1 /**
2  * Copyright (C) 2014 Google Inc.
3  * Licensed to The Android Open Source Project.
4  *
5  * Licensed under the Apache License, Version 2.0 (the "License");
6  * you may not use this file except in compliance with the License.
7  * You may obtain a copy of the License at
8  *
9  *      http://www.apache.org/licenses/LICENSE-2.0
10  *
11  * Unless required by applicable law or agreed to in writing, software
12  * distributed under the License is distributed on an "AS IS" BASIS,
13  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14  * See the License for the specific language governing permissions and
15  * limitations under the License.
16  */
17 
18 package com.android.mail.ui;
19 
20 import android.content.Context;
21 import android.support.annotation.Nullable;
22 import android.view.MotionEvent;
23 import android.view.VelocityTracker;
24 import android.view.ViewConfiguration;
25 
26 /**
27  * Generic utility class that deals with capturing drag events in a particular horizontal direction
28  * and calls the callback interface for drag events.
29  *
30  * Usage:
31  *
32  * <code>
33  *
34  *  class CustomView extends ... {
35  *      private boolean mShouldInterceptDrag;
36  *      private int mDragMode;
37  *
38  *      public boolean onInterceptTouchEvent(MotionEvent ev) {
39  *          switch (ev.getAction()) {
40  *              case MotionEvent.ACTION_DOWN:
41  *                  // Check if the event is in the draggable area
42  *                  mShouldInterceptDrag = ...;
43  *                  mDragMode = ...;
44  *          }
45  *          return mShouldInterceptDrag && GmailDragHelper.processTouchEvent(ev, mDragMode);
46  *      }
47  *
48  *      public boolean onTouchEvent(MotionEvent ev) {
49  *          if (mShouldInterceptDrag) {
50  *              GmailDragHelper.processTouchEvent(ev, mDragMode);
51  *              return true;
52  *          }
53  *          return super.onTouchEvent(ev);
54  *      }
55  *  }
56  *
57  * </code>
58  */
59 public class GmailDragHelper {
60     public static final int CAPTURE_LEFT_TO_RIGHT = 0;
61     public static final int CAPTURE_RIGHT_TO_LEFT = 1;
62 
63     private final GmailDragHelperCallback mCallback;
64     private final ViewConfiguration mConfiguration;
65 
66     private boolean mDragging;
67     private VelocityTracker mVelocityTracker;
68 
69     private float mInitialInterceptedX;
70     private float mInitialInterceptedY;
71 
72     private float mStartDragX;
73 
74     public interface GmailDragHelperCallback {
onDragStarted()75         public void onDragStarted();
onDrag(float deltaX)76         public void onDrag(float deltaX);
onDragEnded(float deltaX, float velocityX, boolean isFling)77         public void onDragEnded(float deltaX, float velocityX, boolean isFling);
78     }
79 
80     /**
81      */
GmailDragHelper(Context context, GmailDragHelperCallback callback)82     public GmailDragHelper(Context context, GmailDragHelperCallback callback) {
83         mCallback = callback;
84         mConfiguration = ViewConfiguration.get(context);
85     }
86 
87     /**
88      * Process incoming MotionEvent to compute the new drag state and coordinates.
89      *
90      * @param ev the captured MotionEvent
91      * @param dragMode either {@link GmailDragHelper#CAPTURE_LEFT_TO_RIGHT} or
92      *   {@link GmailDragHelper#CAPTURE_RIGHT_TO_LEFT}
93      * @return whether if drag is happening
94      */
processTouchEvent(MotionEvent ev, int dragMode)95     public boolean processTouchEvent(MotionEvent ev, int dragMode) {
96         return processTouchEvent(ev, dragMode, null);
97     }
98 
99     /**
100      * @param xThreshold optional parameter to specify that the drag can only happen if it crosses
101      *   the threshold coordinate. This can be used to only start the drag once the user hits the
102      *   edge of the view.
103      */
processTouchEvent(MotionEvent ev, int dragMode, @Nullable Float xThreshold)104     public boolean processTouchEvent(MotionEvent ev, int dragMode, @Nullable Float xThreshold) {
105         if (mVelocityTracker == null) {
106             mVelocityTracker = VelocityTracker.obtain();
107         }
108         mVelocityTracker.addMovement(ev);
109 
110         switch (ev.getAction()) {
111             case MotionEvent.ACTION_DOWN:
112                 mInitialInterceptedX = ev.getX();
113                 mInitialInterceptedY = ev.getY();
114                 break;
115             case MotionEvent.ACTION_MOVE:
116                 if (mDragging) {
117                     mCallback.onDrag(ev.getX() - mStartDragX);
118                 } else {
119                     // Try to start dragging
120                     final float evX = ev.getX();
121                     // Check for directional drag
122                     if ((dragMode == CAPTURE_LEFT_TO_RIGHT && evX <= mInitialInterceptedX) ||
123                             (dragMode == CAPTURE_RIGHT_TO_LEFT && evX >= mInitialInterceptedX)) {
124                         break;
125                     }
126 
127                     // Check for optional threshold
128                     boolean passedThreshold = true;
129                     if (xThreshold != null) {
130                         if (dragMode == CAPTURE_LEFT_TO_RIGHT) {
131                             passedThreshold = evX > xThreshold;
132                         } else {
133                             passedThreshold = evX < xThreshold;
134                         }
135                     }
136 
137                     // Check for drag threshold
138                     final float deltaX = Math.abs(evX - mInitialInterceptedX);
139                     final float deltaY = Math.abs(ev.getY() - mInitialInterceptedY);
140                     if (deltaX >= mConfiguration.getScaledTouchSlop() && deltaX >= deltaY
141                             && passedThreshold) {
142                         setDragging(true, evX);
143                     }
144                 }
145                 break;
146             case MotionEvent.ACTION_UP:
147                 if (mDragging) {
148                     setDragging(false, ev.getX());
149                 }
150                 break;
151         }
152 
153         return mDragging;
154     }
155 
156     /**
157      * Set the internal dragging state and calls the appropriate callbacks.
158      */
setDragging(boolean dragging, float evX)159     private void setDragging(boolean dragging, float evX) {
160         mDragging = dragging;
161 
162         if (mDragging) {
163             mStartDragX = evX;
164             mCallback.onDragStarted();
165         } else {
166             // Here velocity is in pixel/second, let's take that into account for evX.
167             mVelocityTracker.computeCurrentVelocity(1000);
168             // Check for fling
169             final float xVelocity = mVelocityTracker.getXVelocity();
170             final boolean isFling =
171                     Math.abs(xVelocity) > mConfiguration.getScaledMinimumFlingVelocity();
172             mVelocityTracker.clear();
173 
174             mCallback.onDragEnded(evX - mStartDragX, xVelocity, isFling);
175         }
176     }
177 }
178