ObservableCombiner.java revision 81308a22b0f64c6667f6c23adee9da520415bcb6 
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
17package com.android.camera.async;
18
19import com.google.common.base.Function;
20import com.google.common.base.Supplier;
21import com.google.common.collect.ImmutableList;
22
23import java.util.ArrayList;
24import java.util.List;
25import java.util.concurrent.Executor;
26
27import javax.annotation.CheckReturnValue;
28import javax.annotation.Nonnull;
29import javax.annotation.ParametersAreNonnullByDefault;
30import 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
41final class ObservableCombiner<T> implements Observable<T> {
42    private final ImmutableList<Observable<?>> mInputs;
43    private final Supplier<T> mOutput;
44
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     */
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