1 /*
2  * Copyright (C) 2020 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 
17 package com.android.internal.listeners;
18 
19 import android.annotation.Nullable;
20 
21 import java.util.Objects;
22 import java.util.concurrent.Executor;
23 import java.util.function.Consumer;
24 
25 /**
26  * A listener transport object which can run listener operations on an executor.
27  *
28  * @param <TListener> listener type
29  */
30 public interface ListenerTransport<TListener> {
31 
32     /**
33      * Should return a valid listener until {@link #unregister()} is invoked, and must return
34      * null after that. Recommended (but not required) that this is implemented via a volatile
35      * variable.
36      */
getListener()37     @Nullable TListener getListener();
38 
39     /**
40      * Must be implemented so that {@link #getListener()} returns null after this is invoked.
41      */
unregister()42     void unregister();
43 
44     /**
45      * Executes the given operation for the listener.
46      */
execute(Executor executor, Consumer<TListener> operation)47     default void execute(Executor executor, Consumer<TListener> operation) {
48         Objects.requireNonNull(operation);
49 
50         if (getListener() == null) {
51             return;
52         }
53 
54         executor.execute(() -> {
55             TListener listener = getListener();
56             if (listener == null) {
57                 return;
58             }
59 
60             operation.accept(listener);
61         });
62     }
63 }
64