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.google.common.base.Function;
20 import com.google.common.base.Supplier;
21 import com.google.common.collect.ImmutableList;
22 
23 import java.util.ArrayList;
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 import javax.annotation.concurrent.ThreadSafe;
31 
32 /**
33  * Enables combining multiple {@link Observable}s together with a given
34  * function.
35  * <p>
36  * Callbacks added to the resulting observable are notified when any of the
37  * dependencies change.
38  */
39 @ThreadSafe
40 @ParametersAreNonnullByDefault
41 final class ObservableCombiner<T> implements Observable<T> {
42     private final ImmutableList<Observable<?>> mInputs;
43     private final Supplier<T> mOutput;
44 
ObservableCombiner(List<? extends Observable<?>> inputs, Supplier<T> output)45     private ObservableCombiner(List<? extends Observable<?>> inputs,
46             Supplier<T> output) {
47         mInputs = ImmutableList.copyOf(inputs);
48         mOutput = output;
49     }
50 
51     /**
52      * Transforms a set of input observables with a function.
53      *
54      * @param inputs The input observables.
55      * @param function The function to apply to all of the inputs.
56      * @param <I> The type of all inputs values.
57      * @param <O> The type of the output values.
58      * @return An observable which will reflect the combination of all inputs
59      *         with the given function. Changes in the output value will result
60      *         in calls to any callbacks registered with the output.
61      */
transform(final List<? extends Observable<I>> inputs, final Function<List<I>, O> function)62     static <I, O> Observable<O> transform(final List<? extends Observable<I>> inputs,
63             final Function<List<I>, O> function) {
64         return new ObservableCombiner<>(inputs, new Supplier<O>() {
65             @Override
66             public O get() {
67                 ArrayList<I> deps = new ArrayList<>();
68                 for (Observable<? extends I> dependency : inputs) {
69                     deps.add(dependency.get());
70                 }
71                 return function.apply(deps);
72             }
73         });
74     }
75 
76     static <O> Observable<O> transform(final List<? extends Observable<?>> inputs,
77                                        final Supplier<O> output) {
78         return new ObservableCombiner<>(inputs, output);
79     }
80 
81     @Nonnull
82     @Override
83     @CheckReturnValue
84     public SafeCloseable addCallback(Runnable callback, Executor executor) {
85         Lifetime callbackLifetime = new Lifetime();
86 
87         for (Observable<?> input : mInputs) {
88             callbackLifetime.add(input.addCallback(callback, executor));
89         }
90 
91         return callbackLifetime;
92     }
93 
94     @Nonnull
95     @Override
96     public T get() {
97         return mOutput.get();
98     }
99 }
100