com.google.gwt.touch.client.DefaultMomentum Maven / Gradle / Ivy
/*
* Copyright 2011 Google Inc.
*
* 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.google.gwt.touch.client;
/**
* Default implementation of momentum.
*/
public class DefaultMomentum implements Momentum {
/**
* The constant factor applied to velocity every millisecond to simulate
* deceleration.
*/
private static final double DECELERATION_FACTOR = 0.9993;
/**
* The velocity threshold at which declereration will end.
*/
private static final double DECELERATION_STOP_VELOCITY = 0.02;
/**
* The minimum deceleration rate in pixels per millisecond^2.
*/
private static final double MIN_DECELERATION = 0.0005;
public State createState(Point initialPosition, Point initialVelocity) {
return new State(initialPosition, initialVelocity);
}
public boolean updateState(State state) {
// Calculate the new velocity.
int ellapsedMillis = state.getElapsedMillis();
int totalEllapsedMillis = state.getCumulativeElapsedMillis();
Point initialVelocity = state.getInitialVelocity();
Point oldVelocity = state.getVelocity();
double decelFactor = Math.pow(DECELERATION_FACTOR, totalEllapsedMillis);
double minDecel = ellapsedMillis * MIN_DECELERATION;
double newVelocityX =
calcNewVelocity(initialVelocity.getX(), decelFactor, oldVelocity.getX(), minDecel);
double newVelocityY =
calcNewVelocity(initialVelocity.getY(), decelFactor, oldVelocity.getY(), minDecel);
// Save the new velocity.
Point newVelocity = new Point(newVelocityX, newVelocityY);
state.setVelocity(newVelocity);
// Calculate the distance traveled.
int elapsedMillis = state.getElapsedMillis();
Point dist = newVelocity.mult(new Point(elapsedMillis, elapsedMillis));
// Update the state with the new point.
Point position = state.getPosition();
state.setPosition(position.plus(dist));
// End momentum when we reach the threshold.
if (Math.abs(newVelocity.getX()) < DECELERATION_STOP_VELOCITY
&& Math.abs(newVelocity.getY()) < DECELERATION_STOP_VELOCITY) {
return false;
}
return true;
}
/**
* Calculate the new velocity.
*
* @param initialVelocity the initial velocity
* @param decelFactor the deceleration factor based on the cumulative elapsed
* time
* @param oldVelocity the previous velocity
* @param minDecel the absolute value of the minimum deceleration over the
* elapsed time since the last update
* @return the new velocity
*/
private double calcNewVelocity(double initialVelocity, double decelFactor, double oldVelocity,
double minDecel) {
// Calculate the new velocity based on the deceleration factor.
double newVelocity = initialVelocity * decelFactor;
// Ensure that we are decelerating faster than the minimum rate.
if (oldVelocity >= 0) {
double maxVelocityX = Math.max(0.0, oldVelocity - minDecel);
newVelocity = Math.min(newVelocity, maxVelocityX);
} else {
double minVelocityX = Math.min(0.0, oldVelocity + minDecel);
newVelocity = Math.max(newVelocity, minVelocityX);
}
return newVelocity;
}
}