1 /*
2  * Copyright (C) 2015 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.messaging.ui;
17 
18 import android.os.Parcelable;
19 import android.view.View;
20 import android.view.ViewGroup;
21 
22 /**
23  * The base pager view holder implementation that handles basic view creation/destruction logic,
24  * as well as logic to save/restore instance state that's persisted not only for activity
25  * reconstruction (e.g. during a configuration change), but also cases where the individual
26  * page view gets destroyed and recreated.
27  *
28  * To opt into saving/restoring instance state behavior for a particular page view, let the
29  * PageView implement PersistentInstanceState to save and restore instance states to/from a
30  * Parcelable.
31  */
32 public abstract class BasePagerViewHolder implements PagerViewHolder {
33     protected View mView;
34     protected Parcelable mSavedState;
35 
36     /**
37      * This is called when the entire view pager is being torn down (due to configuration change
38      * for example) that will be restored later.
39      */
40     @Override
saveState()41     public Parcelable saveState() {
42         savePendingState();
43         return mSavedState;
44     }
45 
46     /**
47      * This is called when the view pager is being restored.
48      */
49     @Override
restoreState(final Parcelable restoredState)50     public void restoreState(final Parcelable restoredState) {
51         if (restoredState != null) {
52             mSavedState = restoredState;
53             // If the view is already there, let it restore the state. Otherwise, it will be
54             // restored the next time the view gets created.
55             restorePendingState();
56         }
57     }
58 
59     @Override
resetState()60     public void resetState() {
61         mSavedState = null;
62         if (mView != null && (mView instanceof PersistentInstanceState)) {
63             ((PersistentInstanceState) mView).resetState();
64         }
65     }
66 
67     /**
68      * This is called when the view itself is being torn down. This may happen when the user
69      * has flipped to another page in the view pager, so we want to save the current state if
70      * possible.
71      */
72     @Override
destroyView()73     public View destroyView() {
74         savePendingState();
75         final View retView = mView;
76         mView = null;
77         return retView;
78     }
79 
80     @Override
getView(ViewGroup container)81     public View getView(ViewGroup container) {
82         if (mView == null) {
83             mView = createView(container);
84             // When initially created, check if the view has any saved state. If so restore it.
85             restorePendingState();
86         }
87         return mView;
88     }
89 
savePendingState()90     private void savePendingState() {
91         if (mView != null && (mView instanceof PersistentInstanceState)) {
92             mSavedState = ((PersistentInstanceState) mView).saveState();
93         }
94     }
95 
restorePendingState()96     private void restorePendingState() {
97         if (mView != null && (mView instanceof PersistentInstanceState) && (mSavedState != null)) {
98             ((PersistentInstanceState) mView).restoreState(mSavedState);
99         }
100     }
101 
102     /**
103      * Create and initialize a new page view.
104      */
createView(ViewGroup container)105     protected abstract View createView(ViewGroup container);
106 }
107