xref: /aosp12/frameworks/base/services/accessibility/java/com/android/server/accessibility/AccessibilityInputFilter.java

/*
 * Copyright (C) 2011 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.server.accessibility;

import static android.view.WindowManager.LayoutParams.TYPE_ACCESSIBILITY_MAGNIFICATION_OVERLAY;
import static android.view.WindowManager.LayoutParams.TYPE_MAGNIFICATION_OVERLAY;

import android.accessibilityservice.AccessibilityTrace;
import android.annotation.MainThread;
import android.annotation.NonNull;
import android.content.Context;
import android.graphics.Region;
import android.os.PowerManager;
import android.provider.Settings;
import android.util.Slog;
import android.util.SparseArray;
import android.util.SparseBooleanArray;
import android.view.Display;
import android.view.InputDevice;
import android.view.InputEvent;
import android.view.InputFilter;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.accessibility.AccessibilityEvent;

import com.android.server.LocalServices;
import com.android.server.accessibility.gestures.TouchExplorer;
import com.android.server.accessibility.magnification.FullScreenMagnificationGestureHandler;
import com.android.server.accessibility.magnification.MagnificationGestureHandler;
import com.android.server.accessibility.magnification.WindowMagnificationGestureHandler;
import com.android.server.accessibility.magnification.WindowMagnificationPromptController;
import com.android.server.policy.WindowManagerPolicy;

import java.io.FileDescriptor;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringJoiner;

/**
 * This class is an input filter for implementing accessibility features such
 * as display magnification and explore by touch.
 *
 * NOTE: This class has to be created and poked only from the main thread.
 */
class AccessibilityInputFilter extends InputFilter implements EventStreamTransformation {

    private static final String TAG = AccessibilityInputFilter.class.getSimpleName();

    private static final boolean DEBUG = false;

    /**
     * Flag for enabling the screen magnification feature.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_SCREEN_MAGNIFIER = 0x00000001;

    /**
     * Flag for enabling the touch exploration feature.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_TOUCH_EXPLORATION = 0x00000002;

    /**
     * Flag for enabling the filtering key events feature.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_FILTER_KEY_EVENTS = 0x00000004;

    /**
     * Flag for enabling "Automatically click on mouse stop" feature.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_AUTOCLICK = 0x00000008;

    /**
     * Flag for enabling motion event injection.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_INJECT_MOTION_EVENTS = 0x00000010;

    /**
     * Flag for enabling the feature to control the screen magnifier. If
     * {@link #FLAG_FEATURE_SCREEN_MAGNIFIER} is set this flag is ignored
     * as the screen magnifier feature performs a super set of the work
     * performed by this feature.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_FEATURE_CONTROL_SCREEN_MAGNIFIER = 0x00000020;

    /**
     * Flag for enabling the feature to trigger the screen magnifier
     * from another on-device interaction.
     */
    static final int FLAG_FEATURE_TRIGGERED_SCREEN_MAGNIFIER = 0x00000040;

    /**
     * Flag for dispatching double tap and double tap and hold to the service.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_SERVICE_HANDLES_DOUBLE_TAP = 0x00000080;

/**
     * Flag for enabling multi-finger gestures.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_REQUEST_MULTI_FINGER_GESTURES = 0x00000100;

    /**
     * Flag for enabling two-finger passthrough when multi-finger gestures are enabled.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_REQUEST_2_FINGER_PASSTHROUGH = 0x00000200;

    /**
     * Flag for including motion events when dispatching a gesture.
     *
     * @see #setUserAndEnabledFeatures(int, int)
     */
    static final int FLAG_SEND_MOTION_EVENTS = 0x00000400;

    static final int FEATURES_AFFECTING_MOTION_EVENTS =
            FLAG_FEATURE_INJECT_MOTION_EVENTS
                    | FLAG_FEATURE_AUTOCLICK
                    | FLAG_FEATURE_TOUCH_EXPLORATION
                    | FLAG_FEATURE_SCREEN_MAGNIFIER
                    | FLAG_FEATURE_TRIGGERED_SCREEN_MAGNIFIER
                    | FLAG_SERVICE_HANDLES_DOUBLE_TAP
                    | FLAG_REQUEST_MULTI_FINGER_GESTURES
                    | FLAG_REQUEST_2_FINGER_PASSTHROUGH;

    private final Context mContext;

    private final PowerManager mPm;

    private final AccessibilityManagerService mAms;

    private final SparseArray<EventStreamTransformation> mEventHandler;

    private final SparseArray<TouchExplorer> mTouchExplorer = new SparseArray<>(0);

    private final SparseArray<MagnificationGestureHandler> mMagnificationGestureHandler =
            new SparseArray<>(0);

    private final SparseArray<MotionEventInjector> mMotionEventInjectors = new SparseArray<>(0);

    private AutoclickController mAutoclickController;

    private KeyboardInterceptor mKeyboardInterceptor;

    private boolean mInstalled;

    private int mUserId;

    private int mEnabledFeatures;

    private final SparseArray<EventStreamState> mMouseStreamStates = new SparseArray<>(0);

    private final SparseArray<EventStreamState> mTouchScreenStreamStates = new SparseArray<>(0);

    private EventStreamState mKeyboardStreamState;

    AccessibilityInputFilter(Context context, AccessibilityManagerService service) {
        this(context, service, new SparseArray<>(0));
    }

    AccessibilityInputFilter(Context context, AccessibilityManagerService service,
            SparseArray<EventStreamTransformation> eventHandler) {
        super(context.getMainLooper());
        mContext = context;
        mAms = service;
        mPm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        mEventHandler = eventHandler;
    }

    @Override
    public void onInstalled() {
        if (DEBUG) {
            Slog.d(TAG, "Accessibility input filter installed.");
        }
        mInstalled = true;
        disableFeatures();
        enableFeatures();
        super.onInstalled();
    }

    @Override
    public void onUninstalled() {
        if (DEBUG) {
            Slog.d(TAG, "Accessibility input filter uninstalled.");
        }
        mInstalled = false;
        disableFeatures();
        super.onUninstalled();
    }

    void onDisplayAdded(@NonNull Display display) {
        if (mInstalled) {
            resetStreamStateForDisplay(display.getDisplayId());
            enableFeaturesForDisplay(display);
        }
    }

    void onDisplayRemoved(int displayId) {
        if (mInstalled) {
            disableFeaturesForDisplay(displayId);
            resetStreamStateForDisplay(displayId);
        }
    }

    @Override
    public void onInputEvent(InputEvent event, int policyFlags) {
        if (DEBUG) {
            Slog.d(TAG, "Received event: " + event + ", policyFlags=0x"
                    + Integer.toHexString(policyFlags));
        }
        if (mAms.getTraceManager().isA11yTracingEnabledForTypes(
                AccessibilityTrace.FLAGS_INPUT_FILTER)) {
            mAms.getTraceManager().logTrace(TAG + ".onInputEvent",
                    AccessibilityTrace.FLAGS_INPUT_FILTER,
                    "event=" + event + ";policyFlags=" + policyFlags);
        }
        if (mEventHandler.size() == 0) {
            if (DEBUG) Slog.d(TAG, "No mEventHandler for event " + event);
            super.onInputEvent(event, policyFlags);
            return;
        }

        EventStreamState state = getEventStreamState(event);
        if (state == null) {
            super.onInputEvent(event, policyFlags);
            return;
        }

        final int eventSource = event.getSource();
        final int displayId = event.getDisplayId();
        if ((policyFlags & WindowManagerPolicy.FLAG_PASS_TO_USER) == 0) {
            state.reset();
            clearEventStreamHandler(displayId, eventSource);
            super.onInputEvent(event, policyFlags);
            return;
        }

        if (state.updateInputSource(event.getSource())) {
            clearEventStreamHandler(displayId, eventSource);
        }

        if (!state.inputSourceValid()) {
            super.onInputEvent(event, policyFlags);
            return;
        }

        if (event instanceof MotionEvent) {
            if ((mEnabledFeatures & FEATURES_AFFECTING_MOTION_EVENTS) != 0) {
                MotionEvent motionEvent = (MotionEvent) event;
                processMotionEvent(state, motionEvent, policyFlags);
                return;
            } else {
                super.onInputEvent(event, policyFlags);
            }
        } else if (event instanceof KeyEvent) {
            KeyEvent keyEvent = (KeyEvent) event;
            processKeyEvent(state, keyEvent, policyFlags);
        }
    }

    /**
     * Gets current event stream state associated with an input event.
     * @return The event stream state that should be used for the event. Null if the event should
     *     not be handled by #AccessibilityInputFilter.
     */
    private EventStreamState getEventStreamState(InputEvent event) {
        if (event instanceof MotionEvent) {
            final int displayId = event.getDisplayId();

            if (event.isFromSource(InputDevice.SOURCE_TOUCHSCREEN)) {
                EventStreamState touchScreenStreamState = mTouchScreenStreamStates.get(displayId);
                if (touchScreenStreamState == null) {
                    touchScreenStreamState = new TouchScreenEventStreamState();
                    mTouchScreenStreamStates.put(displayId, touchScreenStreamState);
                }
                return touchScreenStreamState;
            }
            if (event.isFromSource(InputDevice.SOURCE_MOUSE)) {
                EventStreamState mouseStreamState = mMouseStreamStates.get(displayId);
                if (mouseStreamState == null) {
                    mouseStreamState = new MouseEventStreamState();
                    mMouseStreamStates.put(displayId, mouseStreamState);
                }
                return mouseStreamState;
            }
        } else if (event instanceof KeyEvent) {
            if (event.isFromSource(InputDevice.SOURCE_KEYBOARD)) {
                if (mKeyboardStreamState == null) {
                    mKeyboardStreamState = new KeyboardEventStreamState();
                }
                return mKeyboardStreamState;
            }
        }
        return null;
    }

    private void clearEventStreamHandler(int displayId, int eventSource) {
        final EventStreamTransformation eventHandler = mEventHandler.get(displayId);
        if (eventHandler != null) {
            eventHandler.clearEvents(eventSource);
        }
    }

    private void processMotionEvent(EventStreamState state, MotionEvent event, int policyFlags) {
        if (!state.shouldProcessScroll() && event.getActionMasked() == MotionEvent.ACTION_SCROLL) {
            super.onInputEvent(event, policyFlags);
            return;
        }

        if (!state.shouldProcessMotionEvent(event)) {
            return;
        }

        handleMotionEvent(event, policyFlags);
    }

    private void processKeyEvent(EventStreamState state, KeyEvent event, int policyFlags) {
        if (!state.shouldProcessKeyEvent(event)) {
            super.onInputEvent(event, policyFlags);
            return;
        }
        // Since the display id of KeyEvent always would be -1 and there is only one
        // KeyboardInterceptor for all display, pass KeyEvent to the mEventHandler of
        // DEFAULT_DISPLAY to handle.
        mEventHandler.get(Display.DEFAULT_DISPLAY).onKeyEvent(event, policyFlags);
    }

    private void handleMotionEvent(MotionEvent event, int policyFlags) {
        if (DEBUG) {
            Slog.i(TAG, "Handling motion event: " + event + ", policyFlags: " + policyFlags);
        }
        mPm.userActivity(event.getEventTime(), false);
        MotionEvent transformedEvent = MotionEvent.obtain(event);
        final int displayId = event.getDisplayId();
        mEventHandler.get(isDisplayIdValid(displayId) ? displayId : Display.DEFAULT_DISPLAY)
                .onMotionEvent(transformedEvent, event, policyFlags);
        transformedEvent.recycle();
    }

    private boolean isDisplayIdValid(int displayId) {
        return mEventHandler.get(displayId) != null;
    }

    @Override
    public void onMotionEvent(MotionEvent transformedEvent, MotionEvent rawEvent,
            int policyFlags) {
        sendInputEvent(transformedEvent, policyFlags);
    }

    @Override
    public void onKeyEvent(KeyEvent event, int policyFlags) {
        sendInputEvent(event, policyFlags);
    }

    @Override
    public void onAccessibilityEvent(AccessibilityEvent event) {
        // TODO Implement this to inject the accessibility event
        //      into the accessibility manager service similarly
        //      to how this is done for input events.
    }

    @Override
    public void setNext(EventStreamTransformation sink) {
        /* do nothing */
    }

    @Override
    public EventStreamTransformation getNext() {
        return null;
    }

    @Override
    public void clearEvents(int inputSource) {
        /* do nothing */
    }

    void setUserAndEnabledFeatures(int userId, int enabledFeatures) {
        if (DEBUG) {
            Slog.i(TAG, "setUserAndEnabledFeatures(userId = " + userId + ", enabledFeatures = 0x"
                    + Integer.toHexString(enabledFeatures) + ")");
        }
        if (mEnabledFeatures == enabledFeatures && mUserId == userId) {
            return;
        }
        if (mInstalled) {
            disableFeatures();
        }
        mUserId = userId;
        mEnabledFeatures = enabledFeatures;
        if (mInstalled) {
            enableFeatures();
        }
    }

    void notifyAccessibilityEvent(AccessibilityEvent event) {
        for (int i = 0; i < mEventHandler.size(); i++) {
            final EventStreamTransformation eventHandler = mEventHandler.valueAt(i);
            if (eventHandler != null) {
                eventHandler.onAccessibilityEvent(event);
            }
        }
    }

    void notifyAccessibilityButtonClicked(int displayId) {
        if (mMagnificationGestureHandler.size() != 0) {
            final MagnificationGestureHandler handler = mMagnificationGestureHandler.get(displayId);
            if (handler != null) {
                handler.notifyShortcutTriggered();
            }
        }
    }

    private void enableFeatures() {
        if (DEBUG) Slog.i(TAG, "enableFeatures()");

        resetAllStreamState();

        final ArrayList<Display> displaysList = mAms.getValidDisplayList();

        for (int i = displaysList.size() - 1; i >= 0; i--) {
            enableFeaturesForDisplay(displaysList.get(i));
        }
        enableDisplayIndependentFeatures();
    }

    private void enableFeaturesForDisplay(Display display) {
        if (DEBUG) {
            Slog.i(TAG, "enableFeaturesForDisplay() : display Id = " + display.getDisplayId());
        }

        final Context displayContext = mContext.createDisplayContext(display);
        final int displayId = display.getDisplayId();

        if ((mEnabledFeatures & FLAG_FEATURE_AUTOCLICK) != 0) {
            if (mAutoclickController == null) {
                mAutoclickController = new AutoclickController(
                        mContext, mUserId, mAms.getTraceManager());
            }
            addFirstEventHandler(displayId, mAutoclickController);
        }

        if ((mEnabledFeatures & FLAG_FEATURE_TOUCH_EXPLORATION) != 0) {
            TouchExplorer explorer = new TouchExplorer(displayContext, mAms);
            if ((mEnabledFeatures & FLAG_SERVICE_HANDLES_DOUBLE_TAP) != 0) {
                explorer.setServiceHandlesDoubleTap(true);
            }
            if ((mEnabledFeatures & FLAG_REQUEST_MULTI_FINGER_GESTURES) != 0) {
                explorer.setMultiFingerGesturesEnabled(true);
            }
            if ((mEnabledFeatures & FLAG_REQUEST_2_FINGER_PASSTHROUGH) != 0) {
                explorer.setTwoFingerPassthroughEnabled(true);
            }
            if ((mEnabledFeatures & FLAG_SEND_MOTION_EVENTS) != 0) {
                explorer.setSendMotionEventsEnabled(true);
            }
            addFirstEventHandler(displayId, explorer);
            mTouchExplorer.put(displayId, explorer);
        }

        if ((mEnabledFeatures & FLAG_FEATURE_CONTROL_SCREEN_MAGNIFIER) != 0
                || ((mEnabledFeatures & FLAG_FEATURE_SCREEN_MAGNIFIER) != 0)
                || ((mEnabledFeatures & FLAG_FEATURE_TRIGGERED_SCREEN_MAGNIFIER) != 0)) {
            final MagnificationGestureHandler magnificationGestureHandler =
                    createMagnificationGestureHandler(displayId,
                            displayContext);
            addFirstEventHandler(displayId, magnificationGestureHandler);
            mMagnificationGestureHandler.put(displayId, magnificationGestureHandler);
        }

        if ((mEnabledFeatures & FLAG_FEATURE_INJECT_MOTION_EVENTS) != 0) {
            MotionEventInjector injector = new MotionEventInjector(
                    mContext.getMainLooper(), mAms.getTraceManager());
            addFirstEventHandler(displayId, injector);
            mMotionEventInjectors.put(displayId, injector);
        }
    }

    private void enableDisplayIndependentFeatures() {
        if ((mEnabledFeatures & FLAG_FEATURE_INJECT_MOTION_EVENTS) != 0) {
            mAms.setMotionEventInjectors(mMotionEventInjectors);
        }

        if ((mEnabledFeatures & FLAG_FEATURE_FILTER_KEY_EVENTS) != 0) {
            mKeyboardInterceptor = new KeyboardInterceptor(mAms,
                    LocalServices.getService(WindowManagerPolicy.class));
            // Since the display id of KeyEvent always would be -1 and it would be dispatched to
            // the display with input focus directly, we only need one KeyboardInterceptor for
            // default display.
            addFirstEventHandler(Display.DEFAULT_DISPLAY, mKeyboardInterceptor);
        }
    }

    /**
     * Adds an event handler to the event handler chain for giving display. The handler is added at
     * the beginning of the chain.
     *
     * @param displayId The logical display id.
     * @param handler The handler to be added to the event handlers list.
     */
    private void addFirstEventHandler(int displayId, EventStreamTransformation handler) {
        EventStreamTransformation eventHandler = mEventHandler.get(displayId);
        if (eventHandler != null) {
            handler.setNext(eventHandler);
        } else {
            handler.setNext(this);
        }
        eventHandler = handler;
        mEventHandler.put(displayId, eventHandler);
    }

    private void disableFeatures() {
        final ArrayList<Display> displaysList = mAms.getValidDisplayList();

        for (int i = displaysList.size() - 1; i >= 0; i--) {
            disableFeaturesForDisplay(displaysList.get(i).getDisplayId());
        }
        mAms.setMotionEventInjectors(null);
        disableDisplayIndependentFeatures();

        resetAllStreamState();
    }

    private void disableFeaturesForDisplay(int displayId) {
        if (DEBUG) {
            Slog.i(TAG, "disableFeaturesForDisplay() : display Id = " + displayId);
        }

        final MotionEventInjector injector = mMotionEventInjectors.get(displayId);
        if (injector != null) {
            injector.onDestroy();
            mMotionEventInjectors.remove(displayId);
        }

        final TouchExplorer explorer = mTouchExplorer.get(displayId);
        if (explorer != null) {
            explorer.onDestroy();
            mTouchExplorer.remove(displayId);
        }

        final MagnificationGestureHandler handler = mMagnificationGestureHandler.get(displayId);
        if (handler != null) {
            handler.onDestroy();
            mMagnificationGestureHandler.remove(displayId);
        }

        final EventStreamTransformation eventStreamTransformation = mEventHandler.get(displayId);
        if (eventStreamTransformation != null) {
            mEventHandler.remove(displayId);
        }
    }

    private void disableDisplayIndependentFeatures() {
        if (mAutoclickController != null) {
            mAutoclickController.onDestroy();
            mAutoclickController = null;
        }

        if (mKeyboardInterceptor != null) {
            mKeyboardInterceptor.onDestroy();
            mKeyboardInterceptor = null;
        }
    }

    private MagnificationGestureHandler createMagnificationGestureHandler(
            int displayId, Context displayContext) {
        final boolean detectControlGestures = (mEnabledFeatures
                & FLAG_FEATURE_SCREEN_MAGNIFIER) != 0;
        final boolean triggerable = (mEnabledFeatures
                & FLAG_FEATURE_TRIGGERED_SCREEN_MAGNIFIER) != 0;
        MagnificationGestureHandler magnificationGestureHandler;
        if (mAms.getMagnificationMode(displayId)
                == Settings.Secure.ACCESSIBILITY_MAGNIFICATION_MODE_WINDOW) {
            final Context uiContext = displayContext.createWindowContext(
                    TYPE_ACCESSIBILITY_MAGNIFICATION_OVERLAY, null /* options */);
            magnificationGestureHandler = new WindowMagnificationGestureHandler(uiContext,
                    mAms.getWindowMagnificationMgr(), mAms.getTraceManager(),
                    mAms.getMagnificationController(), detectControlGestures, triggerable,
                    displayId);
        } else {
            final Context uiContext = displayContext.createWindowContext(
                    TYPE_MAGNIFICATION_OVERLAY, null /* options */);
            magnificationGestureHandler = new FullScreenMagnificationGestureHandler(uiContext,
                    mAms.getFullScreenMagnificationController(), mAms.getTraceManager(),
                    mAms.getMagnificationController(), detectControlGestures, triggerable,
                    new WindowMagnificationPromptController(displayContext, mUserId), displayId);
        }
        return magnificationGestureHandler;
    }

    void resetAllStreamState() {
        final ArrayList<Display> displaysList = mAms.getValidDisplayList();

        for (int i = displaysList.size() - 1; i >= 0; i--) {
            resetStreamStateForDisplay(displaysList.get(i).getDisplayId());
        }

        if (mKeyboardStreamState != null) {
            mKeyboardStreamState.reset();
        }
    }

    void resetStreamStateForDisplay(int displayId) {
        final EventStreamState touchScreenStreamState = mTouchScreenStreamStates.get(displayId);
        if (touchScreenStreamState != null) {
            touchScreenStreamState.reset();
            mTouchScreenStreamStates.remove(displayId);
        }

        final EventStreamState mouseStreamState = mMouseStreamStates.get(displayId);
        if (mouseStreamState != null) {
            mouseStreamState.reset();
            mMouseStreamStates.remove(displayId);
        }
    }

    @Override
    public void onDestroy() {
        /* ignore */
    }

    /**
     * Called to refresh the magnification mode on the given display.
     * It's responsible for changing {@link MagnificationGestureHandler} based on the current mode.
     *
     * @param display The logical display
     */
    @MainThread
    public void refreshMagnificationMode(Display display) {
        final int displayId = display.getDisplayId();
        final MagnificationGestureHandler magnificationGestureHandler =
                mMagnificationGestureHandler.get(displayId);
        if (magnificationGestureHandler == null) {
            return;
        }
        if (magnificationGestureHandler.getMode() == mAms.getMagnificationMode(displayId)) {
            return;
        }
        magnificationGestureHandler.onDestroy();
        final MagnificationGestureHandler currentMagnificationGestureHandler =
                createMagnificationGestureHandler(displayId,
                        mContext.createDisplayContext(display));
        switchEventStreamTransformation(displayId, magnificationGestureHandler,
                currentMagnificationGestureHandler);
        mMagnificationGestureHandler.put(displayId, currentMagnificationGestureHandler);
    }

    @MainThread
    private void switchEventStreamTransformation(int displayId,
            EventStreamTransformation oldStreamTransformation,
            EventStreamTransformation currentStreamTransformation) {
        EventStreamTransformation eventStreamTransformation = mEventHandler.get(displayId);
        if (eventStreamTransformation == null) {
            return;
        }
        if (eventStreamTransformation == oldStreamTransformation) {
            currentStreamTransformation.setNext(oldStreamTransformation.getNext());
            mEventHandler.put(displayId, currentStreamTransformation);
        } else {
            while (eventStreamTransformation != null) {
                if (eventStreamTransformation.getNext() == oldStreamTransformation) {
                    eventStreamTransformation.setNext(currentStreamTransformation);
                    currentStreamTransformation.setNext(oldStreamTransformation.getNext());
                    return;
                } else {
                    eventStreamTransformation = eventStreamTransformation.getNext();
                }
            }
        }
    }

    /**
     * Keeps state of event streams observed for an input device with a certain source.
     * Provides information about whether motion and key events should be processed by accessibility
     * #EventStreamTransformations. Base implementation describes behaviour for event sources that
     * whose events should not be handled by a11y event stream transformations.
     */
    private static class EventStreamState {
        private int mSource;

        EventStreamState() {
            mSource = -1;
        }

        /**
         * Updates the input source of the device associated with the state. If the source changes,
         * resets internal state.
         *
         * @param source Updated input source.
         * @return Whether the input source has changed.
         */
        public boolean updateInputSource(int source) {
            if (mSource == source) {
                return false;
            }
            // Reset clears internal state, so make sure it's called before |mSource| is updated.
            reset();
            mSource = source;
            return true;
        }

        /**
         * @return Whether input source is valid.
         */
        public boolean inputSourceValid() {
            return mSource >= 0;
        }

        /**
         * Resets the event stream state.
         */
        public void reset() {
            mSource = -1;
        }

        /**
         * @return Whether scroll events for device should be handled by event transformations.
         */
        public boolean shouldProcessScroll() {
            return false;
        }

        /**
         * @param event An observed motion event.
         * @return Whether the event should be handled by event transformations.
         */
        public boolean shouldProcessMotionEvent(MotionEvent event) {
            return false;
        }

        /**
         * @param event An observed key event.
         * @return Whether the event should be handled by event transformations.
         */
        public boolean shouldProcessKeyEvent(KeyEvent event) {
            return false;
        }
    }

    /**
     * Keeps state of stream of events from a mouse device.
     */
    private static class MouseEventStreamState extends EventStreamState {
        private boolean mMotionSequenceStarted;

        public MouseEventStreamState() {
            reset();
        }

        @Override
        final public void reset() {
            super.reset();
            mMotionSequenceStarted = false;
        }

        @Override
        final public boolean shouldProcessScroll() {
            return true;
        }

        @Override
        final public boolean shouldProcessMotionEvent(MotionEvent event) {
            if (mMotionSequenceStarted) {
                return true;
            }
            // Wait for down or move event to start processing mouse events.
            int action = event.getActionMasked();
            mMotionSequenceStarted =
                    action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_HOVER_MOVE;
            return mMotionSequenceStarted;
        }
    }

    /**
     * Keeps state of stream of events from a touch screen device.
     */
    private static class TouchScreenEventStreamState extends EventStreamState {
        private boolean mTouchSequenceStarted;
        private boolean mHoverSequenceStarted;

        public TouchScreenEventStreamState() {
            reset();
        }

        @Override
        final public void reset() {
            super.reset();
            mTouchSequenceStarted = false;
            mHoverSequenceStarted = false;
        }

        @Override
        final public boolean shouldProcessMotionEvent(MotionEvent event) {
            // Wait for a down touch event to start processing.
            if (event.isTouchEvent()) {
                if (mTouchSequenceStarted) {
                    return true;
                }
                mTouchSequenceStarted = event.getActionMasked() == MotionEvent.ACTION_DOWN;
                return mTouchSequenceStarted;
            }

            // Wait for an enter hover event to start processing.
            if (mHoverSequenceStarted) {
                return true;
            }
            mHoverSequenceStarted = event.getActionMasked() == MotionEvent.ACTION_HOVER_ENTER;
            return mHoverSequenceStarted;
        }
    }

    /**
     * Keeps state of streams of events from all keyboard devices.
     */
    private static class KeyboardEventStreamState extends EventStreamState {
        private SparseBooleanArray mEventSequenceStartedMap = new SparseBooleanArray();

        public KeyboardEventStreamState() {
            reset();
        }

        @Override
        final public void reset() {
            super.reset();
            mEventSequenceStartedMap.clear();
        }

        /*
         * Key events from different devices may be interleaved. For example, the volume up and
         * down keys can come from different input sources.
         */
        @Override
        public boolean updateInputSource(int deviceId) {
            return false;
        }

        // We manage all input source simultaneously; there is no concept of validity.
        @Override
        public boolean inputSourceValid() {
            return true;
        }

        @Override
        final public boolean shouldProcessKeyEvent(KeyEvent event) {
            // For each keyboard device, wait for a down event from a device to start processing
            int deviceId = event.getDeviceId();
            if (mEventSequenceStartedMap.get(deviceId, false)) {
                return true;
            }
            boolean shouldProcess = event.getAction() == KeyEvent.ACTION_DOWN;
            mEventSequenceStartedMap.put(deviceId, shouldProcess);
            return shouldProcess;
        }
    }

    public void setGestureDetectionPassthroughRegion(int displayId, Region region) {
        if (region != null && mTouchExplorer.contains(displayId)) {
            mTouchExplorer.get(displayId).setGestureDetectionPassthroughRegion(region);
        }
    }

    public void setTouchExplorationPassthroughRegion(int displayId, Region region) {
        if (region != null && mTouchExplorer.contains(displayId)) {
            mTouchExplorer.get(displayId).setTouchExplorationPassthroughRegion(region);
        }
    }

    /**
     * Dumps all {@link AccessibilityInputFilter}s here.
     */
    public void dump(FileDescriptor fd, final PrintWriter pw, String[] args) {
        if (mEventHandler == null) {
            return;
        }
        pw.append("A11yInputFilter Info : ");
        pw.println();

        final ArrayList<Display> displaysList = mAms.getValidDisplayList();
        for (int i = 0; i < displaysList.size(); i++) {
            final int displayId = displaysList.get(i).getDisplayId();
            EventStreamTransformation next = mEventHandler.get(displayId);
            if (next != null) {
                pw.append("Enabled features of Display [");
                pw.append(Integer.toString(displayId));
                pw.append("] = ");

                final StringJoiner joiner = new StringJoiner(",", "[", "]");

                while (next != null) {
                    if (next instanceof MagnificationGestureHandler) {
                        joiner.add("MagnificationGesture");
                    } else if (next instanceof KeyboardInterceptor) {
                        joiner.add("KeyboardInterceptor");
                    } else if (next instanceof TouchExplorer) {
                        joiner.add("TouchExplorer");
                    } else if (next instanceof AutoclickController) {
                        joiner.add("AutoclickController");
                    } else if (next instanceof MotionEventInjector) {
                        joiner.add("MotionEventInjector");
                    }
                    next = next.getNext();
                }
                pw.append(joiner.toString());
            }
            pw.println();
        }
    }
}