android/app/ActivityView.java

/**
 * Copyright (c) 2017 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 android.app;

import static android.hardware.display.DisplayManager.VIRTUAL_DISPLAY_FLAG_DESTROY_CONTENT_ON_REMOVAL;
import static android.hardware.display.DisplayManager.VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY;
import static android.hardware.display.DisplayManager.VIRTUAL_DISPLAY_FLAG_PUBLIC;
import static android.view.Display.INVALID_DISPLAY;

import android.annotation.NonNull;
import android.annotation.TestApi;
import android.app.ActivityManager.StackInfo;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.graphics.Insets;
import android.graphics.Matrix;
import android.graphics.Region;
import android.hardware.display.DisplayManager;
import android.hardware.display.VirtualDisplay;
import android.hardware.input.InputManager;
import android.os.RemoteException;
import android.os.SystemClock;
import android.os.UserHandle;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.util.Log;
import android.view.IWindowManager;
import android.view.InputDevice;
import android.view.KeyCharacterMap;
import android.view.KeyEvent;
import android.view.SurfaceControl;
import android.view.SurfaceHolder;
import android.view.SurfaceSession;
import android.view.SurfaceView;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.WindowManager;
import android.view.WindowManagerGlobal;
import android.view.inputmethod.InputMethodManager;

import dalvik.system.CloseGuard;

import java.util.List;

/**
 * Activity container that allows launching activities into itself.
 * <p>Activity launching into this container is restricted by the same rules that apply to launching
 * on VirtualDisplays.
 * @hide
 */
@TestApi
public class ActivityView extends ViewGroup {

    private static final String DISPLAY_NAME = "ActivityViewVirtualDisplay";
    private static final String TAG = "ActivityView";

    private VirtualDisplay mVirtualDisplay;
    private final SurfaceView mSurfaceView;

    /**
     * This is the root surface for the VirtualDisplay. The VirtualDisplay child surfaces will be
     * re-parented to this surface. This will also be a child of the SurfaceView's SurfaceControl.
     */
    private SurfaceControl mRootSurfaceControl;

    private final SurfaceCallback mSurfaceCallback;
    private StateCallback mActivityViewCallback;

    private IActivityTaskManager mActivityTaskManager;
    // Temp container to store view coordinates in window.
    private final int[] mLocationInWindow = new int[2];

    // The latest tap exclude region that we've sent to WM.
    private final Region mTapExcludeRegion = new Region();

    private TaskStackListener mTaskStackListener;

    private final CloseGuard mGuard = CloseGuard.get();
    private boolean mOpened; // Protected by mGuard.

    private final SurfaceControl.Transaction mTmpTransaction = new SurfaceControl.Transaction();

    /** The ActivityView is only allowed to contain one task. */
    private final boolean mSingleTaskInstance;

    private Insets mForwardedInsets;

    public ActivityView(Context context) {
        this(context, null /* attrs */);
    }

    public ActivityView(Context context, AttributeSet attrs) {
        this(context, attrs, 0 /* defStyle */);
    }

    public ActivityView(Context context, AttributeSet attrs, int defStyle) {
        this(context, attrs, defStyle, false /*singleTaskInstance*/);
    }

    public ActivityView(
            Context context, AttributeSet attrs, int defStyle, boolean singleTaskInstance) {
        super(context, attrs, defStyle);
        mSingleTaskInstance = singleTaskInstance;

        mActivityTaskManager = ActivityTaskManager.getService();
        mSurfaceView = new SurfaceView(context);
        mSurfaceCallback = new SurfaceCallback();
        mSurfaceView.getHolder().addCallback(mSurfaceCallback);
        addView(mSurfaceView);

        mOpened = true;
        mGuard.open("release");
    }

    /** Callback that notifies when the container is ready or destroyed. */
    public abstract static class StateCallback {

        /**
         * Called when the container is ready for launching activities. Calling
         * {@link #startActivity(Intent)} prior to this callback will result in an
         * {@link IllegalStateException}.
         *
         * @see #startActivity(Intent)
         */
        public abstract void onActivityViewReady(ActivityView view);

        /**
         * Called when the container can no longer launch activities. Calling
         * {@link #startActivity(Intent)} after this callback will result in an
         * {@link IllegalStateException}.
         *
         * @see #startActivity(Intent)
         */
        public abstract void onActivityViewDestroyed(ActivityView view);

        /**
         * Called when a task is created inside the container.
         * This is a filtered version of {@link TaskStackListener}
         */
        public void onTaskCreated(int taskId, ComponentName componentName) { }

        /**
         * Called when a task is moved to the front of the stack inside the container.
         * This is a filtered version of {@link TaskStackListener}
         */
        public void onTaskMovedToFront(int taskId) { }

        /**
         * Called when a task is about to be removed from the stack inside the container.
         * This is a filtered version of {@link TaskStackListener}
         */
        public void onTaskRemovalStarted(int taskId) { }
    }

    /**
     * Set the callback to be notified about state changes.
     * <p>This class must finish initializing before {@link #startActivity(Intent)} can be called.
     * <p>Note: If the instance was ready prior to this call being made, then
     * {@link StateCallback#onActivityViewReady(ActivityView)} will be called from within
     * this method call.
     *
     * @param callback The callback to report events to.
     *
     * @see StateCallback
     * @see #startActivity(Intent)
     */
    public void setCallback(StateCallback callback) {
        mActivityViewCallback = callback;

        if (mVirtualDisplay != null && mActivityViewCallback != null) {
            mActivityViewCallback.onActivityViewReady(this);
        }
    }

    /**
     * Sets the corner radius for the Activity displayed here. The corners will be
     * cropped from the window painted by the contained Activity.
     *
     * @param cornerRadius the radius for the corners, in pixels
     * @hide
     */
    public void setCornerRadius(float cornerRadius) {
        mSurfaceView.setCornerRadius(cornerRadius);
    }

    /**
     * Launch a new activity into this container.
     * <p>Activity resolved by the provided {@link Intent} must have
     * {@link android.R.attr#resizeableActivity} attribute set to {@code true} in order to be
     * launched here. Also, if activity is not owned by the owner of this container, it must allow
     * embedding and the caller must have permission to embed.
     * <p>Note: This class must finish initializing and
     * {@link StateCallback#onActivityViewReady(ActivityView)} callback must be triggered before
     * this method can be called.
     *
     * @param intent Intent used to launch an activity.
     *
     * @see StateCallback
     * @see #startActivity(PendingIntent)
     */
    public void startActivity(@NonNull Intent intent) {
        final ActivityOptions options = prepareActivityOptions();
        getContext().startActivity(intent, options.toBundle());
    }

    /**
     * Launch a new activity into this container.
     * <p>Activity resolved by the provided {@link Intent} must have
     * {@link android.R.attr#resizeableActivity} attribute set to {@code true} in order to be
     * launched here. Also, if activity is not owned by the owner of this container, it must allow
     * embedding and the caller must have permission to embed.
     * <p>Note: This class must finish initializing and
     * {@link StateCallback#onActivityViewReady(ActivityView)} callback must be triggered before
     * this method can be called.
     *
     * @param intent Intent used to launch an activity.
     * @param user The UserHandle of the user to start this activity for.
     *
     *
     * @see StateCallback
     * @see #startActivity(PendingIntent)
     */
    public void startActivity(@NonNull Intent intent, UserHandle user) {
        final ActivityOptions options = prepareActivityOptions();
        getContext().startActivityAsUser(intent, options.toBundle(), user);
    }

    /**
     * Launch a new activity into this container.
     * <p>Activity resolved by the provided {@link PendingIntent} must have
     * {@link android.R.attr#resizeableActivity} attribute set to {@code true} in order to be
     * launched here. Also, if activity is not owned by the owner of this container, it must allow
     * embedding and the caller must have permission to embed.
     * <p>Note: This class must finish initializing and
     * {@link StateCallback#onActivityViewReady(ActivityView)} callback must be triggered before
     * this method can be called.
     *
     * @param pendingIntent Intent used to launch an activity.
     *
     * @see StateCallback
     * @see #startActivity(Intent)
     */
    public void startActivity(@NonNull PendingIntent pendingIntent) {
        final ActivityOptions options = prepareActivityOptions();
        try {
            pendingIntent.send(null /* context */, 0 /* code */, null /* intent */,
                    null /* onFinished */, null /* handler */, null /* requiredPermission */,
                    options.toBundle());
        } catch (PendingIntent.CanceledException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * Launch a new activity into this container.
     * <p>Activity resolved by the provided {@link PendingIntent} must have
     * {@link android.R.attr#resizeableActivity} attribute set to {@code true} in order to be
     * launched here. Also, if activity is not owned by the owner of this container, it must allow
     * embedding and the caller must have permission to embed.
     * <p>Note: This class must finish initializing and
     * {@link StateCallback#onActivityViewReady(ActivityView)} callback must be triggered before
     * this method can be called.
     *
     * @param pendingIntent Intent used to launch an activity.
     * @param options options for the activity
     *
     * @see StateCallback
     * @see #startActivity(Intent)
     */
    public void startActivity(@NonNull PendingIntent pendingIntent,
            @NonNull ActivityOptions options) {
        options.setLaunchDisplayId(mVirtualDisplay.getDisplay().getDisplayId());
        try {
            pendingIntent.send(null /* context */, 0 /* code */, null /* intent */,
                    null /* onFinished */, null /* handler */, null /* requiredPermission */,
                    options.toBundle());
        } catch (PendingIntent.CanceledException e) {
            throw new RuntimeException(e);
        }
    }

    /**
     * Check if container is ready to launch and create {@link ActivityOptions} to target the
     * virtual display.
     */
    private ActivityOptions prepareActivityOptions() {
        if (mVirtualDisplay == null) {
            throw new IllegalStateException(
                    "Trying to start activity before ActivityView is ready.");
        }
        final ActivityOptions options = ActivityOptions.makeBasic();
        options.setLaunchDisplayId(mVirtualDisplay.getDisplay().getDisplayId());
        return options;
    }

    /**
     * Release this container. Activity launching will no longer be permitted.
     * <p>Note: Calling this method is allowed after
     * {@link StateCallback#onActivityViewReady(ActivityView)} callback was triggered and before
     * {@link StateCallback#onActivityViewDestroyed(ActivityView)}.
     *
     * @see StateCallback
     */
    public void release() {
        if (mVirtualDisplay == null) {
            throw new IllegalStateException(
                    "Trying to release container that is not initialized.");
        }
        performRelease();
    }

    /**
     * Triggers an update of {@link ActivityView}'s location in window to properly set tap exclude
     * regions and avoid focus switches by touches on this view.
     */
    public void onLocationChanged() {
        updateLocationAndTapExcludeRegion();
    }

    private void clearActivityViewGeometryForIme() {
        if (mVirtualDisplay == null) {
            return;
        }
        final int displayId = mVirtualDisplay.getDisplay().getDisplayId();
        mContext.getSystemService(InputMethodManager.class).reportActivityView(displayId, null);
    }

    @Override
    public void onLayout(boolean changed, int l, int t, int r, int b) {
        mSurfaceView.layout(0 /* left */, 0 /* top */, r - l /* right */, b - t /* bottom */);
    }

    @Override
    public boolean gatherTransparentRegion(Region region) {
        // The tap exclude region may be affected by any view on top of it, so we detect the
        // possible change by monitoring this function.
        updateLocationAndTapExcludeRegion();
        return super.gatherTransparentRegion(region);
    }

    /**
     * Sends current location in window and tap exclude region to WM for this view.
     */
    private void updateLocationAndTapExcludeRegion() {
        if (mVirtualDisplay == null || !isAttachedToWindow()) {
            return;
        }
        try {
            int x = mLocationInWindow[0];
            int y = mLocationInWindow[1];
            getLocationInWindow(mLocationInWindow);
            if (x != mLocationInWindow[0] || y != mLocationInWindow[1]) {
                x = mLocationInWindow[0];
                y = mLocationInWindow[1];
                final int displayId = mVirtualDisplay.getDisplay().getDisplayId();
                WindowManagerGlobal.getWindowSession().updateDisplayContentLocation(
                        getWindow(), x, y, displayId);

                // Also report this geometry information to InputMethodManagerService.
                // TODO(b/115693908): Unify this logic into the above WMS-based one.
                final Matrix matrix = new Matrix();
                matrix.set(getMatrix());
                matrix.postTranslate(x, y);
                mContext.getSystemService(InputMethodManager.class)
                        .reportActivityView(displayId, matrix);
            }
            updateTapExcludeRegion(x, y);
        } catch (RemoteException e) {
            e.rethrowAsRuntimeException();
        }
    }

    /** Computes and sends current tap exclude region to WM for this view. */
    private void updateTapExcludeRegion(int x, int y) throws RemoteException {
        if (!canReceivePointerEvents()) {
            cleanTapExcludeRegion();
            return;
        }
        mTapExcludeRegion.set(x, y, x + getWidth(), y + getHeight());

        // There might be views on top of us. We need to subtract those areas from the tap
        // exclude region.
        final ViewParent parent = getParent();
        if (parent != null) {
            parent.subtractObscuredTouchableRegion(mTapExcludeRegion, this);
        }

        WindowManagerGlobal.getWindowSession().updateTapExcludeRegion(getWindow(), hashCode(),
                mTapExcludeRegion);
    }

    private class SurfaceCallback implements SurfaceHolder.Callback {
        @Override
        public void surfaceCreated(SurfaceHolder surfaceHolder) {
            if (mVirtualDisplay == null) {
                initVirtualDisplay(new SurfaceSession());
                if (mVirtualDisplay != null && mActivityViewCallback != null) {
                    mActivityViewCallback.onActivityViewReady(ActivityView.this);
                }
            } else {
                mTmpTransaction.reparent(mRootSurfaceControl,
                        mSurfaceView.getSurfaceControl()).apply();
            }

            if (mVirtualDisplay != null) {
                mVirtualDisplay.setDisplayState(true);
            }

            updateLocationAndTapExcludeRegion();
        }

        @Override
        public void surfaceChanged(SurfaceHolder surfaceHolder, int format, int width, int height) {
            if (mVirtualDisplay != null) {
                mVirtualDisplay.resize(width, height, getBaseDisplayDensity());
            }
            updateLocationAndTapExcludeRegion();
        }

        @Override
        public void surfaceDestroyed(SurfaceHolder surfaceHolder) {
            if (mVirtualDisplay != null) {
                mVirtualDisplay.setDisplayState(false);
            }
            clearActivityViewGeometryForIme();
            cleanTapExcludeRegion();
        }
    }

    @Override
    protected void onVisibilityChanged(View changedView, int visibility) {
        super.onVisibilityChanged(changedView, visibility);
        mSurfaceView.setVisibility(visibility);
    }

    /**
     * @return the display id of the virtual display.
     */
    public int getVirtualDisplayId() {
        if (mVirtualDisplay != null) {
            return mVirtualDisplay.getDisplay().getDisplayId();
        }
        return INVALID_DISPLAY;
    }

    /**
     * Injects a pair of down/up key events with keycode {@link KeyEvent#KEYCODE_BACK} to the
     * virtual display.
     */
    public void performBackPress() {
        if (mVirtualDisplay == null) {
            return;
        }
        final int displayId = mVirtualDisplay.getDisplay().getDisplayId();
        final InputManager im = InputManager.getInstance();
        im.injectInputEvent(createKeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_BACK, displayId),
                InputManager.INJECT_INPUT_EVENT_MODE_ASYNC);
        im.injectInputEvent(createKeyEvent(KeyEvent.ACTION_UP, KeyEvent.KEYCODE_BACK, displayId),
                InputManager.INJECT_INPUT_EVENT_MODE_ASYNC);
    }

    private static KeyEvent createKeyEvent(int action, int code, int displayId) {
        long when = SystemClock.uptimeMillis();
        final KeyEvent ev = new KeyEvent(when, when, action, code, 0 /* repeat */,
                0 /* metaState */, KeyCharacterMap.VIRTUAL_KEYBOARD, 0 /* scancode */,
                KeyEvent.FLAG_FROM_SYSTEM | KeyEvent.FLAG_VIRTUAL_HARD_KEY,
                InputDevice.SOURCE_KEYBOARD);
        ev.setDisplayId(displayId);
        return ev;
    }

    private void initVirtualDisplay(SurfaceSession surfaceSession) {
        if (mVirtualDisplay != null) {
            throw new IllegalStateException("Trying to initialize for the second time.");
        }

        final int width = mSurfaceView.getWidth();
        final int height = mSurfaceView.getHeight();
        final DisplayManager displayManager = mContext.getSystemService(DisplayManager.class);

        mVirtualDisplay = displayManager.createVirtualDisplay(
                DISPLAY_NAME + "@" + System.identityHashCode(this), width, height,
                getBaseDisplayDensity(), null,
                VIRTUAL_DISPLAY_FLAG_PUBLIC | VIRTUAL_DISPLAY_FLAG_OWN_CONTENT_ONLY
                        | VIRTUAL_DISPLAY_FLAG_DESTROY_CONTENT_ON_REMOVAL);
        if (mVirtualDisplay == null) {
            Log.e(TAG, "Failed to initialize ActivityView");
            return;
        }

        final int displayId = mVirtualDisplay.getDisplay().getDisplayId();
        final IWindowManager wm = WindowManagerGlobal.getWindowManagerService();

        mRootSurfaceControl = new SurfaceControl.Builder(surfaceSession)
                .setContainerLayer()
                .setParent(mSurfaceView.getSurfaceControl())
                .setName(DISPLAY_NAME)
                .build();

        try {
            // TODO: Find a way to consolidate these calls to the server.
            WindowManagerGlobal.getWindowSession().reparentDisplayContent(
                    getWindow(), mRootSurfaceControl, displayId);
            wm.dontOverrideDisplayInfo(displayId);
            if (mSingleTaskInstance) {
                mActivityTaskManager.setDisplayToSingleTaskInstance(displayId);
            }
            wm.setForwardedInsets(displayId, mForwardedInsets);
        } catch (RemoteException e) {
            e.rethrowAsRuntimeException();
        }

        mTmpTransaction.show(mRootSurfaceControl).apply();
        mTaskStackListener = new TaskStackListenerImpl();
        try {
            mActivityTaskManager.registerTaskStackListener(mTaskStackListener);
        } catch (RemoteException e) {
            Log.e(TAG, "Failed to register task stack listener", e);
        }
    }

    private void performRelease() {
        if (!mOpened) {
            return;
        }

        mSurfaceView.getHolder().removeCallback(mSurfaceCallback);

        cleanTapExcludeRegion();

        if (mTaskStackListener != null) {
            try {
                mActivityTaskManager.unregisterTaskStackListener(mTaskStackListener);
            } catch (RemoteException e) {
                Log.e(TAG, "Failed to unregister task stack listener", e);
            }
            mTaskStackListener = null;
        }

        final boolean displayReleased;
        if (mVirtualDisplay != null) {
            mVirtualDisplay.release();
            mVirtualDisplay = null;
            displayReleased = true;
        } else {
            displayReleased = false;
        }

        if (displayReleased && mActivityViewCallback != null) {
            mActivityViewCallback.onActivityViewDestroyed(this);
        }

        mGuard.close();
        mOpened = false;
    }

    /** Report to server that tap exclude region on hosting display should be cleared. */
    private void cleanTapExcludeRegion() {
        if (!isAttachedToWindow() || mTapExcludeRegion.isEmpty()) {
            return;
        }
        // Update tap exclude region with a null region to clean the state on server.
        try {
            WindowManagerGlobal.getWindowSession().updateTapExcludeRegion(getWindow(), hashCode(),
                    null /* region */);
            mTapExcludeRegion.setEmpty();
        } catch (RemoteException e) {
            e.rethrowAsRuntimeException();
        }
    }

    /** Get density of the hosting display. */
    private int getBaseDisplayDensity() {
        final WindowManager wm = mContext.getSystemService(WindowManager.class);
        final DisplayMetrics metrics = new DisplayMetrics();
        wm.getDefaultDisplay().getMetrics(metrics);
        return metrics.densityDpi;
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            if (mGuard != null) {
                mGuard.warnIfOpen();
                performRelease();
            }
        } finally {
            super.finalize();
        }
    }

    /**
     * Set forwarded insets on the virtual display.
     *
     * @see IWindowManager#setForwardedInsets
     */
    public void setForwardedInsets(Insets insets) {
        mForwardedInsets = insets;
        if (mVirtualDisplay == null) {
            return;
        }
        try {
            final IWindowManager wm = WindowManagerGlobal.getWindowManagerService();
            wm.setForwardedInsets(mVirtualDisplay.getDisplay().getDisplayId(), mForwardedInsets);
        } catch (RemoteException e) {
            e.rethrowAsRuntimeException();
        }
    }

    /**
     * A task change listener that detects background color change of the topmost stack on our
     * virtual display and updates the background of the surface view. This background will be shown
     * when surface view is resized, but the app hasn't drawn its content in new size yet.
     * It also calls StateCallback.onTaskMovedToFront to notify interested parties that the stack
     * associated with the {@link ActivityView} has had a Task moved to the front. This is useful
     * when needing to also bring the host Activity to the foreground at the same time.
     */
    private class TaskStackListenerImpl extends TaskStackListener {

        @Override
        public void onTaskDescriptionChanged(ActivityManager.RunningTaskInfo taskInfo)
                throws RemoteException {
            if (mVirtualDisplay == null
                    || taskInfo.displayId != mVirtualDisplay.getDisplay().getDisplayId()) {
                return;
            }

            StackInfo stackInfo = getTopMostStackInfo();
            if (stackInfo == null) {
                return;
            }
            // Found the topmost stack on target display. Now check if the topmost task's
            // description changed.
            if (taskInfo.taskId == stackInfo.taskIds[stackInfo.taskIds.length - 1]) {
                mSurfaceView.setResizeBackgroundColor(
                        taskInfo.taskDescription.getBackgroundColor());
            }
        }

        @Override
        public void onTaskMovedToFront(ActivityManager.RunningTaskInfo taskInfo)
                throws RemoteException {
            if (mActivityViewCallback  == null || mVirtualDisplay == null
                    || taskInfo.displayId != mVirtualDisplay.getDisplay().getDisplayId()) {
                return;
            }

            StackInfo stackInfo = getTopMostStackInfo();
            // if StackInfo was null or unrelated to the "move to front" then there's no use
            // notifying the callback
            if (stackInfo != null
                    && taskInfo.taskId == stackInfo.taskIds[stackInfo.taskIds.length - 1]) {
                mActivityViewCallback.onTaskMovedToFront(taskInfo.taskId);
            }
        }

        @Override
        public void onTaskCreated(int taskId, ComponentName componentName) throws RemoteException {
            if (mActivityViewCallback == null || mVirtualDisplay == null) {
                return;
            }

            StackInfo stackInfo = getTopMostStackInfo();
            // if StackInfo was null or unrelated to the task creation then there's no use
            // notifying the callback
            if (stackInfo != null
                    && taskId == stackInfo.taskIds[stackInfo.taskIds.length - 1]) {
                mActivityViewCallback.onTaskCreated(taskId, componentName);
            }
        }

        @Override
        public void onTaskRemovalStarted(ActivityManager.RunningTaskInfo taskInfo)
                throws RemoteException {
            if (mActivityViewCallback == null || mVirtualDisplay == null
                    || taskInfo.displayId != mVirtualDisplay.getDisplay().getDisplayId()) {
                return;
            }
            mActivityViewCallback.onTaskRemovalStarted(taskInfo.taskId);
        }

        private StackInfo getTopMostStackInfo() throws RemoteException {
            // Find the topmost task on our virtual display - it will define the background
            // color of the surface view during resizing.
            final int displayId = mVirtualDisplay.getDisplay().getDisplayId();
            final List<StackInfo> stackInfoList = mActivityTaskManager.getAllStackInfos();

            // Iterate through stacks from top to bottom.
            final int stackCount = stackInfoList.size();
            for (int i = 0; i < stackCount; i++) {
                final StackInfo stackInfo = stackInfoList.get(i);
                // Only look for stacks on our virtual display.
                if (stackInfo.displayId != displayId) {
                    continue;
                }
                // Found the topmost stack on target display.
                return stackInfo;
            }
            return null;
        }
    }
}