Differential motion fling for platform widgets

This change adds a DifferentialMotionFlingHelper class to control
flinging from differential motion, and uses it in AbsListView and
ScrollView.

The helper class is currently hidden. Unless we have a strong incentive
to open up the API for developers, we will keep the API hidden.

Demo video: http://shortn/_0quHhjPtCV

Interaction with stretch effect is also demonstrated in the demo.

Bug: 293332089
Test: manual, DifferentialMotionFlingHelperTest
Test: no CTS regression for ScrollViewTest, ListViewTest, GridViewTest
Change-Id: I7acd1aa63da4c64d72cca3c8ac6e10dea1f5bd0b

core/java/android/widget/AbsListView.java

@@ -916,6 +916,8 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
         }
     }
 
+    private DifferentialMotionFlingHelper mDifferentialMotionFlingHelper;
+
     public AbsListView(Context context) {
         super(context);
         setupDeviceConfigProperties();
@@ -4488,17 +4490,22 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
     public boolean onGenericMotionEvent(MotionEvent event) {
         switch (event.getAction()) {
             case MotionEvent.ACTION_SCROLL:
-                final float axisValue;
+                final int axis;
                 if (event.isFromSource(InputDevice.SOURCE_CLASS_POINTER)) {
-                    axisValue = event.getAxisValue(MotionEvent.AXIS_VSCROLL);
+                    axis = MotionEvent.AXIS_VSCROLL;
                 } else if (event.isFromSource(InputDevice.SOURCE_ROTARY_ENCODER)) {
-                    axisValue = event.getAxisValue(MotionEvent.AXIS_SCROLL);
+                    axis = MotionEvent.AXIS_SCROLL;
                 } else {
-                    axisValue = 0;
+                    axis = -1;
                 }
 
+                final float axisValue = (axis == -1) ? 0 : event.getAxisValue(axis);
                 final int delta = Math.round(axisValue * mVerticalScrollFactor);
                 if (delta != 0) {
+                    // Tracks whether or not we should attempt fling for this event.
+                    // Fling should not be attempted if the view is already at the limit of scroll,
+                    // since it conflicts with EdgeEffect.
+                    boolean shouldAttemptFling = true;
                     // If we're moving down, we want the top item. If we're moving up, bottom item.
                     final int motionIndex = delta > 0 ? 0 : getChildCount() - 1;
 
@@ -4511,6 +4518,10 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
                     final int overscrollMode = getOverScrollMode();
 
                     if (!trackMotionScroll(delta, delta)) {
+                        if (shouldAttemptFling) {
+                            initDifferentialFlingHelperIfNotExists();
+                            mDifferentialMotionFlingHelper.onMotionEvent(event, axis);
+                        }
                         return true;
                     } else if (!event.isFromSource(InputDevice.SOURCE_MOUSE) && motionView != null
                             && (overscrollMode == OVER_SCROLL_ALWAYS
@@ -4677,6 +4688,14 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
         }
     }
 
+    private void initDifferentialFlingHelperIfNotExists() {
+        if (mDifferentialMotionFlingHelper == null) {
+            mDifferentialMotionFlingHelper =
+                    new DifferentialMotionFlingHelper(
+                            mContext, new DifferentialFlingTarget());
+        }
+    }
+
     private void recycleVelocityTracker() {
         if (mVelocityTracker != null) {
             mVelocityTracker.recycle();
@@ -8197,4 +8216,26 @@ public abstract class AbsListView extends AdapterView<ListAdapter> implements Te
             }
         }
     }
+
+    private class DifferentialFlingTarget
+            implements DifferentialMotionFlingHelper.DifferentialMotionFlingTarget {
+        @Override
+        public boolean startDifferentialMotionFling(float velocity) {
+            stopDifferentialMotionFling();
+            fling((int) velocity);
+            return true;
+        }
+
+        @Override
+        public void stopDifferentialMotionFling() {
+            if (mFlingRunnable != null) {
+                mFlingRunnable.endFling();
+            }
+        }
+
+        @Override
+        public float getScaledScrollFactor() {
+            return -mVerticalScrollFactor;
+        }
+    }
 }

core/java/android/widget/DifferentialMotionFlingHelper.java

@@ -0,0 +1,249 @@
+/*
+ * Copyright 2023 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.widget;
+
+import android.annotation.Nullable;
+import android.content.Context;
+import android.view.MotionEvent;
+import android.view.VelocityTracker;
+import android.view.ViewConfiguration;
+
+import com.android.internal.annotations.VisibleForTesting;
+
+/**
+ * Helper for controlling differential motion flings.
+ *
+ * <p><b>Differential motion</b> here refers to motions that report change in position instead of
+ * absolution position. For instance, differential data points of 2, -1, 5 represent: there was
+ * a movement by "2" units, then by "-1" units, then by "5" units. Examples of motions reported
+ * differentially include motions from {@link MotionEvent#AXIS_SCROLL}.
+ *
+ * <p>The client should call {@link #onMotionEvent} when a differential motion event happens on
+ * the target View (that is, the View on which we want to fling), and this class processes the event
+ * to orchestrate fling.
+ *
+ * <p>Note that this helper class currently works to control fling only in one direction at a time.
+ * As such, it works independently of horizontal/vertical orientations. It requests its client to
+ * start/stop fling, and it's up to the client to choose the fling direction based on its specific
+ * internal configurations and/or preferences.
+ *
+ * @hide
+ */
+public class DifferentialMotionFlingHelper {
+    private final Context mContext;
+    private final DifferentialMotionFlingTarget mTarget;
+
+    private final FlingVelocityThresholdCalculator mVelocityThresholdCalculator;
+    private final DifferentialVelocityProvider mVelocityProvider;
+
+    @Nullable private VelocityTracker mVelocityTracker;
+
+    private float mLastFlingVelocity;
+
+    private int mLastProcessedAxis = -1;
+    private int mLastProcessedSource = -1;
+    private int mLastProcessedDeviceId = -1;
+
+    // Initialize min and max to +infinity and 0, to effectively disable fling at start.
+    private final int[] mFlingVelocityThresholds = new int[] {Integer.MAX_VALUE, 0};
+
+    /** Interface to calculate the fling velocity thresholds. Helps fake during testing. */
+    @VisibleForTesting
+    public interface FlingVelocityThresholdCalculator {
+        /**
+         * Calculates the fling velocity thresholds (in pixels/second) and puts them in a provided
+         * store.
+         *
+         * @param context the context associated with the View that may be flung.
+         * @param store an at-least size-2 int array. The method will overwrite positions 0 and 1
+         *             with the min and max fling velocities, respectively.
+         * @param event the event that may trigger fling.
+         * @param axis the axis being processed for the event.
+         */
+        void calculateFlingVelocityThresholds(
+                Context context, int[] store, MotionEvent event, int axis);
+    }
+
+    /**
+     * Interface to provide velocity. Helps fake during testing.
+     *
+     * <p>The client should call {@link #getCurrentVelocity(VelocityTracker, MotionEvent, int)} each
+     * time it wants to consider a {@link MotionEvent} towards the latest velocity, and the
+     * interface handles providing velocity that accounts for the latest and all past events.
+     */
+    @VisibleForTesting
+    public interface DifferentialVelocityProvider {
+        /**
+         * Returns the latest velocity.
+         *
+         * @param vt the {@link VelocityTracker} to be used to compute velocity.
+         * @param event the latest event to be considered in the velocity computations.
+         * @param axis the axis being processed for the event.
+         * @return the calculated, latest velocity.
+         */
+        float getCurrentVelocity(VelocityTracker vt, MotionEvent event, int axis);
+    }
+
+    /**
+     * Represents an entity that may be flung by a differential motion or an entity that initiates
+     * fling on a target View.
+     */
+    public interface DifferentialMotionFlingTarget {
+        /**
+         * Start flinging on the target View by a given velocity.
+         *
+         * @param velocity the fling velocity, in pixels/second.
+         * @return {@code true} if fling was successfully initiated, {@code false} otherwise.
+         */
+        boolean startDifferentialMotionFling(float velocity);
+
+        /** Stop any ongoing fling on the target View that is caused by a differential motion. */
+        void stopDifferentialMotionFling();
+
+        /**
+         * Returns the scaled scroll factor to be used for differential motions. This is the
+         * value that the raw {@link MotionEvent} values should be multiplied with to get pixels.
+         *
+         * <p>This usually is one of the values provided by {@link ViewConfiguration}. It is
+         * up to the client to choose and provide any value as per its internal configuration.
+         *
+         * @see ViewConfiguration#getScaledHorizontalScrollFactor()
+         * @see ViewConfiguration#getScaledVerticalScrollFactor()
+         */
+        float getScaledScrollFactor();
+    }
+
+    /** Constructs an instance for a given {@link DifferentialMotionFlingTarget}. */
+    public DifferentialMotionFlingHelper(
+            Context context,
+            DifferentialMotionFlingTarget target) {
+        this(context,
+                target,
+                DifferentialMotionFlingHelper::calculateFlingVelocityThresholds,
+                DifferentialMotionFlingHelper::getCurrentVelocity);
+    }
+
+    @VisibleForTesting
+    public DifferentialMotionFlingHelper(
+            Context context,
+            DifferentialMotionFlingTarget target,
+            FlingVelocityThresholdCalculator velocityThresholdCalculator,
+            DifferentialVelocityProvider velocityProvider) {
+        mContext = context;
+        mTarget = target;
+        mVelocityThresholdCalculator = velocityThresholdCalculator;
+        mVelocityProvider = velocityProvider;
+    }
+
+    /**
+     * Called to report when a differential motion happens on the View that's the target for fling.
+     *
+     * @param event the {@link MotionEvent} being reported.
+     * @param axis the axis being processed by the target View.
+     */
+    public void onMotionEvent(MotionEvent event, int axis) {
+        boolean flingParamsChanged = calculateFlingVelocityThresholds(event, axis);
+        if (mFlingVelocityThresholds[0] == Integer.MAX_VALUE) {
+            // Integer.MAX_VALUE means that the device does not support fling for the current
+            // configuration. Do not proceed any further.
+            recycleVelocityTracker();
+            return;
+        }
+
+        float scaledVelocity =
+                getCurrentVelocity(event, axis) * mTarget.getScaledScrollFactor();
+
+        float velocityDirection = Math.signum(scaledVelocity);
+        // Stop ongoing fling if there has been state changes affecting fling, or if the current
+        // velocity (if non-zero) is opposite of the velocity that last caused fling.
+        if (flingParamsChanged
+                || (velocityDirection != Math.signum(mLastFlingVelocity)
+                    && velocityDirection != 0)) {
+            mTarget.stopDifferentialMotionFling();
+        }
+
+        if (Math.abs(scaledVelocity) < mFlingVelocityThresholds[0]) {
+            return;
+        }
+
+        // Clamp the scaled velocity between [-max, max].
+        // e.g. if max=100, and vel=200
+        // vel = max(-100, min(200, 100)) = max(-100, 100) = 100
+        // e.g. if max=100, and vel=-200
+        // vel = max(-100, min(-200, 100)) = max(-100, -200) = -100
+        scaledVelocity =
+                Math.max(
+                        -mFlingVelocityThresholds[1],
+                        Math.min(scaledVelocity, mFlingVelocityThresholds[1]));
+
+        boolean flung = mTarget.startDifferentialMotionFling(scaledVelocity);
+        mLastFlingVelocity = flung ? scaledVelocity : 0;
+    }
+
+    /**
+     * Calculates fling velocity thresholds based on the provided event and axis, and returns {@code
+     * true} if there has been a change of any params that may affect fling velocity thresholds.
+     */
+    private boolean calculateFlingVelocityThresholds(MotionEvent event, int axis) {
+        int source = event.getSource();
+        int deviceId = event.getDeviceId();
+        if (mLastProcessedSource != source
+                || mLastProcessedDeviceId != deviceId
+                || mLastProcessedAxis != axis) {
+            mVelocityThresholdCalculator.calculateFlingVelocityThresholds(
+                    mContext, mFlingVelocityThresholds, event, axis);
+            // Save data about this processing so that we don't have to re-process fling thresholds
+            // for similar parameters.
+            mLastProcessedSource = source;
+            mLastProcessedDeviceId = deviceId;
+            mLastProcessedAxis = axis;
+            return true;
+        }
+        return false;
+    }
+
+    private static void calculateFlingVelocityThresholds(
+            Context context, int[] buffer, MotionEvent event, int axis) {
+        int source = event.getSource();
+        int deviceId = event.getDeviceId();
+
+        ViewConfiguration vc = ViewConfiguration.get(context);
+        buffer[0] = vc.getScaledMinimumFlingVelocity(deviceId, axis, source);
+        buffer[1] = vc.getScaledMaximumFlingVelocity(deviceId, axis, source);
+    }
+
+    private float getCurrentVelocity(MotionEvent event, int axis) {
+        if (mVelocityTracker == null) {
+            mVelocityTracker = VelocityTracker.obtain();
+        }
+
+        return mVelocityProvider.getCurrentVelocity(mVelocityTracker, event, axis);
+    }
+
+    private void recycleVelocityTracker() {
+        if (mVelocityTracker != null) {
+            mVelocityTracker.recycle();
+            mVelocityTracker = null;
+        }
+    }
+
+    private static float getCurrentVelocity(VelocityTracker vt, MotionEvent event, int axis) {
+        vt.addMovement(event);
+        vt.computeCurrentVelocity(1000);
+        return vt.getAxisVelocity(axis);
+    }
+}

core/java/android/widget/ScrollView.java

@@ -204,6 +204,8 @@ public class ScrollView extends FrameLayout {
     @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
     private StrictMode.Span mFlingStrictSpan = null;
 
+    private DifferentialMotionFlingHelper mDifferentialMotionFlingHelper;
+
     /**
      * Sentinel value for no current active pointer.
      * Used by {@link #mActivePointerId}.
@@ -594,6 +596,14 @@ public class ScrollView extends FrameLayout {
         }
     }
 
+    private void initDifferentialFlingHelperIfNotExists() {
+        if (mDifferentialMotionFlingHelper == null) {
+            mDifferentialMotionFlingHelper =
+                    new DifferentialMotionFlingHelper(
+                            mContext, new DifferentialFlingTarget());
+        }
+    }
+
     private void recycleVelocityTracker() {
         if (mVelocityTracker != null) {
             mVelocityTracker.recycle();
@@ -942,17 +952,22 @@ public class ScrollView extends FrameLayout {
     public boolean onGenericMotionEvent(MotionEvent event) {
         switch (event.getAction()) {
             case MotionEvent.ACTION_SCROLL:
-                final float axisValue;
+                final int axis;
                 if (event.isFromSource(InputDevice.SOURCE_CLASS_POINTER)) {
-                    axisValue = event.getAxisValue(MotionEvent.AXIS_VSCROLL);
+                    axis = MotionEvent.AXIS_VSCROLL;
                 } else if (event.isFromSource(InputDevice.SOURCE_ROTARY_ENCODER)) {
-                    axisValue = event.getAxisValue(MotionEvent.AXIS_SCROLL);
+                    axis = MotionEvent.AXIS_SCROLL;
                 } else {
-                    axisValue = 0;
+                    axis = -1;
                 }
 
+                final float axisValue = (axis == -1) ? 0 : event.getAxisValue(axis);
                 final int delta = Math.round(axisValue * mVerticalScrollFactor);
                 if (delta != 0) {
+                    // Tracks whether or not we should attempt fling for this event.
+                    // Fling should not be attempted if the view is already at the limit of scroll,
+                    // since it conflicts with EdgeEffect.
+                    boolean shouldAttemptFling = true;
                     final int range = getScrollRange();
                     int oldScrollY = mScrollY;
                     int newScrollY = oldScrollY - delta;
@@ -971,6 +986,7 @@ public class ScrollView extends FrameLayout {
                             absorbed = true;
                         }
                         newScrollY = 0;
+                        shouldAttemptFling = false;
                     } else if (newScrollY > range) {
                         if (canOverscroll) {
                             mEdgeGlowBottom.onPullDistance(
@@ -980,9 +996,14 @@ public class ScrollView extends FrameLayout {
                             absorbed = true;
                         }
                         newScrollY = range;
+                        shouldAttemptFling = false;
                     }
                     if (newScrollY != oldScrollY) {
                         super.scrollTo(mScrollX, newScrollY);
+                        if (shouldAttemptFling) {
+                            initDifferentialFlingHelperIfNotExists();
+                            mDifferentialMotionFlingHelper.onMotionEvent(event, axis);
+                        }
                         return true;
                     }
                     if (absorbed) {
@@ -2126,4 +2147,23 @@ public class ScrollView extends FrameLayout {
         };
     }
 
+    private class DifferentialFlingTarget
+            implements DifferentialMotionFlingHelper.DifferentialMotionFlingTarget {
+        @Override
+        public boolean startDifferentialMotionFling(float velocity) {
+            stopDifferentialMotionFling();
+            fling((int) velocity);
+            return true;
+        }
+
+        @Override
+        public void stopDifferentialMotionFling() {
+            mScroller.abortAnimation();
+        }
+
+        @Override
+        public float getScaledScrollFactor() {
+            return -mVerticalScrollFactor;
+        }
+    }
 }

core/tests/coretests/src/android/widget/DifferentialMotionFlingHelperTest.java

@@ -0,0 +1,186 @@
+/*
+ * Copyright 2023 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.widget;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
+import android.view.MotionEvent;
+
+import androidx.test.core.app.ApplicationProvider;
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.filters.SmallTest;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+@SmallTest
+@RunWith(AndroidJUnit4.class)
+public class DifferentialMotionFlingHelperTest {
+    private int mMinVelocity = 0;
+    private int mMaxVelocity = Integer.MAX_VALUE;
+    /** A fake velocity value that's going to be returned from the velocity provider. */
+    private float mVelocity;
+    private boolean mVelocityCalculated;
+
+    private final DifferentialMotionFlingHelper.DifferentialVelocityProvider mVelocityProvider =
+            (vt, event, axis) -> {
+                mVelocityCalculated = true;
+                return mVelocity;
+            };
+
+    private final DifferentialMotionFlingHelper.FlingVelocityThresholdCalculator
+            mVelocityThresholdCalculator =
+                    (ctx, buffer, event, axis) -> {
+                        buffer[0] = mMinVelocity;
+                        buffer[1] = mMaxVelocity;
+                    };
+
+    private final TestDifferentialMotionFlingTarget mFlingTarget =
+            new TestDifferentialMotionFlingTarget();
+
+    private DifferentialMotionFlingHelper mFlingHelper;
+
+    @Before
+    public void setUp() throws Exception {
+        mFlingHelper = new DifferentialMotionFlingHelper(
+                ApplicationProvider.getApplicationContext(),
+                mFlingTarget,
+                mVelocityThresholdCalculator,
+                mVelocityProvider);
+    }
+
+    @Test
+    public void deviceDoesNotSupportFling_noVelocityCalculated() {
+        mMinVelocity = Integer.MAX_VALUE;
+        mMaxVelocity = Integer.MIN_VALUE;
+
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, 60);
+
+        assertFalse(mVelocityCalculated);
+    }
+
+    @Test
+    public void flingVelocityOppositeToPrevious_stopsOngoingFling() {
+        deliverEventWithVelocity(createRotaryEncoderEvent(), MotionEvent.AXIS_SCROLL, 50);
+        deliverEventWithVelocity(createRotaryEncoderEvent(), MotionEvent.AXIS_SCROLL, -10);
+
+        // One stop on the initial event, and second stop due to opposite velocities.
+        assertEquals(2, mFlingTarget.mNumStops);
+    }
+
+    @Test
+    public void flingParamsChanged_stopsOngoingFling() {
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, 50);
+        deliverEventWithVelocity(createRotaryEncoderEvent(), MotionEvent.AXIS_SCROLL, 10);
+
+        // One stop on the initial event, and second stop due to changed axis/source.
+        assertEquals(2, mFlingTarget.mNumStops);
+    }
+
+    @Test
+    public void positiveFlingVelocityTooLow_doesNotGenerateFling() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, 20);
+
+        assertEquals(0, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    @Test
+    public void negativeFlingVelocityTooLow_doesNotGenerateFling() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, -20);
+
+        assertEquals(0, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    @Test
+    public void positiveFlingVelocityAboveMinimum_generateFlings() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, 60);
+
+        assertEquals(60, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    @Test
+    public void negativeFlingVelocityAboveMinimum_generateFlings() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, -60);
+
+        assertEquals(-60, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    @Test
+    public void positiveFlingVelocityAboveMaximum_velocityClamped() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, 3000);
+
+        assertEquals(100, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    @Test
+    public void negativeFlingVelocityAboveMaximum_velocityClamped() {
+        mMinVelocity = 50;
+        mMaxVelocity = 100;
+        deliverEventWithVelocity(createPointerEvent(), MotionEvent.AXIS_VSCROLL, -3000);
+
+        assertEquals(-100, mFlingTarget.mLastFlingVelocity, /* delta= */ 0);
+    }
+
+    private MotionEvent createRotaryEncoderEvent() {
+        return MotionEventUtils.createRotaryEvent(-2);
+    }
+
+    private MotionEvent createPointerEvent() {
+        return MotionEventUtils.createGenericPointerEvent(/* hScroll= */ 0, /* vScroll= */ -1);
+
+    }
+
+    private void deliverEventWithVelocity(MotionEvent ev, int axis, float velocity) {
+        mVelocity = velocity;
+        mFlingHelper.onMotionEvent(ev, axis);
+        ev.recycle();
+    }
+
+    private static class TestDifferentialMotionFlingTarget
+            implements DifferentialMotionFlingHelper.DifferentialMotionFlingTarget {
+        float mLastFlingVelocity = 0;
+        int mNumStops = 0;
+
+        @Override
+        public boolean startDifferentialMotionFling(float velocity) {
+            mLastFlingVelocity = velocity;
+            return true;
+        }
+
+        @Override
+        public void stopDifferentialMotionFling() {
+            mNumStops++;
+        }
+
+        @Override
+        public float getScaledScrollFactor() {
+            return 1;
+        }
+    }
+}

core/tests/coretests/src/android/widget/MotionEventUtils.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2023 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.widget;
+
+import static android.view.InputDevice.SOURCE_CLASS_POINTER;
+import static android.view.InputDevice.SOURCE_ROTARY_ENCODER;
+import static android.view.MotionEvent.ACTION_SCROLL;
+import static android.view.MotionEvent.AXIS_HSCROLL;
+import static android.view.MotionEvent.AXIS_SCROLL;
+import static android.view.MotionEvent.AXIS_VSCROLL;
+
+import android.view.MotionEvent;
+
+/** Test utilities for {@link MotionEvent}s. */
+public class MotionEventUtils {
+
+    /** Creates a test {@link MotionEvent} from a {@link SOURCE_ROTARY_ENCODER}. */
+    public static MotionEvent createRotaryEvent(float scroll) {
+        MotionEvent.PointerCoords coords = new MotionEvent.PointerCoords();
+        coords.setAxisValue(AXIS_SCROLL, scroll);
+
+        return createGenericMotionEvent(SOURCE_ROTARY_ENCODER, ACTION_SCROLL, coords);
+    }
+
+    /** Creates a test {@link MotionEvent} from a {@link SOURCE_CLASS_POINTER}. */
+    public static MotionEvent createGenericPointerEvent(float hScroll, float vScroll) {
+        MotionEvent.PointerCoords coords = new MotionEvent.PointerCoords();
+        coords.setAxisValue(AXIS_HSCROLL, hScroll);
+        coords.setAxisValue(AXIS_VSCROLL, vScroll);
+
+        return createGenericMotionEvent(SOURCE_CLASS_POINTER, ACTION_SCROLL, coords);
+    }
+
+    private static MotionEvent createGenericMotionEvent(
+            int source, int action, MotionEvent.PointerCoords coords) {
+        MotionEvent.PointerProperties props = new MotionEvent.PointerProperties();
+        props.id = 0;
+
+        return MotionEvent.obtain(
+                /* downTime= */ 0,
+                /* eventTime= */ 100,
+                action,
+                /* pointerCount= */ 1,
+                new MotionEvent.PointerProperties[] {props},
+                new MotionEvent.PointerCoords[] {coords},
+                /* metaState= */ 0,
+                /* buttonState= */ 0,
+                /* xPrecision= */ 0,
+                /* yPrecision= */ 0,
+                /* deviceId= */ 1,
+                /* edgeFlags= */ 0,
+                source,
+                /* flags= */ 0);
+    }
+}