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 
17 package com.android.camera.async;
18 
19 import com.android.camera.util.Callback;
20 import com.google.common.base.Function;
21 import com.google.common.base.Supplier;
22 import com.google.common.util.concurrent.MoreExecutors;
23 
24 import java.util.List;
25 import java.util.concurrent.Executor;
26 
27 import javax.annotation.CheckReturnValue;
28 import javax.annotation.Nonnull;
29 import javax.annotation.ParametersAreNonnullByDefault;
30 
31 /**
32  * Helper methods for {@link Observable}.
33  */
34 @ParametersAreNonnullByDefault
35 public class Observables {
36     private static final SafeCloseable NOOP_CALLBACK_HANDLE = new SafeCloseable() {
37         @Override
38         public void close() {
39             // Do Nothing.
40         }
41     };
42 
Observables()43     private Observables() {
44     }
45 
46     /**
47      * Transforms an observable with a function.
48      *
49      * @return The transformed observable.
50      */
transform(final Observable<F> input, final Function<F, T> function)51     public static <F, T> Observable<T> transform(final Observable<F> input,
52             final Function<F, T> function) {
53         return new Observable<T>() {
54             @Nonnull
55             @Override
56             public T get() {
57                 return function.apply(input.get());
58             }
59 
60             @CheckReturnValue
61             @Nonnull
62             @Override
63             public SafeCloseable addCallback(Runnable callback, Executor executor) {
64                 return input.addCallback(callback, executor);
65             }
66         };
67     }
68 
69     /**
70      * @return An observable which recomputes its output every time an input changes.
71      */
72     public static <T> Observable<T> transform(final List<? extends Observable<?>> inputs,
73                                               final Supplier<T> output) {
74         return ObservableCombiner.transform(inputs, output);
75     }
76 
77     /**
78      * Transforms a set of observables with a function.
79      *
80      * @return The transformed observable.
81      */
82     public static <F, T> Observable<T> transform(final List<? extends Observable<F>> input,
83             Function<List<F>, T> function) {
84         return ObservableCombiner.transform(input, function);
85     }
86 
87     /**
88      * @return An observable which has the given constant value.
89      */
90     @Nonnull
91     public static <T> Observable<T> of(final @Nonnull T constant) {
92         return new Observable<T>() {
93             @Nonnull
94             @Override
95             public T get() {
96                 return constant;
97             }
98 
99             @CheckReturnValue
100             @Nonnull
101             @Override
102             public SafeCloseable addCallback(Runnable callback, Executor executor) {
103                 return NOOP_CALLBACK_HANDLE;
104             }
105         };
106     }
107 
108     @Nonnull
109     @CheckReturnValue
110     public static <T> SafeCloseable addThreadSafeCallback(final Observable<T> observable,
111             final Updatable<T> callback) {
112         return observable.addCallback(new Runnable() {
113             @Override
114             public void run() {
115                 callback.update(observable.get());
116             }
117         }, MoreExecutors.sameThreadExecutor());
118     }
119 }
120