android/view/VelocityTracker.java

/*
 * Copyright (C) 2006 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.view;

import android.annotation.IntDef;
import android.compat.annotation.UnsupportedAppUsage;
import android.hardware.input.InputManagerGlobal;
import android.os.IInputConstants;
import android.util.ArrayMap;
import android.util.Pools.SynchronizedPool;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.util.Map;

/**
 * Helper for tracking the velocity of motion events, for implementing
 * flinging and other such gestures.
 *
 * Use {@link #obtain} to retrieve a new instance of the class when you are going
 * to begin tracking.  Put the motion events you receive into it with
 * {@link #addMovement(MotionEvent)}.  When you want to determine the velocity, call
 * {@link #computeCurrentVelocity(int)} and then call the velocity-getter methods like
 * {@link #getXVelocity(int)}, {@link #getYVelocity(int)}, or {@link #getAxisVelocity(int, int)}
 * to retrieve velocity for different axes and/or pointer IDs.
 */
public final class VelocityTracker {
    private static final SynchronizedPool<VelocityTracker> sPool =
            new SynchronizedPool<VelocityTracker>(2);

    private static final int ACTIVE_POINTER_ID = -1;

    /** @hide */
    @IntDef(value = {
            MotionEvent.AXIS_X,
            MotionEvent.AXIS_Y,
            MotionEvent.AXIS_SCROLL
    })
    @Retention(RetentionPolicy.SOURCE)
    public @interface VelocityTrackableMotionEventAxis {}

    /**
     * Use the default Velocity Tracker Strategy. Different axes may use different default
     * strategies.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_DEFAULT =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_DEFAULT;

    /**
     * Velocity Tracker Strategy: Impulse.
     * Physical model of pushing an object.  Quality: VERY GOOD.
     * Works with duplicate coordinates, unclean finger liftoff.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_IMPULSE =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_IMPULSE;

    /**
     * Velocity Tracker Strategy: LSQ1.
     * 1st order least squares.  Quality: POOR.
     * Frequently underfits the touch data especially when the finger accelerates
     * or changes direction.  Often underestimates velocity.  The direction
     * is overly influenced by historical touch points.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_LSQ1 =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_LSQ1;

    /**
     * Velocity Tracker Strategy: LSQ2.
     * 2nd order least squares.  Quality: VERY GOOD.
     * Pretty much ideal, but can be confused by certain kinds of touch data,
     * particularly if the panel has a tendency to generate delayed,
     * duplicate or jittery touch coordinates when the finger is released.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_LSQ2 =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_LSQ2;

    /**
     * Velocity Tracker Strategy: LSQ3.
     * 3rd order least squares.  Quality: UNUSABLE.
     * Frequently overfits the touch data yielding wildly divergent estimates
     * of the velocity when the finger is released.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_LSQ3 =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_LSQ3;

    /**
     * Velocity Tracker Strategy: WLSQ2_DELTA.
     * 2nd order weighted least squares, delta weighting.  Quality: EXPERIMENTAL
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_WLSQ2_DELTA =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_WLSQ2_DELTA;

    /**
     * Velocity Tracker Strategy: WLSQ2_CENTRAL.
     * 2nd order weighted least squares, central weighting.  Quality: EXPERIMENTAL
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_WLSQ2_CENTRAL =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_WLSQ2_CENTRAL;

    /**
     * Velocity Tracker Strategy: WLSQ2_RECENT.
     * 2nd order weighted least squares, recent weighting.  Quality: EXPERIMENTAL
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_WLSQ2_RECENT =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_WLSQ2_RECENT;

    /**
     * Velocity Tracker Strategy: INT1.
     * 1st order integrating filter.  Quality: GOOD.
     * Not as good as 'lsq2' because it cannot estimate acceleration but it is
     * more tolerant of errors.  Like 'lsq1', this strategy tends to underestimate
     * the velocity of a fling but this strategy tends to respond to changes in
     * direction more quickly and accurately.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_INT1 =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_INT1;

    /**
     * Velocity Tracker Strategy: INT2.
     * 2nd order integrating filter.  Quality: EXPERIMENTAL.
     * For comparison purposes only.  Unlike 'int1' this strategy can compensate
     * for acceleration but it typically overestimates the effect.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_INT2 =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_INT2;

    /**
     * Velocity Tracker Strategy: Legacy.
     * Legacy velocity tracker algorithm.  Quality: POOR.
     * For comparison purposes only.  This algorithm is strongly influenced by
     * old data points, consistently underestimates velocity and takes a very long
     * time to adjust to changes in direction.
     *
     * @hide
     */
    public static final int VELOCITY_TRACKER_STRATEGY_LEGACY =
            IInputConstants.VELOCITY_TRACKER_STRATEGY_LEGACY;


    /**
     * Velocity Tracker Strategy look up table.
     */
    private static final Map<String, Integer> STRATEGIES = new ArrayMap<>();

    /** @hide */
    @Retention(RetentionPolicy.SOURCE)
    @IntDef(prefix = {"VELOCITY_TRACKER_STRATEGY_"}, value = {
            VELOCITY_TRACKER_STRATEGY_DEFAULT,
            VELOCITY_TRACKER_STRATEGY_IMPULSE,
            VELOCITY_TRACKER_STRATEGY_LSQ1,
            VELOCITY_TRACKER_STRATEGY_LSQ2,
            VELOCITY_TRACKER_STRATEGY_LSQ3,
            VELOCITY_TRACKER_STRATEGY_WLSQ2_DELTA,
            VELOCITY_TRACKER_STRATEGY_WLSQ2_CENTRAL,
            VELOCITY_TRACKER_STRATEGY_WLSQ2_RECENT,
            VELOCITY_TRACKER_STRATEGY_INT1,
            VELOCITY_TRACKER_STRATEGY_INT2,
            VELOCITY_TRACKER_STRATEGY_LEGACY
    })
    public @interface VelocityTrackerStrategy {}

    private long mPtr;
    @VelocityTrackerStrategy
    private final int mStrategy;

    private static native long nativeInitialize(int strategy);
    private static native void nativeDispose(long ptr);
    private static native void nativeClear(long ptr);
    private static native void nativeAddMovement(long ptr, MotionEvent event);
    private static native void nativeComputeCurrentVelocity(long ptr, int units, float maxVelocity);
    private static native float nativeGetVelocity(long ptr, int axis, int id);
    private static native boolean nativeIsAxisSupported(int axis);

    static {
        // Strategy string and IDs mapping lookup.
        STRATEGIES.put("impulse", VELOCITY_TRACKER_STRATEGY_IMPULSE);
        STRATEGIES.put("lsq1", VELOCITY_TRACKER_STRATEGY_LSQ1);
        STRATEGIES.put("lsq2", VELOCITY_TRACKER_STRATEGY_LSQ2);
        STRATEGIES.put("lsq3", VELOCITY_TRACKER_STRATEGY_LSQ3);
        STRATEGIES.put("wlsq2-delta", VELOCITY_TRACKER_STRATEGY_WLSQ2_DELTA);
        STRATEGIES.put("wlsq2-central", VELOCITY_TRACKER_STRATEGY_WLSQ2_CENTRAL);
        STRATEGIES.put("wlsq2-recent", VELOCITY_TRACKER_STRATEGY_WLSQ2_RECENT);
        STRATEGIES.put("int1", VELOCITY_TRACKER_STRATEGY_INT1);
        STRATEGIES.put("int2", VELOCITY_TRACKER_STRATEGY_INT2);
        STRATEGIES.put("legacy", VELOCITY_TRACKER_STRATEGY_LEGACY);
    }

    /**
     * Return a strategy ID from string.
     */
    private static int toStrategyId(String strStrategy) {
        if (STRATEGIES.containsKey(strStrategy)) {
            return STRATEGIES.get(strStrategy);
        }
        return VELOCITY_TRACKER_STRATEGY_DEFAULT;
    }

    /**
     * Retrieve a new VelocityTracker object to watch the velocity of a
     * motion.  Be sure to call {@link #recycle} when done.  You should
     * generally only maintain an active object while tracking a movement,
     * so that the VelocityTracker can be re-used elsewhere.
     *
     * @return Returns a new VelocityTracker.
     */
    static public VelocityTracker obtain() {
        VelocityTracker instance = sPool.acquire();
        return (instance != null) ? instance
                : new VelocityTracker(VELOCITY_TRACKER_STRATEGY_DEFAULT);
    }

    /**
     * Obtains a velocity tracker with the specified strategy as string.
     * For testing and comparison purposes only.
     * @deprecated Use {@link obtain(int strategy)} instead.
     *
     * @param strategy The strategy, or null to use the default.
     * @return The velocity tracker.
     *
     * @hide
     */
    @UnsupportedAppUsage
    @Deprecated
    public static VelocityTracker obtain(String strategy) {
        if (strategy == null) {
            return obtain();
        }
        return new VelocityTracker(toStrategyId(strategy));
    }

    /**
     * Obtains a velocity tracker with the specified strategy.
     *
     * @param strategy The strategy Id, VELOCITY_TRACKER_STRATEGY_DEFAULT to use the default.
     * @return The velocity tracker.
     *
     * @hide
     */
    public static VelocityTracker obtain(int strategy) {
        if (strategy == VELOCITY_TRACKER_STRATEGY_DEFAULT) {
            return obtain();
        }
        return new VelocityTracker(strategy);
    }

    /**
     * Return a VelocityTracker object back to be re-used by others.  You must
     * not touch the object after calling this function.
     */
    public void recycle() {
        if (mStrategy == VELOCITY_TRACKER_STRATEGY_DEFAULT) {
            clear();
            sPool.release(this);
        }
    }

    /**
     * Return strategy Id of VelocityTracker object.
     * @return The velocity tracker strategy Id.
     *
     * @hide
     */
    public int getStrategyId() {
        return mStrategy;
    }

    private VelocityTracker(@VelocityTrackerStrategy int strategy) {
        // If user has not selected a specific strategy
        if (strategy == VELOCITY_TRACKER_STRATEGY_DEFAULT) {
            final String strategyProperty = InputManagerGlobal.getInstance()
                    .getVelocityTrackerStrategy();
            // Check if user specified strategy by overriding system property.
            if (strategyProperty == null || strategyProperty.isEmpty()) {
                mStrategy = strategy;
            } else {
                mStrategy = toStrategyId(strategyProperty);
            }
        } else {
            // User specified strategy
            mStrategy = strategy;
        }
        mPtr = nativeInitialize(mStrategy);
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            if (mPtr != 0) {
                nativeDispose(mPtr);
                mPtr = 0;
            }
        } finally {
            super.finalize();
        }
    }

    /**
     * Checks whether a given velocity-trackable {@link MotionEvent} axis is supported for velocity
     * tracking by this {@link VelocityTracker} instance (refer to
     * {@link #getAxisVelocity(int, int)} for a list of potentially velocity-trackable axes).
     *
     * <p>Note that the value returned from this method will stay the same for a given instance, so
     * a single check for axis support is enough per a {@link VelocityTracker} instance.
     *
     * @param axis The axis to check for velocity support.
     * @return {@code true} if {@code axis} is supported for velocity tracking, or {@code false}
     * otherwise.
     * @see #getAxisVelocity(int, int)
     * @see #getAxisVelocity(int)
     */
    public boolean isAxisSupported(@VelocityTrackableMotionEventAxis int axis) {
        return nativeIsAxisSupported(axis);
    }

    /**
     * Reset the velocity tracker back to its initial state.
     */
    public void clear() {
        nativeClear(mPtr);
    }

    /**
     * Add a user's movement to the tracker.  You should call this for the
     * initial {@link MotionEvent#ACTION_DOWN}, the following
     * {@link MotionEvent#ACTION_MOVE} events that you receive, and the
     * final {@link MotionEvent#ACTION_UP}.  You can, however, call this
     * for whichever events you desire.
     *
     * @param event The MotionEvent you received and would like to track.
     */
    public void addMovement(MotionEvent event) {
        if (event == null) {
            throw new IllegalArgumentException("event must not be null");
        }
        nativeAddMovement(mPtr, event);
    }

    /**
     * Equivalent to invoking {@link #computeCurrentVelocity(int, float)} with a maximum
     * velocity of Float.MAX_VALUE.
     *
     * @see #computeCurrentVelocity(int, float)
     */
    public void computeCurrentVelocity(int units) {
        nativeComputeCurrentVelocity(mPtr, units, Float.MAX_VALUE);
    }

    /**
     * Compute the current velocity based on the points that have been
     * collected.  Only call this when you actually want to retrieve velocity
     * information, as it is relatively expensive.  You can then retrieve
     * the velocity with {@link #getXVelocity()} and
     * {@link #getYVelocity()}.
     *
     * @param units The units you would like the velocity in.  A value of 1
     * provides units per millisecond, 1000 provides units per second, etc.
     * Note that the units referred to here are the same units with which motion is reported. For
     * axes X and Y, the units are pixels.
     * @param maxVelocity The maximum velocity that can be computed by this method.
     * This value must be declared in the same unit as the units parameter. This value
     * must be positive.
     */
    public void computeCurrentVelocity(int units, float maxVelocity) {
        nativeComputeCurrentVelocity(mPtr, units, maxVelocity);
    }

    /**
     * Retrieve the last computed X velocity.  You must first call
     * {@link #computeCurrentVelocity(int)} before calling this function.
     *
     * @return The previously computed X velocity.
     */
    public float getXVelocity() {
        return getXVelocity(ACTIVE_POINTER_ID);
    }

    /**
     * Retrieve the last computed Y velocity.  You must first call
     * {@link #computeCurrentVelocity(int)} before calling this function.
     *
     * @return The previously computed Y velocity.
     */
    public float getYVelocity() {
        return getYVelocity(ACTIVE_POINTER_ID);
    }

    /**
     * Retrieve the last computed X velocity.  You must first call
     * {@link #computeCurrentVelocity(int)} before calling this function.
     *
     * @param id Which pointer's velocity to return.
     * @return The previously computed X velocity.
     */
    public float getXVelocity(int id) {
        return nativeGetVelocity(mPtr, MotionEvent.AXIS_X, id);
    }

    /**
     * Retrieve the last computed Y velocity.  You must first call
     * {@link #computeCurrentVelocity(int)} before calling this function.
     *
     * @param id Which pointer's velocity to return.
     * @return The previously computed Y velocity.
     */
    public float getYVelocity(int id) {
        return nativeGetVelocity(mPtr, MotionEvent.AXIS_Y, id);
    }

    /**
     * Retrieve the last computed velocity for a given motion axis. You must first call
     * {@link #computeCurrentVelocity(int)} or {@link #computeCurrentVelocity(int, float)} before
     * calling this function.
     *
     * <p>In addition to {@link MotionEvent#AXIS_X} and {@link MotionEvent#AXIS_Y} which have been
     * supported since the introduction of this class, the following axes can be candidates for this
     * method:
     * <ul>
     *   <li> {@link MotionEvent#AXIS_SCROLL}: supported starting
     *        {@link android.os.Build.VERSION_CODES#UPSIDE_DOWN_CAKE}
     * </ul>
     *
     * <p>Before accessing velocities of an axis using this method, check that your
     * {@link VelocityTracker} instance supports the axis by using {@link #isAxisSupported(int)}.
     *
     * @param axis Which axis' velocity to return.
     * @param id Which pointer's velocity to return.
     * @return The previously computed velocity for {@code axis} for pointer ID of {@code id} if
     * {@code axis} is supported for velocity tracking, or 0 if velocity tracking is not supported
     * for the axis.
     * @see #isAxisSupported(int)
     */
    public float getAxisVelocity(@VelocityTrackableMotionEventAxis int axis, int id) {
        return nativeGetVelocity(mPtr, axis, id);
    }

    /**
     * Equivalent to calling {@link #getAxisVelocity(int, int)} for {@code axis} and the active
     * pointer.
     *
     * @param axis Which axis' velocity to return.
     * @return The previously computed velocity for {@code axis} for the active pointer if
     * {@code axis} is supported for velocity tracking, or 0 if velocity tracking is not supported
     * for the axis.
     * @see #isAxisSupported(int)
     * @see #getAxisVelocity(int, int)
     */
    public float getAxisVelocity(@VelocityTrackableMotionEventAxis int axis) {
        return nativeGetVelocity(mPtr, axis, ACTIVE_POINTER_ID);
    }
}