ObservableCombiner.java revision e919a48fb40b9d6c698a495acf40adbc0e320431 
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.android.camera.util.Callback;
20import com.google.common.base.Function;
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.GuardedBy;
31import javax.annotation.concurrent.ThreadSafe;
32
33/**
34 * Enables combining multiple {@link Observable}s together with a given
35 * function.
36 * <p>
37 * Callbacks added to the resulting observable are notified when any of the
38 * dependencies change.
39 */
40@ThreadSafe
41@ParametersAreNonnullByDefault
42final class ObservableCombiner<I, O> implements Observable<O> {
43    private final ImmutableList<Observable<I>> mInputs;
44    private final Function<List<I>, O> mFunction;
45
46    private final Object mLock;
47
48    @GuardedBy("mLock")
49    private final ConcurrentState<O> mListenerNotifier;
50
51    @GuardedBy("mLock")
52    private final List<SafeCloseable> mInputCallbackHandles;
53
54    @GuardedBy("mLock")
55    private int mNumRegisteredCallbacks;
56
57    /**
58     * The thread-safe callback to be registered with each input.
59     */
60    private final Updatable<I> mInputCallback = new Updatable<I>() {
61        public void update(I ignored) {
62            mListenerNotifier.update(get());
63        }
64    };
65
66    private ObservableCombiner(List<? extends Observable<I>> inputs,
67            Function<List<I>, O> function, O initialValue) {
68        mInputs = ImmutableList.copyOf(inputs);
69        mFunction = function;
70        mListenerNotifier = new ConcurrentState<>(initialValue);
71        mLock = new Object();
72        mInputCallbackHandles = new ArrayList<>();
73        mNumRegisteredCallbacks = 0;
74    }
75
76    /**
77     * Transforms a set of input observables with a function.
78     *
79     * @param inputs The input observables.
80     * @param function The function to apply to all of the inputs.
81     * @param <I> The type of all inputs values.
82     * @param <O> The type of the output values.
83     * @return An observable which will reflect the combination of all inputs
84     *         with the given function. Changes in the output value will result
85     *         in calls to any callbacks registered with the output.
86     */
87    static <I, O> Observable<O> transform(List<? extends Observable<I>> inputs,
88            Function<List<I>, O> function) {
89        // Compute the initial value.
90        ArrayList<I> deps = new ArrayList<>();
91        for (Observable<? extends I> input : inputs) {
92            deps.add(input.get());
93        }
94        O initialValue = function.apply(deps);
95
96        return new ObservableCombiner<>(inputs, function, initialValue);
97    }
98
99    @GuardedBy("mLock")
100    private void addCallbacksToInputs() {
101        for (Observable<I> observable : mInputs) {
102            final SafeCloseable callbackHandle =
103                    Observables.addThreadSafeCallback(observable, mInputCallback);
104
105            mInputCallbackHandles.add(callbackHandle);
106        }
107    }
108
109    @GuardedBy("mLock")
110    private void removeCallbacksFromInputs() {
111        for (SafeCloseable callbackHandle : mInputCallbackHandles) {
112            callbackHandle.close();
113        }
114    }
115
116    @Nonnull
117    @Override
118    @CheckReturnValue
119    public SafeCloseable addCallback(final Callback<O> callback, Executor executor) {
120        // When a callback is added to this, the "output", we must ensure that
121        // callbacks are registered with all of the inputs so that they can be
122        // forwarded properly.
123        // Instead of adding another callback to each input for each callback
124        // registered with the output, callbacks are registered when the first
125        // output callback is added, and removed when the last output callback
126        // is removed.
127
128        synchronized (mLock) {
129            if (mNumRegisteredCallbacks == 0) {
130                addCallbacksToInputs();
131            }
132            mNumRegisteredCallbacks++;
133        }
134
135        // Wrap the callback in a {@link FilteredCallback} to prevent many
136        // duplicate/cascading updates even if the output does not change.
137        final SafeCloseable resultingCallbackHandle = mListenerNotifier.addCallback(
138                new FilteredCallback<O>(callback), executor);
139
140        return new SafeCloseable() {
141            @Override
142            public void close() {
143                resultingCallbackHandle.close();
144
145                synchronized (mLock) {
146                    mNumRegisteredCallbacks--;
147                    if (mNumRegisteredCallbacks == 0) {
148                        removeCallbacksFromInputs();
149                    }
150                }
151            }
152        };
153    }
154
155    @Nonnull
156    @Override
157    public O get() {
158        ArrayList<I> deps = new ArrayList<>();
159        for (Observable<? extends I> dependency : mInputs) {
160            deps.add(dependency.get());
161        }
162        return mFunction.apply(deps);
163    }
164}
165