From 3a55579cf2ddea6877a52b9f5d8127baa008ca00 Mon Sep 17 00:00:00 2001
From: Arthur Hung <arthurhung@google.com>
Date: Mon, 14 Nov 2022 03:55:51 +0000
Subject: [PATCH] Intruduce Cross Activity Animator

Add a new animator to support `TYPE_CROSS_ACTIVITY` when starting a back
navigation and it will return to another activity.

This also introduces the background controller that could show the
animation background surface attached to the display area.

Bug: 236759828
Bug: 238474994
Test: atest BackAnimationControllerTest BackNavigationControllerTests
Change-Id: I6574b6ce969cb905ba8b4a797560216aed2ee3cb
---
 .../wm/shell/animation/Interpolators.java     |  16 +
 .../shell/back/BackAnimationBackground.java   |  69 ++++
 .../shell/back/BackAnimationController.java   |  29 +-
 .../wm/shell/back/CrossActivityAnimation.java | 373 ++++++++++++++++++
 .../wm/shell/back/CrossTaskBackAnimation.java | 114 +++---
 .../wm/shell/dagger/WMShellBaseModule.java    |  18 +-
 .../back/BackAnimationControllerTest.java     |  15 +-
 .../server/wm/BackNavigationController.java   |  34 +-
 8 files changed, 576 insertions(+), 92 deletions(-)
 create mode 100644 libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationBackground.java
 create mode 100644 libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossActivityAnimation.java

diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/animation/Interpolators.java b/libs/WindowManager/Shell/src/com/android/wm/shell/animation/Interpolators.java
index a0dde6ad168d0..2ec9e8b12fc6d 100644
--- a/libs/WindowManager/Shell/src/com/android/wm/shell/animation/Interpolators.java
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/animation/Interpolators.java
@@ -16,6 +16,7 @@
 
 package com.android.wm.shell.animation;
 
+import android.graphics.Path;
 import android.view.animation.Interpolator;
 import android.view.animation.LinearInterpolator;
 import android.view.animation.PathInterpolator;
@@ -52,6 +53,11 @@ public class Interpolators {
      */
     public static final Interpolator LINEAR_OUT_SLOW_IN = new PathInterpolator(0f, 0f, 0.2f, 1f);
 
+    /**
+     * The default emphasized interpolator. Used for hero / emphasized movement of content.
+     */
+    public static final Interpolator EMPHASIZED = createEmphasizedInterpolator();
+
     /**
      * The accelerated emphasized interpolator. Used for hero / emphasized movement of content that
      * is disappearing e.g. when moving off screen.
@@ -81,4 +87,14 @@ public class Interpolators {
 
     public static final PathInterpolator DIM_INTERPOLATOR =
             new PathInterpolator(.23f, .87f, .52f, -0.11f);
+
+    // Create the default emphasized interpolator
+    private static PathInterpolator createEmphasizedInterpolator() {
+        Path path = new Path();
+        // Doing the same as fast_out_extra_slow_in
+        path.moveTo(0f, 0f);
+        path.cubicTo(0.05f, 0f, 0.133333f, 0.06f, 0.166666f, 0.4f);
+        path.cubicTo(0.208333f, 0.82f, 0.25f, 1f, 1f, 1f);
+        return new PathInterpolator(path);
+    }
 }
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationBackground.java b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationBackground.java
new file mode 100644
index 0000000000000..36cf29a4c4f3d
--- /dev/null
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationBackground.java
@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2022 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.wm.shell.back;
+
+import static android.view.Display.DEFAULT_DISPLAY;
+
+import android.annotation.NonNull;
+import android.graphics.Color;
+import android.view.SurfaceControl;
+
+import com.android.wm.shell.RootTaskDisplayAreaOrganizer;
+
+/**
+ * Controls background surface for the back animations
+ */
+public class BackAnimationBackground {
+    private static final int BACKGROUND_LAYER = -1;
+    private final RootTaskDisplayAreaOrganizer mRootTaskDisplayAreaOrganizer;
+    private SurfaceControl mBackgroundSurface;
+
+    public BackAnimationBackground(RootTaskDisplayAreaOrganizer rootTaskDisplayAreaOrganizer) {
+        mRootTaskDisplayAreaOrganizer = rootTaskDisplayAreaOrganizer;
+    }
+
+    void ensureBackground(int color, @NonNull SurfaceControl.Transaction transaction) {
+        if (mBackgroundSurface != null) {
+            return;
+        }
+
+        final float[] colorComponents = new float[] { Color.red(color) / 255.f,
+                Color.green(color) / 255.f, Color.blue(color) / 255.f };
+
+        final SurfaceControl.Builder colorLayerBuilder = new SurfaceControl.Builder()
+                .setName("back-animation-background")
+                .setCallsite("BackAnimationBackground")
+                .setColorLayer();
+
+        mRootTaskDisplayAreaOrganizer.attachToDisplayArea(DEFAULT_DISPLAY, colorLayerBuilder);
+        mBackgroundSurface = colorLayerBuilder.build();
+        transaction.setColor(mBackgroundSurface, colorComponents)
+                .setLayer(mBackgroundSurface, BACKGROUND_LAYER)
+                .show(mBackgroundSurface);
+    }
+
+    void removeBackground(@NonNull SurfaceControl.Transaction transaction) {
+        if (mBackgroundSurface == null) {
+            return;
+        }
+
+        if (mBackgroundSurface.isValid()) {
+            transaction.remove(mBackgroundSurface);
+        }
+        mBackgroundSurface = null;
+    }
+}
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
index f811940fd3040..0133f6b44d2bf 100644
--- a/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/back/BackAnimationController.java
@@ -138,14 +138,18 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
         }
     };
 
+    private final BackAnimationBackground mAnimationBackground;
+
     public BackAnimationController(
             @NonNull ShellInit shellInit,
             @NonNull ShellController shellController,
             @NonNull @ShellMainThread ShellExecutor shellExecutor,
             @NonNull @ShellBackgroundThread Handler backgroundHandler,
-            Context context) {
+            Context context,
+            @NonNull BackAnimationBackground backAnimationBackground) {
         this(shellInit, shellController, shellExecutor, backgroundHandler,
-                ActivityTaskManager.getService(), context, context.getContentResolver());
+                ActivityTaskManager.getService(), context, context.getContentResolver(),
+                backAnimationBackground);
     }
 
     @VisibleForTesting
@@ -155,7 +159,8 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
             @NonNull @ShellMainThread ShellExecutor shellExecutor,
             @NonNull @ShellBackgroundThread Handler bgHandler,
             @NonNull IActivityTaskManager activityTaskManager,
-            Context context, ContentResolver contentResolver) {
+            Context context, ContentResolver contentResolver,
+            @NonNull BackAnimationBackground backAnimationBackground) {
         mShellController = shellController;
         mShellExecutor = shellExecutor;
         mActivityTaskManager = activityTaskManager;
@@ -163,6 +168,7 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
         mContentResolver = contentResolver;
         mBgHandler = bgHandler;
         shellInit.addInitCallback(this::onInit, this);
+        mAnimationBackground = backAnimationBackground;
     }
 
     @VisibleForTesting
@@ -184,10 +190,14 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
             return;
         }
 
-        final CrossTaskBackAnimation crossTaskAnimation = new CrossTaskBackAnimation(mContext);
+        final CrossTaskBackAnimation crossTaskAnimation =
+                new CrossTaskBackAnimation(mContext, mAnimationBackground);
         mAnimationDefinition.set(BackNavigationInfo.TYPE_CROSS_TASK,
-                new BackAnimationRunner(crossTaskAnimation.mCallback, crossTaskAnimation.mRunner));
-        // TODO (238474994): register cross activity animation when it's completed.
+                crossTaskAnimation.mBackAnimationRunner);
+        final CrossActivityAnimation crossActivityAnimation =
+                new CrossActivityAnimation(mContext, mAnimationBackground);
+        mAnimationDefinition.set(BackNavigationInfo.TYPE_CROSS_ACTIVITY,
+                crossActivityAnimation.mBackAnimationRunner);
         // TODO (236760237): register dialog close animation when it's completed.
     }
 
@@ -275,7 +285,8 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
         @Override
         public void clearBackToLauncherCallback() {
             executeRemoteCallWithTaskPermission(mController, "clearBackToLauncherCallback",
-                    (controller) -> controller.clearBackToLauncherCallback());
+                    (controller) -> controller.unregisterAnimation(
+                            BackNavigationInfo.TYPE_RETURN_TO_HOME));
         }
 
         @Override
@@ -289,8 +300,8 @@ public class BackAnimationController implements RemoteCallable<BackAnimationCont
         mAnimationDefinition.set(type, runner);
     }
 
-    private void clearBackToLauncherCallback() {
-        mAnimationDefinition.remove(BackNavigationInfo.TYPE_RETURN_TO_HOME);
+    void unregisterAnimation(@BackNavigationInfo.BackTargetType int type) {
+        mAnimationDefinition.remove(type);
     }
 
     /**
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossActivityAnimation.java b/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossActivityAnimation.java
new file mode 100644
index 0000000000000..9f6bc5d89e101
--- /dev/null
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossActivityAnimation.java
@@ -0,0 +1,373 @@
+/*
+ * Copyright (C) 2022 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.wm.shell.back;
+
+import static android.view.RemoteAnimationTarget.MODE_CLOSING;
+import static android.view.RemoteAnimationTarget.MODE_OPENING;
+
+import static com.android.wm.shell.protolog.ShellProtoLogGroup.WM_SHELL_BACK_PREVIEW;
+
+import android.animation.Animator;
+import android.animation.AnimatorListenerAdapter;
+import android.animation.ValueAnimator;
+import android.annotation.NonNull;
+import android.content.Context;
+import android.graphics.Matrix;
+import android.graphics.PointF;
+import android.graphics.Rect;
+import android.graphics.RectF;
+import android.os.RemoteException;
+import android.view.IRemoteAnimationFinishedCallback;
+import android.view.IRemoteAnimationRunner;
+import android.view.RemoteAnimationTarget;
+import android.view.SurfaceControl;
+import android.view.animation.AccelerateInterpolator;
+import android.view.animation.DecelerateInterpolator;
+import android.view.animation.Interpolator;
+import android.window.BackEvent;
+import android.window.BackProgressAnimator;
+import android.window.IOnBackInvokedCallback;
+
+import com.android.internal.policy.ScreenDecorationsUtils;
+import com.android.internal.protolog.common.ProtoLog;
+import com.android.wm.shell.animation.Interpolators;
+import com.android.wm.shell.common.annotations.ShellMainThread;
+
+/** Class that defines cross-activity animation. */
+@ShellMainThread
+class CrossActivityAnimation {
+    /**
+     * Minimum scale of the entering/closing window.
+     */
+    private static final float MIN_WINDOW_SCALE = 0.9f;
+
+    /**
+     * Minimum alpha of the closing/entering window.
+     */
+    private static final float CLOSING_MIN_WINDOW_ALPHA = 0.5f;
+
+    /**
+     * Progress value to fly out closing window and fly in entering window.
+     */
+    private static final float SWITCH_ENTERING_WINDOW_PROGRESS = 0.5f;
+
+    /** Max window translation in the Y axis. */
+    private static final int WINDOW_MAX_DELTA_Y = 160;
+
+    /** Duration of fade in/out entering window. */
+    private static final int FADE_IN_DURATION = 100;
+    /** Duration of post animation after gesture committed. */
+    private static final int POST_ANIMATION_DURATION = 350;
+    private static final Interpolator INTERPOLATOR = Interpolators.EMPHASIZED;
+
+    private final Rect mStartTaskRect = new Rect();
+    private final float mCornerRadius;
+
+    // The closing window properties.
+    private final RectF mClosingRect = new RectF();
+
+    // The entering window properties.
+    private final Rect mEnteringStartRect = new Rect();
+    private final RectF mEnteringRect = new RectF();
+
+    private float mCurrentAlpha = 1.0f;
+
+    private float mEnteringMargin = 0;
+    private ValueAnimator mEnteringAnimator;
+    private boolean mEnteringWindowShow = false;
+
+    private final PointF mInitialTouchPos = new PointF();
+
+    private final Matrix mTransformMatrix = new Matrix();
+
+    private final float[] mTmpFloat9 = new float[9];
+
+    private RemoteAnimationTarget mEnteringTarget;
+    private RemoteAnimationTarget mClosingTarget;
+    private SurfaceControl.Transaction mTransaction = new SurfaceControl.Transaction();
+
+    private boolean mBackInProgress = false;
+
+    private PointF mTouchPos = new PointF();
+    private IRemoteAnimationFinishedCallback mFinishCallback;
+
+    private final BackProgressAnimator mProgressAnimator = new BackProgressAnimator();
+    final BackAnimationRunner mBackAnimationRunner;
+
+    private final BackAnimationBackground mBackground;
+
+    CrossActivityAnimation(Context context, BackAnimationBackground background) {
+        mCornerRadius = ScreenDecorationsUtils.getWindowCornerRadius(context);
+        mBackAnimationRunner = new BackAnimationRunner(new Callback(), new Runner());
+        mBackground = background;
+    }
+
+    private static float mapRange(float value, float min, float max) {
+        return min + (value * (max - min));
+    }
+
+    private float getInterpolatedProgress(float backProgress) {
+        return INTERPOLATOR.getInterpolation(backProgress);
+    }
+
+    private void startBackAnimation() {
+        if (mEnteringTarget == null || mClosingTarget == null) {
+            ProtoLog.d(WM_SHELL_BACK_PREVIEW, "Entering target or closing target is null.");
+            return;
+        }
+        mTransaction.setAnimationTransaction();
+
+        // Offset start rectangle to align task bounds.
+        mStartTaskRect.set(mClosingTarget.windowConfiguration.getBounds());
+        mStartTaskRect.offsetTo(0, 0);
+
+        // Draw background with task background color.
+        mBackground.ensureBackground(
+                mEnteringTarget.taskInfo.taskDescription.getBackgroundColor(), mTransaction);
+    }
+
+    private void applyTransform(SurfaceControl leash, RectF targetRect, float targetAlpha) {
+        final float scale = targetRect.width() / mStartTaskRect.width();
+        mTransformMatrix.reset();
+        mTransformMatrix.setScale(scale, scale);
+        mTransformMatrix.postTranslate(targetRect.left, targetRect.top);
+        mTransaction.setAlpha(leash, targetAlpha)
+                .setMatrix(leash, mTransformMatrix, mTmpFloat9)
+                .setWindowCrop(leash, mStartTaskRect)
+                .setCornerRadius(leash, mCornerRadius);
+    }
+
+    private void finishAnimation() {
+        if (mEnteringTarget != null) {
+            mEnteringTarget.leash.release();
+            mEnteringTarget = null;
+        }
+        if (mClosingTarget != null) {
+            mClosingTarget.leash.release();
+            mClosingTarget = null;
+        }
+        if (mBackground != null) {
+            mBackground.removeBackground(mTransaction);
+        }
+
+        mTransaction.apply();
+        mBackInProgress = false;
+        mTransformMatrix.reset();
+        mInitialTouchPos.set(0, 0);
+        mEnteringWindowShow = false;
+        mEnteringMargin = 0;
+
+        if (mFinishCallback != null) {
+            try {
+                mFinishCallback.onAnimationFinished();
+            } catch (RemoteException e) {
+                e.printStackTrace();
+            }
+            mFinishCallback = null;
+        }
+    }
+
+    private void onGestureProgress(@NonNull BackEvent backEvent) {
+        if (!mBackInProgress) {
+            mInitialTouchPos.set(backEvent.getTouchX(), backEvent.getTouchY());
+            mBackInProgress = true;
+        }
+        mTouchPos.set(backEvent.getTouchX(), backEvent.getTouchY());
+
+        if (mEnteringTarget == null || mClosingTarget == null) {
+            return;
+        }
+
+        final float progress = getInterpolatedProgress(backEvent.getProgress());
+        final float touchY = mTouchPos.y;
+
+        final int width = mStartTaskRect.width();
+        final int height = mStartTaskRect.height();
+
+        final float closingScale = mapRange(progress, 1, MIN_WINDOW_SCALE);
+
+        final float closingWidth = closingScale * width;
+        final float closingHeight = (float) height / width * closingWidth;
+
+        // Move the window along the X axis.
+        final float closingLeft = mStartTaskRect.left + (width - closingWidth) / 2;
+
+        // Move the window along the Y axis.
+        final float deltaYRatio = (touchY - mInitialTouchPos.y) / height;
+        final float deltaY = (float) Math.sin(deltaYRatio * Math.PI * 0.5f) * WINDOW_MAX_DELTA_Y;
+        final float closingTop = (height - closingHeight) * 0.5f + deltaY;
+        mClosingRect.set(
+                closingLeft, closingTop, closingLeft + closingWidth, closingTop + closingHeight);
+        mEnteringRect.set(mClosingRect);
+
+        // Switch closing/entering targets while reach to the threshold progress.
+        if (showEnteringWindow(progress > SWITCH_ENTERING_WINDOW_PROGRESS)) {
+            return;
+        }
+
+        // Present windows and update the alpha.
+        mCurrentAlpha = Math.max(mapRange(progress, 1.0f, 0), CLOSING_MIN_WINDOW_ALPHA);
+        mClosingRect.offset(mEnteringMargin, 0);
+        mEnteringRect.offset(mEnteringMargin - width, 0);
+
+        applyTransform(
+                mClosingTarget.leash, mClosingRect, mEnteringWindowShow ? 0.01f : mCurrentAlpha);
+        applyTransform(
+                mEnteringTarget.leash, mEnteringRect, mEnteringWindowShow ? mCurrentAlpha : 0.01f);
+        mTransaction.apply();
+    }
+
+    private boolean showEnteringWindow(boolean show) {
+        if (mEnteringAnimator == null) {
+            mEnteringAnimator = ValueAnimator.ofFloat(1f, 0f).setDuration(FADE_IN_DURATION);
+            mEnteringAnimator.setInterpolator(new AccelerateInterpolator());
+            mEnteringAnimator.addUpdateListener(animation -> {
+                float progress = animation.getAnimatedFraction();
+                final int width = mStartTaskRect.width();
+                mEnteringMargin = width * progress;
+                // We don't animate to 0 or the surface would become invisible and lose focus.
+                final float alpha = progress >= 0.5f ? 0.01f
+                        : mapRange(progress * 2, mCurrentAlpha, 0.01f);
+                mClosingRect.offset(mEnteringMargin, 0);
+                mEnteringRect.offset(mEnteringMargin - width, 0);
+
+                applyTransform(mClosingTarget.leash, mClosingRect, alpha);
+                applyTransform(mEnteringTarget.leash, mEnteringRect, mCurrentAlpha);
+                mTransaction.apply();
+            });
+        }
+
+        if (mEnteringAnimator.isRunning()) {
+            return true;
+        }
+
+        if (mEnteringWindowShow == show) {
+            return false;
+        }
+
+        mEnteringWindowShow = show;
+        if (show) {
+            mEnteringAnimator.start();
+        } else {
+            mEnteringAnimator.reverse();
+        }
+        return true;
+    }
+
+    private void onGestureCommitted() {
+        if (mEnteringTarget == null || mClosingTarget == null) {
+            finishAnimation();
+            return;
+        }
+
+        // End the fade in animation.
+        if (mEnteringAnimator.isRunning()) {
+            mEnteringAnimator.cancel();
+        }
+
+        // We enter phase 2 of the animation, the starting coordinates for phase 2 are the current
+        // coordinate of the gesture driven phase.
+        mEnteringRect.round(mEnteringStartRect);
+        mTransaction.hide(mClosingTarget.leash);
+
+        ValueAnimator valueAnimator =
+                ValueAnimator.ofFloat(1f, 0f).setDuration(POST_ANIMATION_DURATION);
+        valueAnimator.setInterpolator(new DecelerateInterpolator());
+        valueAnimator.addUpdateListener(animation -> {
+            float progress = animation.getAnimatedFraction();
+            updatePostCommitEnteringAnimation(progress);
+            mTransaction.apply();
+        });
+
+        valueAnimator.addListener(new AnimatorListenerAdapter() {
+            @Override
+            public void onAnimationEnd(Animator animation) {
+                finishAnimation();
+            }
+        });
+        valueAnimator.start();
+    }
+
+    private void updatePostCommitEnteringAnimation(float progress) {
+        float left = mapRange(progress, mEnteringStartRect.left, mStartTaskRect.left);
+        float top = mapRange(progress, mEnteringStartRect.top, mStartTaskRect.top);
+        float width = mapRange(progress, mEnteringStartRect.width(), mStartTaskRect.width());
+        float height = mapRange(progress, mEnteringStartRect.height(), mStartTaskRect.height());
+        float alpha = mapRange(progress, mCurrentAlpha, 1.0f);
+
+        mEnteringRect.set(left, top, left + width, top + height);
+        applyTransform(mEnteringTarget.leash, mEnteringRect, alpha);
+    }
+
+    private final class Callback extends IOnBackInvokedCallback.Default {
+        @Override
+        public void onBackStarted(BackEvent backEvent) {
+            mProgressAnimator.onBackStarted(backEvent,
+                    CrossActivityAnimation.this::onGestureProgress);
+        }
+
+        @Override
+        public void onBackProgressed(@NonNull BackEvent backEvent) {
+            mProgressAnimator.onBackProgressed(backEvent);
+        }
+
+        @Override
+        public void onBackCancelled() {
+            // End the fade in animation.
+            if (mEnteringAnimator.isRunning()) {
+                mEnteringAnimator.cancel();
+            }
+            // TODO (b259608500): Let BackProgressAnimator could play cancel animation.
+            mProgressAnimator.reset();
+            finishAnimation();
+        }
+
+        @Override
+        public void onBackInvoked() {
+            mProgressAnimator.reset();
+            onGestureCommitted();
+        }
+    }
+
+    private final class Runner extends IRemoteAnimationRunner.Default {
+        @Override
+        public void onAnimationStart(
+                int transit,
+                RemoteAnimationTarget[] apps,
+                RemoteAnimationTarget[] wallpapers,
+                RemoteAnimationTarget[] nonApps,
+                IRemoteAnimationFinishedCallback finishedCallback) {
+            ProtoLog.d(WM_SHELL_BACK_PREVIEW, "Start back to activity animation.");
+            for (RemoteAnimationTarget a : apps) {
+                if (a.mode == MODE_CLOSING) {
+                    mClosingTarget = a;
+                }
+                if (a.mode == MODE_OPENING) {
+                    mEnteringTarget = a;
+                }
+            }
+
+            startBackAnimation();
+            mFinishCallback = finishedCallback;
+        }
+
+        @Override
+        public void onAnimationCancelled(boolean isKeyguardOccluded) {
+            finishAnimation();
+        }
+    }
+}
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossTaskBackAnimation.java b/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossTaskBackAnimation.java
index 2074b6ab44f29..a9a7b7742d2f1 100644
--- a/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossTaskBackAnimation.java
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/back/CrossTaskBackAnimation.java
@@ -61,7 +61,7 @@ import com.android.wm.shell.common.annotations.ShellMainThread;
  */
 @ShellMainThread
 class CrossTaskBackAnimation {
-    private static final float[] BACKGROUNDCOLOR = {0.263f, 0.263f, 0.227f};
+    private static final int BACKGROUNDCOLOR = 0x43433A;
 
     /**
      * Minimum scale of the entering window.
@@ -106,7 +106,6 @@ class CrossTaskBackAnimation {
 
     private RemoteAnimationTarget mEnteringTarget;
     private RemoteAnimationTarget mClosingTarget;
-    private SurfaceControl mBackgroundSurface;
     private SurfaceControl.Transaction mTransaction = new SurfaceControl.Transaction();
 
     private boolean mBackInProgress = false;
@@ -115,56 +114,15 @@ class CrossTaskBackAnimation {
     private float mProgress = 0;
     private PointF mTouchPos = new PointF();
     private IRemoteAnimationFinishedCallback mFinishCallback;
-
     private BackProgressAnimator mProgressAnimator = new BackProgressAnimator();
+    final BackAnimationRunner mBackAnimationRunner;
 
-    final IOnBackInvokedCallback mCallback = new IOnBackInvokedCallback.Default() {
-        @Override
-        public void onBackStarted(BackEvent backEvent) {
-            mProgressAnimator.onBackStarted(backEvent,
-                    CrossTaskBackAnimation.this::onGestureProgress);
-        }
+    private final BackAnimationBackground mBackground;
 
-        @Override
-        public void onBackProgressed(@NonNull BackEvent backEvent) {
-            mProgressAnimator.onBackProgressed(backEvent);
-        }
-
-        @Override
-        public void onBackCancelled() {
-            mProgressAnimator.reset();
-            finishAnimation();
-        }
-
-        @Override
-        public void onBackInvoked() {
-            mProgressAnimator.reset();
-            onGestureCommitted();
-        }
-    };
-
-    final IRemoteAnimationRunner mRunner = new IRemoteAnimationRunner.Default() {
-        @Override
-        public void onAnimationStart(int transit, RemoteAnimationTarget[] apps,
-                RemoteAnimationTarget[] wallpapers, RemoteAnimationTarget[] nonApps,
-                IRemoteAnimationFinishedCallback finishedCallback) {
-            ProtoLog.d(WM_SHELL_BACK_PREVIEW, "Start back to task animation.");
-            for (RemoteAnimationTarget a : apps) {
-                if (a.mode == MODE_CLOSING) {
-                    mClosingTarget = a;
-                }
-                if (a.mode == MODE_OPENING) {
-                    mEnteringTarget = a;
-                }
-            }
-
-            startBackAnimation();
-            mFinishCallback = finishedCallback;
-        }
-    };
-
-    CrossTaskBackAnimation(Context context) {
+    CrossTaskBackAnimation(Context context, BackAnimationBackground background) {
         mCornerRadius = ScreenDecorationsUtils.getWindowCornerRadius(context);
+        mBackAnimationRunner = new BackAnimationRunner(new Callback(), new Runner());
+        mBackground = background;
     }
 
     private float getInterpolatedProgress(float backProgress) {
@@ -182,14 +140,7 @@ class CrossTaskBackAnimation {
         mStartTaskRect.offsetTo(0, 0);
 
         // Draw background.
-        mBackgroundSurface = new SurfaceControl.Builder()
-                .setName("Background of Back Navigation")
-                .setColorLayer()
-                .setHidden(false)
-                .build();
-        mTransaction.setColor(mBackgroundSurface, BACKGROUNDCOLOR)
-                .setLayer(mBackgroundSurface, -1);
-        mTransaction.apply();
+        mBackground.ensureBackground(BACKGROUNDCOLOR, mTransaction);
     }
 
     private void updateGestureBackProgress(float progress, BackEvent event) {
@@ -300,11 +251,11 @@ class CrossTaskBackAnimation {
             mClosingTarget = null;
         }
 
-        if (mBackgroundSurface != null) {
-            mBackgroundSurface.release();
-            mBackgroundSurface = null;
+        if (mBackground != null) {
+            mBackground.removeBackground(mTransaction);
         }
 
+        mTransaction.apply();
         mBackInProgress = false;
         mTransformMatrix.reset();
         mClosingCurrentRect.setEmpty();
@@ -362,4 +313,49 @@ class CrossTaskBackAnimation {
     private static float mapRange(float value, float min, float max) {
         return min + (value * (max - min));
     }
+
+    private final class Callback extends IOnBackInvokedCallback.Default  {
+        @Override
+        public void onBackStarted(BackEvent backEvent) {
+            mProgressAnimator.onBackStarted(backEvent,
+                    CrossTaskBackAnimation.this::onGestureProgress);
+        }
+
+        @Override
+        public void onBackProgressed(@NonNull BackEvent backEvent) {
+            mProgressAnimator.onBackProgressed(backEvent);
+        }
+
+        @Override
+        public void onBackCancelled() {
+            mProgressAnimator.reset();
+            finishAnimation();
+        }
+
+        @Override
+        public void onBackInvoked() {
+            mProgressAnimator.reset();
+            onGestureCommitted();
+        }
+    };
+
+    private final class Runner extends IRemoteAnimationRunner.Default {
+        @Override
+        public void onAnimationStart(int transit, RemoteAnimationTarget[] apps,
+                RemoteAnimationTarget[] wallpapers, RemoteAnimationTarget[] nonApps,
+                IRemoteAnimationFinishedCallback finishedCallback) {
+            ProtoLog.d(WM_SHELL_BACK_PREVIEW, "Start back to task animation.");
+            for (RemoteAnimationTarget a : apps) {
+                if (a.mode == MODE_CLOSING) {
+                    mClosingTarget = a;
+                }
+                if (a.mode == MODE_OPENING) {
+                    mEnteringTarget = a;
+                }
+            }
+
+            startBackAnimation();
+            mFinishCallback = finishedCallback;
+        }
+    };
 }
diff --git a/libs/WindowManager/Shell/src/com/android/wm/shell/dagger/WMShellBaseModule.java b/libs/WindowManager/Shell/src/com/android/wm/shell/dagger/WMShellBaseModule.java
index 962be9da2111b..4ea8a5dbc5a99 100644
--- a/libs/WindowManager/Shell/src/com/android/wm/shell/dagger/WMShellBaseModule.java
+++ b/libs/WindowManager/Shell/src/com/android/wm/shell/dagger/WMShellBaseModule.java
@@ -38,6 +38,7 @@ import com.android.wm.shell.TaskViewTransitions;
 import com.android.wm.shell.WindowManagerShellWrapper;
 import com.android.wm.shell.activityembedding.ActivityEmbeddingController;
 import com.android.wm.shell.back.BackAnimation;
+import com.android.wm.shell.back.BackAnimationBackground;
 import com.android.wm.shell.back.BackAnimationController;
 import com.android.wm.shell.bubbles.BubbleController;
 import com.android.wm.shell.bubbles.Bubbles;
@@ -93,13 +94,13 @@ import com.android.wm.shell.unfold.UnfoldAnimationController;
 import com.android.wm.shell.unfold.UnfoldTransitionHandler;
 import com.android.wm.shell.windowdecor.WindowDecorViewModel;
 
-import java.util.Optional;
-
 import dagger.BindsOptionalOf;
 import dagger.Lazy;
 import dagger.Module;
 import dagger.Provides;
 
+import java.util.Optional;
+
 /**
  * Provides basic dependencies from {@link com.android.wm.shell}, these dependencies are only
  * accessible from components within the WM subcomponent (can be explicitly exposed to the
@@ -253,6 +254,13 @@ public abstract class WMShellBaseModule {
         return backAnimationController.map(BackAnimationController::getBackAnimationImpl);
     }
 
+    @WMSingleton
+    @Provides
+    static BackAnimationBackground provideBackAnimationBackground(
+            RootTaskDisplayAreaOrganizer rootTaskDisplayAreaOrganizer) {
+        return new BackAnimationBackground(rootTaskDisplayAreaOrganizer);
+    }
+
     @WMSingleton
     @Provides
     static Optional<BackAnimationController> provideBackAnimationController(
@@ -260,16 +268,18 @@ public abstract class WMShellBaseModule {
             ShellInit shellInit,
             ShellController shellController,
             @ShellMainThread ShellExecutor shellExecutor,
-            @ShellBackgroundThread Handler backgroundHandler
+            @ShellBackgroundThread Handler backgroundHandler,
+            BackAnimationBackground backAnimationBackground
     ) {
         if (BackAnimationController.IS_ENABLED) {
             return Optional.of(
                     new BackAnimationController(shellInit, shellController, shellExecutor,
-                            backgroundHandler, context));
+                            backgroundHandler, context, backAnimationBackground));
         }
         return Optional.empty();
     }
 
+
     //
     // Bubbles (optional feature)
     //
diff --git a/libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/back/BackAnimationControllerTest.java b/libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/back/BackAnimationControllerTest.java
index d75c36c99a4cd..bee9a90ec6b81 100644
--- a/libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/back/BackAnimationControllerTest.java
+++ b/libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/back/BackAnimationControllerTest.java
@@ -110,6 +110,9 @@ public class BackAnimationControllerTest extends ShellTestCase {
     @Mock
     private ShellController mShellController;
 
+    @Mock
+    private BackAnimationBackground mAnimationBackground;
+
     private BackAnimationController mController;
     private TestableContentResolver mContentResolver;
     private TestableLooper mTestableLooper;
@@ -127,7 +130,7 @@ public class BackAnimationControllerTest extends ShellTestCase {
         mController = new BackAnimationController(mShellInit, mShellController,
                 mShellExecutor, new Handler(mTestableLooper.getLooper()),
                 mActivityTaskManager, mContext,
-                mContentResolver);
+                mContentResolver, mAnimationBackground);
         mController.setEnableUAnimation(true);
         mShellInit.init();
         mShellExecutor.flushAll();
@@ -239,7 +242,7 @@ public class BackAnimationControllerTest extends ShellTestCase {
         mController = new BackAnimationController(shellInit, mShellController,
                 mShellExecutor, new Handler(mTestableLooper.getLooper()),
                 mActivityTaskManager, mContext,
-                mContentResolver);
+                mContentResolver, mAnimationBackground);
         shellInit.init();
         registerAnimation(BackNavigationInfo.TYPE_RETURN_TO_HOME);
 
@@ -353,6 +356,10 @@ public class BackAnimationControllerTest extends ShellTestCase {
                 BackNavigationInfo.TYPE_CROSS_ACTIVITY,
                 BackNavigationInfo.TYPE_DIALOG_CLOSE};
 
+        for (int type: testTypes) {
+            unregisterAnimation(type);
+        }
+
         for (int type: testTypes) {
             final ResultListener result = new ResultListener();
             createNavigationInfo(new BackNavigationInfo.Builder()
@@ -431,6 +438,10 @@ public class BackAnimationControllerTest extends ShellTestCase {
                 new BackAnimationRunner(mAnimatorCallback, mBackAnimationRunner));
     }
 
+    private void unregisterAnimation(int type) {
+        mController.unregisterAnimation(type);
+    }
+
     private static class ResultListener implements RemoteCallback.OnResultListener {
         boolean mBackNavigationDone = false;
         boolean mTriggerBack = false;
diff --git a/services/core/java/com/android/server/wm/BackNavigationController.java b/services/core/java/com/android/server/wm/BackNavigationController.java
index 14d6d7bdfb7dd..798e73906761e 100644
--- a/services/core/java/com/android/server/wm/BackNavigationController.java
+++ b/services/core/java/com/android/server/wm/BackNavigationController.java
@@ -241,6 +241,7 @@ class BackNavigationController {
                 // We have another Activity in the same currentTask to go to
                 backType = BackNavigationInfo.TYPE_CROSS_ACTIVITY;
                 removedWindowContainer = currentActivity;
+                prevTask = prevActivity.getTask();
             } else if (currentTask.returnsToHomeRootTask()) {
                 // Our Task should bring back to home
                 removedWindowContainer = currentTask;
@@ -608,26 +609,23 @@ class BackNavigationController {
                 // reset leash after animation finished.
                 leashes.add(screenshotSurface);
             }
-        } else if (prevTask != null) {
-            prevActivity = prevTask.getTopNonFinishingActivity();
-            if (prevActivity != null) {
-                // Make previous task show from behind by marking its top activity as visible
-                // and launch-behind to bump its visibility for the duration of the back gesture.
-                setLaunchBehind(prevActivity);
+        } else if (prevTask != null && prevActivity != null) {
+            // Make previous task show from behind by marking its top activity as visible
+            // and launch-behind to bump its visibility for the duration of the back gesture.
+            setLaunchBehind(prevActivity);
 
-                final SurfaceControl leash = prevActivity.makeAnimationLeash()
-                        .setName("BackPreview Leash for " + prevActivity)
-                        .setHidden(false)
-                        .build();
-                prevActivity.reparentSurfaceControl(startedTransaction, leash);
-                behindAppTarget = createRemoteAnimationTargetLocked(
-                        prevTask, leash, MODE_OPENING);
+            final SurfaceControl leash = prevActivity.makeAnimationLeash()
+                    .setName("BackPreview Leash for " + prevActivity)
+                    .setHidden(false)
+                    .build();
+            prevActivity.reparentSurfaceControl(startedTransaction, leash);
+            behindAppTarget = createRemoteAnimationTargetLocked(
+                    prevTask, leash, MODE_OPENING);
 
-                // reset leash after animation finished.
-                leashes.add(leash);
-                prevActivity.reparentSurfaceControl(finishedTransaction,
-                        prevActivity.getParentSurfaceControl());
-            }
+            // reset leash after animation finished.
+            leashes.add(leash);
+            prevActivity.reparentSurfaceControl(finishedTransaction,
+                    prevActivity.getParentSurfaceControl());
         }
 
         if (mShowWallpaper) {