/*
* 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.util.Config;
import android.util.Log;
/**
* Helper for tracking the velocity of touch 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)},
* and when you want to determine the velocity call
* {@link #computeCurrentVelocity(int)} and then {@link #getXVelocity()}
* and {@link #getXVelocity()}.
*/
public final class VelocityTracker {
static final String TAG = "VelocityTracker";
static final boolean DEBUG = false;
static final boolean localLOGV = DEBUG || Config.LOGV;
static final int NUM_PAST = 10;
static final int LONGEST_PAST_TIME = 200;
static final VelocityTracker[] mPool = new VelocityTracker[1];
final float mPastX[] = new float[NUM_PAST];
final float mPastY[] = new float[NUM_PAST];
final long mPastTime[] = new long[NUM_PAST];
float mYVelocity;
float mXVelocity;
/**
* 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() {
synchronized (mPool) {
VelocityTracker vt = mPool[0];
if (vt != null) {
vt.clear();
mPool[0] = null;
return vt;
}
return new VelocityTracker();
}
}
/**
* Return a VelocityTracker object back to be re-used by others. You must
* not touch the object after calling this function.
*/
public void recycle() {
synchronized (mPool) {
mPool[0] = this;
}
}
private VelocityTracker() {
}
/**
* Reset the velocity tracker back to its initial state.
*/
public void clear() {
mPastTime[0] = 0;
}
/**
* 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 ev The MotionEvent you received and would like to track.
*/
public void addMovement(MotionEvent ev) {
long time = ev.getEventTime();
final int N = ev.getHistorySize();
for (int i=0; i<N; i++) {
addPoint(ev.getHistoricalX(i), ev.getHistoricalY(i),
ev.getHistoricalEventTime(i));
}
addPoint(ev.getX(), ev.getY(), time);
}
private void addPoint(float x, float y, long time) {
int drop = -1;
int i;
if (localLOGV) Log.v(TAG, "Adding past y=" + y + " time=" + time);
final long[] pastTime = mPastTime;
for (i=0; i<NUM_PAST; i++) {
if (pastTime[i] == 0) {
break;
} else if (pastTime[i] < time-LONGEST_PAST_TIME) {
if (localLOGV) Log.v(TAG, "Dropping past too old at "
+ i + " time=" + pastTime[i]);
drop = i;
}
}
if (localLOGV) Log.v(TAG, "Add index: " + i);
if (i == NUM_PAST && drop < 0) {
drop = 0;
}
if (drop == i) drop--;
final float[] pastX = mPastX;
final float[] pastY = mPastY;
if (drop >= 0) {
if (localLOGV) Log.v(TAG, "Dropping up to #" + drop);
final int start = drop+1;
final int count = NUM_PAST-drop-1;
System.arraycopy(pastX, start, pastX, 0, count);
System.arraycopy(pastY, start, pastY, 0, count);
System.arraycopy(pastTime, start, pastTime, 0, count);
i -= (drop+1);
}
pastX[i] = x;
pastY[i] = y;
pastTime[i] = time;
i++;
if (i < NUM_PAST) {
pastTime[i] = 0;
}
}
/**
* 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 pixels per millisecond, 1000 provides pixels per second, etc.
*/
public void computeCurrentVelocity(int units) {
final float[] pastX = mPastX;
final float[] pastY = mPastY;
final long[] pastTime = mPastTime;
// Kind-of stupid.
final float oldestX = pastX[0];
final float oldestY = pastY[0];
final long oldestTime = pastTime[0];
float accumX = 0;
float accumY = 0;
int N=0;
while (N < NUM_PAST) {
if (pastTime[N] == 0) {
break;
}
N++;
}
// Skip the last received event, since it is probably pretty noisy.
if (N > 3) N--;
for (int i=1; i < N; i++) {
final int dur = (int)(pastTime[i] - oldestTime);
if (dur == 0) continue;
float dist = pastX[i] - oldestX;
float vel = (dist/dur) * units; // pixels/frame.
if (accumX == 0) accumX = vel;
else accumX = (accumX + vel) * .5f;
dist = pastY[i] - oldestY;
vel = (dist/dur) * units; // pixels/frame.
if (accumY == 0) accumY = vel;
else accumY = (accumY + vel) * .5f;
}
mXVelocity = accumX;
mYVelocity = accumY;
if (localLOGV) Log.v(TAG, "Y velocity=" + mYVelocity +" X velocity="
+ mXVelocity + " N=" + N);
}
/**
* 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 mXVelocity;
}
/**
* 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 mYVelocity;
}
}