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/*
* Copyright 2022 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.android.libraries.mobiledatadownload.internal.util;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import com.google.android.libraries.mobiledatadownload.internal.annotations.SequentialControlExecutor;
import com.google.android.libraries.mobiledatadownload.tracing.PropagatedFutures;
import com.google.common.base.Function;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Executor;
/** Utilities for manipulating futures. */
public final class FuturesUtil {
private final Executor sequentialExecutor;
public FuturesUtil(@SequentialControlExecutor Executor sequentialExecutor) {
this.sequentialExecutor = sequentialExecutor;
}
/**
* Returns a SequentialFutureChain which can takes a number of asynchronous operation and turns
* them into a single asynchronous operation.
*
* <p>SequentialFutureChain provides a clearer way of writing a common idiom used to sequence a
* number of asynchrounous operations. The fragment
*
* <pre>{@code
* ListenableFuture<T> future = immediateFuture(init);
* future = transformAsync(future, arg -> asyncOp1, sequentialExecutor);
* future = transform(future, arg -> op2, sequentialExecutor);
* future = transformAsync(future, arg -> asyncOp3, sequentialExecutor);
* return future;
* }</pre>
*
* <p>can be rewritten as
*
* <pre>{@code
* return new FuturesUtil(sequentialExecutor)
* .newSequentialChain(init)
* .chainAsync(arg -> asyncOp1)
* .chain(arg -> op2)
* .chainAsync(arg -> asyncOp3)
* .start();
* }</pre>
*
* <p>If any intermediate operation raises an exception, the whole chain raises an exception.
*
* <p>Note that sequentialExecutor must be a sequential executor, i.e. provide the sequentiality
* guarantees provided by {@link com.google.common.util.concurrent.SequentialExecutor}.
*/
public <T> SequentialFutureChain<T> newSequentialChain(T init) {
return new SequentialFutureChain<>(init);
}
/**
* Create a SequentialFutureChain that doesn't compute a result.
*
* <p>If any intermediate operation raises an exception, the whole chain raises an exception.
*
* <p>Note that sequentialExecutor must be a sequential executor, i.e. provide the sequentiality
* guarantees provided by {@link com.google.common.util.concurrent.SequentialExecutor}.
*/
public SequentialFutureChain<Void> newSequentialChain() {
return new SequentialFutureChain<>(null);
}
/** Builds a list of Futurse to be executed sequentially. */
public final class SequentialFutureChain<T> {
private final List<FutureChainElement<T>> operations;
private final T init;
private SequentialFutureChain(T init) {
this.operations = new ArrayList<>();
this.init = init;
}
@CanIgnoreReturnValue
public SequentialFutureChain<T> chain(Function<T, T> operation) {
operations.add(new DirectFutureChainElement<>(operation));
return this;
}
@CanIgnoreReturnValue
public SequentialFutureChain<T> chainAsync(Function<T, ListenableFuture<T>> operation) {
operations.add(new AsyncFutureChainElement<>(operation));
return this;
}
public ListenableFuture<T> start() {
ListenableFuture<T> result = immediateFuture(init);
for (FutureChainElement<T> operation : operations) {
result = operation.apply(result);
}
return result;
}
}
private interface FutureChainElement<T> {
abstract ListenableFuture<T> apply(ListenableFuture<T> input);
}
private final class DirectFutureChainElement<T> implements FutureChainElement<T> {
private final Function<T, T> operation;
private DirectFutureChainElement(Function<T, T> operation) {
this.operation = operation;
}
@Override
public ListenableFuture<T> apply(ListenableFuture<T> input) {
return PropagatedFutures.transform(input, operation, sequentialExecutor);
}
}
private final class AsyncFutureChainElement<T> implements FutureChainElement<T> {
private final Function<T, ListenableFuture<T>> operation;
private AsyncFutureChainElement(Function<T, ListenableFuture<T>> operation) {
this.operation = operation;
}
@Override
public ListenableFuture<T> apply(ListenableFuture<T> input) {
return PropagatedFutures.transformAsync(input, operation::apply, sequentialExecutor);
}
}
}
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