY_SYSTEM_NAVIGATION_OBSERVER)
+    public void testNoUiCallback_reregistrations() {
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback1);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 0, /* observer */ 1);
+        assertEquals(mCallback1, mDispatcher.mSystemNavigationObserverCallback);
+
+        // test reregistration of observer-callback as observer-callback
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback2);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 0, /* observer */ 1);
+        assertEquals(mCallback2, mDispatcher.mSystemNavigationObserverCallback);
+
+        // test reregistration of observer-callback as regular callback
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_DEFAULT, mCallback2);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
+
+        // test reregistration of regular callback as observer-callback
+        mDispatcher.registerOnBackInvokedCallback(PRIORITY_SYSTEM_NAVIGATION_OBSERVER, mCallback2);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 0, /* observer */ 1);
+
+        mDispatcher.unregisterOnBackInvokedCallback(mCallback2);
+        assertCallbacksSize(/* default */ 0, /* overlay */ 0, /* observer */ 0);
+    }
+
     private BackMotionEvent backMotionEventFrom(float progress) {
         return new BackMotionEvent(
                 /* touchX = */ 0,
@@ -585,13 +695,13 @@ public class WindowOnBackInvokedDispatcherTest {
     private void verifyImeCallackRegistrations() throws RemoteException {
         // verify default callback is replaced with ImeBackAnimationController
         mDispatcher.registerOnBackInvokedCallbackUnchecked(mDefaultImeCallback, PRIORITY_DEFAULT);
-        assertCallbacksSize(/* default */ 1, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 1, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mImeBackAnimationController);
 
         // verify regular ime callback is successfully registered
         mDispatcher.registerOnBackInvokedCallbackUnchecked(mImeCallback, PRIORITY_DEFAULT);
-        assertCallbacksSize(/* default */ 2, /* overlay */ 0);
+        assertCallbacksSize(/* default */ 2, /* overlay */ 0, /* observer */ 0);
         assertSetCallbackInfo();
         assertTopCallback(mImeCallback);
     }