1 /*
2  * Copyright (C) 2017 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 package com.android.launcher3.touch;
17 
18 /**
19  * Utility methods for overscroll damping and related effect.
20  */
21 public class OverScroll {
22 
23     public static final float OVERSCROLL_DAMP_FACTOR = 0.07f;
24 
25     /**
26      * This curve determines how the effect of scrolling over the limits of the page diminishes
27      * as the user pulls further and further from the bounds
28      *
29      * @param f The percentage of how much the user has overscrolled.
30      * @return A transformed percentage based on the influence curve.
31      */
overScrollInfluenceCurve(float f)32     private static float overScrollInfluenceCurve(float f) {
33         f -= 1.0f;
34         return f * f * f + 1.0f;
35     }
36 
37     /**
38      * @param amount The original amount overscrolled.
39      * @param max The maximum amount that the View can overscroll.
40      * @return The dampened overscroll amount.
41      */
dampedScroll(float amount, int max)42     public static int dampedScroll(float amount, int max) {
43         if (Float.compare(amount, 0) == 0) return 0;
44 
45         float f = amount / max;
46         f = f / (Math.abs(f)) * (overScrollInfluenceCurve(Math.abs(f)));
47 
48         // Clamp this factor, f, to -1 < f < 1
49         if (Math.abs(f) >= 1) {
50             f /= Math.abs(f);
51         }
52 
53         return Math.round(OVERSCROLL_DAMP_FACTOR * f * max);
54     }
55 }
56