VelocityTracker.java revision 2ed2462aa29c564f5231f317c27b3188da875e52 
1/*
2 * Copyright (C) 2006 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package android.view;
18
19import android.util.Poolable;
20import android.util.Pool;
21import android.util.Pools;
22import android.util.PoolableManager;
23
24/**
25 * Helper for tracking the velocity of touch events, for implementing
26 * flinging and other such gestures.
27 *
28 * Use {@link #obtain} to retrieve a new instance of the class when you are going
29 * to begin tracking.  Put the motion events you receive into it with
30 * {@link #addMovement(MotionEvent)}.  When you want to determine the velocity call
31 * {@link #computeCurrentVelocity(int)} and then call {@link #getXVelocity(int)}
32 * and {@link #getXVelocity(int)} to retrieve the velocity for each pointer id.
33 */
34public final class VelocityTracker implements Poolable<VelocityTracker> {
35    private static final Pool<VelocityTracker> sPool = Pools.synchronizedPool(
36            Pools.finitePool(new PoolableManager<VelocityTracker>() {
37                public VelocityTracker newInstance() {
38                    return new VelocityTracker();
39                }
40
41                public void onAcquired(VelocityTracker element) {
42                }
43
44                public void onReleased(VelocityTracker element) {
45                    element.clear();
46                }
47            }, 2));
48
49    private static final int ACTIVE_POINTER_ID = -1;
50
51    private int mPtr;
52    private VelocityTracker mNext;
53
54    private static native int nativeInitialize();
55    private static native void nativeDispose(int ptr);
56    private static native void nativeClear(int ptr);
57    private static native void nativeAddMovement(int ptr, MotionEvent event);
58    private static native void nativeComputeCurrentVelocity(int ptr, int units, float maxVelocity);
59    private static native float nativeGetXVelocity(int ptr, int id);
60    private static native float nativeGetYVelocity(int ptr, int id);
61
62    /**
63     * Retrieve a new VelocityTracker object to watch the velocity of a
64     * motion.  Be sure to call {@link #recycle} when done.  You should
65     * generally only maintain an active object while tracking a movement,
66     * so that the VelocityTracker can be re-used elsewhere.
67     *
68     * @return Returns a new VelocityTracker.
69     */
70    static public VelocityTracker obtain() {
71        return sPool.acquire();
72    }
73
74    /**
75     * Return a VelocityTracker object back to be re-used by others.  You must
76     * not touch the object after calling this function.
77     */
78    public void recycle() {
79        sPool.release(this);
80    }
81
82    /**
83     * @hide
84     */
85    public void setNextPoolable(VelocityTracker element) {
86        mNext = element;
87    }
88
89    /**
90     * @hide
91     */
92    public VelocityTracker getNextPoolable() {
93        return mNext;
94    }
95
96    private VelocityTracker() {
97        mPtr = nativeInitialize();
98    }
99
100    @Override
101    protected void finalize() throws Throwable {
102        try {
103            if (mPtr != 0) {
104                nativeDispose(mPtr);
105                mPtr = 0;
106            }
107        } finally {
108            super.finalize();
109        }
110    }
111
112    /**
113     * Reset the velocity tracker back to its initial state.
114     */
115    public void clear() {
116        nativeClear(mPtr);
117    }
118
119    /**
120     * Add a user's movement to the tracker.  You should call this for the
121     * initial {@link MotionEvent#ACTION_DOWN}, the following
122     * {@link MotionEvent#ACTION_MOVE} events that you receive, and the
123     * final {@link MotionEvent#ACTION_UP}.  You can, however, call this
124     * for whichever events you desire.
125     *
126     * @param event The MotionEvent you received and would like to track.
127     */
128    public void addMovement(MotionEvent event) {
129        if (event == null) {
130            throw new IllegalArgumentException("event must not be null");
131        }
132        nativeAddMovement(mPtr, event);
133    }
134
135    /**
136     * Equivalent to invoking {@link #computeCurrentVelocity(int, float)} with a maximum
137     * velocity of Float.MAX_VALUE.
138     *
139     * @see #computeCurrentVelocity(int, float)
140     */
141    public void computeCurrentVelocity(int units) {
142        nativeComputeCurrentVelocity(mPtr, units, Float.MAX_VALUE);
143    }
144
145    /**
146     * Compute the current velocity based on the points that have been
147     * collected.  Only call this when you actually want to retrieve velocity
148     * information, as it is relatively expensive.  You can then retrieve
149     * the velocity with {@link #getXVelocity()} and
150     * {@link #getYVelocity()}.
151     *
152     * @param units The units you would like the velocity in.  A value of 1
153     * provides pixels per millisecond, 1000 provides pixels per second, etc.
154     * @param maxVelocity The maximum velocity that can be computed by this method.
155     * This value must be declared in the same unit as the units parameter. This value
156     * must be positive.
157     */
158    public void computeCurrentVelocity(int units, float maxVelocity) {
159        nativeComputeCurrentVelocity(mPtr, units, maxVelocity);
160    }
161
162    /**
163     * Retrieve the last computed X velocity.  You must first call
164     * {@link #computeCurrentVelocity(int)} before calling this function.
165     *
166     * @return The previously computed X velocity.
167     */
168    public float getXVelocity() {
169        return nativeGetXVelocity(mPtr, ACTIVE_POINTER_ID);
170    }
171
172    /**
173     * Retrieve the last computed Y velocity.  You must first call
174     * {@link #computeCurrentVelocity(int)} before calling this function.
175     *
176     * @return The previously computed Y velocity.
177     */
178    public float getYVelocity() {
179        return nativeGetYVelocity(mPtr, ACTIVE_POINTER_ID);
180    }
181
182    /**
183     * Retrieve the last computed X velocity.  You must first call
184     * {@link #computeCurrentVelocity(int)} before calling this function.
185     *
186     * @param id Which pointer's velocity to return.
187     * @return The previously computed X velocity.
188     */
189    public float getXVelocity(int id) {
190        return nativeGetXVelocity(mPtr, id);
191    }
192
193    /**
194     * Retrieve the last computed Y velocity.  You must first call
195     * {@link #computeCurrentVelocity(int)} before calling this function.
196     *
197     * @param id Which pointer's velocity to return.
198     * @return The previously computed Y velocity.
199     */
200    public float getYVelocity(int id) {
201        return nativeGetYVelocity(mPtr, id);
202    }
203}
204