/*
* Copyright (c) OSGi Alliance (2014). All Rights Reserved.
*
* 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 org.osgi.util.promise;
import java.lang.reflect.InvocationTargetException;
import org.osgi.annotation.versioning.ProviderType;
import org.osgi.util.function.Function;
import org.osgi.util.function.Predicate;
/**
* A Promise of a value.
*
* <p>
* A Promise represents a future value. It handles the interactions for
* asynchronous processing. A {@link Deferred} object can be used to create a
* Promise and later resolve the Promise. A Promise is used by the caller of an
* asynchronous function to get the result or handle the error. The caller can
* either get a callback when the Promise is resolved with a value or an error,
* or the Promise can be used in chaining. In chaining, callbacks are provided
* that receive the resolved Promise, and a new Promise is generated that
* resolves based upon the result of a callback.
*
* <p>
* Both {@link #onResolve(Runnable) callbacks} and
* {@link #then(Success, Failure) chaining} can be repeated any number of times,
* even after the Promise has been resolved.
* <p>
* Example callback usage:
*
* <pre>
* final Promise<String> foo = foo();
* foo.onResolve(new Runnable() {
* public void run() {
* System.out.println(foo.getValue());
* }
* });
* </pre>
*
* Example chaining usage;
*
* <pre>
* Success<String,String> doubler = new Success<String,String>() {
* public Promise<String> call(Promise<String> p) throws Exception {
* return Promises.resolved(p.getValue()+p.getValue());
* }
* };
* final Promise<String> foo = foo().then(doubler).then(doubler);
* foo.onResolve(new Runnable() {
* public void run() {
* System.out.println(foo.getValue());
* }
* });
* </pre>
*
* @param <T> The value type associated with this Promise.
*
* @ThreadSafe
* @author $Id: ffa193393576e4940a9b38635c5900ddf9067ce2 $
*/
@ProviderType
public interface Promise<T> {
/**
* Returns whether this Promise has been resolved.
*
* <p>
* This Promise may be successfully resolved or resolved with a failure.
*
* @return {@code true} if this Promise was resolved either successfully or
* with a failure; {@code false} if this Promise is unresolved.
*/
boolean isDone();
/**
* Returns the value of this Promise.
*
* <p>
* If this Promise is not {@link #isDone() resolved}, this method must block
* and wait for this Promise to be resolved before completing.
*
* <p>
* If this Promise was successfully resolved, this method returns with the
* value of this Promise. If this Promise was resolved with a failure, this
* method must throw an {@code InvocationTargetException} with the
* {@link #getFailure() failure exception} as the cause.
*
* @return The value of this resolved Promise.
* @throws InvocationTargetException If this Promise was resolved with a
* failure. The cause of the {@code InvocationTargetException} is
* the failure exception.
* @throws InterruptedException If the current thread was interrupted while
* waiting.
*/
T getValue() throws InvocationTargetException, InterruptedException;
/**
* Returns the failure of this Promise.
*
* <p>
* If this Promise is not {@link #isDone() resolved}, this method must block
* and wait for this Promise to be resolved before completing.
*
* <p>
* If this Promise was resolved with a failure, this method returns with the
* failure of this Promise. If this Promise was successfully resolved, this
* method must return {@code null}.
*
* @return The failure of this resolved Promise or {@code null} if this
* Promise was successfully resolved.
* @throws InterruptedException If the current thread was interrupted while
* waiting.
*/
Throwable getFailure() throws InterruptedException;
/**
* Register a callback to be called when this Promise is resolved.
*
* <p>
* The specified callback is called when this Promise is resolved either
* successfully or with a failure.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* <p>
* Resolving this Promise <i>happens-before</i> any registered callback is
* called. That is, in a registered callback, {@link #isDone()} must return
* {@code true} and {@link #getValue()} and {@link #getFailure()} must not
* block.
*
* <p>
* A callback may be called on a different thread than the thread which
* registered the callback. So the callback must be thread safe but can rely
* upon that the registration of the callback <i>happens-before</i> the
* registered callback is called.
*
* @param callback A callback to be called when this Promise is resolved.
* Must not be {@code null}.
* @return This Promise.
*/
Promise<T> onResolve(Runnable callback);
/**
* Chain a new Promise to this Promise with Success and Failure callbacks.
*
* <p>
* The specified {@link Success} callback is called when this Promise is
* successfully resolved and the specified {@link Failure} callback is
* called when this Promise is resolved with a failure.
*
* <p>
* This method returns a new Promise which is chained to this Promise. The
* returned Promise must be resolved when this Promise is resolved after the
* specified Success or Failure callback is executed. The result of the
* executed callback must be used to resolve the returned Promise. Multiple
* calls to this method can be used to create a chain of promises which are
* resolved in sequence.
*
* <p>
* If this Promise is successfully resolved, the Success callback is
* executed and the result Promise, if any, or thrown exception is used to
* resolve the returned Promise from this method. If this Promise is
* resolved with a failure, the Failure callback is executed and the
* returned Promise from this method is failed.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* <p>
* Resolving this Promise <i>happens-before</i> any registered callback is
* called. That is, in a registered callback, {@link #isDone()} must return
* {@code true} and {@link #getValue()} and {@link #getFailure()} must not
* block.
*
* <p>
* A callback may be called on a different thread than the thread which
* registered the callback. So the callback must be thread safe but can rely
* upon that the registration of the callback <i>happens-before</i> the
* registered callback is called.
*
* @param <R> The value type associated with the returned Promise.
* @param success A Success callback to be called when this Promise is
* successfully resolved. May be {@code null} if no Success callback
* is required. In this case, the returned Promise must be resolved
* with the value {@code null} when this Promise is successfully
* resolved.
* @param failure A Failure callback to be called when this Promise is
* resolved with a failure. May be {@code null} if no Failure
* callback is required.
* @return A new Promise which is chained to this Promise. The returned
* Promise must be resolved when this Promise is resolved after the
* specified Success or Failure callback, if any, is executed.
*/
<R> Promise<R> then(Success<? super T, ? extends R> success, Failure failure);
/**
* Chain a new Promise to this Promise with a Success callback.
*
* <p>
* This method performs the same function as calling
* {@link #then(Success, Failure)} with the specified Success callback and
* {@code null} for the Failure callback.
*
* @param <R> The value type associated with the returned Promise.
* @param success A Success callback to be called when this Promise is
* successfully resolved. May be {@code null} if no Success callback
* is required. In this case, the returned Promise must be resolved
* with the value {@code null} when this Promise is successfully
* resolved.
* @return A new Promise which is chained to this Promise. The returned
* Promise must be resolved when this Promise is resolved after the
* specified Success, if any, is executed.
* @see #then(Success, Failure)
*/
<R> Promise<R> then(Success<? super T, ? extends R> success);
/**
* Filter the value of this Promise.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must
* either be resolved with the value of this Promise, if the specified
* Predicate accepts that value, or failed with a
* {@code NoSuchElementException}, if the specified Predicate does not
* accept that value. If the specified Predicate throws an exception, the
* returned Promise must be failed with the exception.
*
* <p>
* If this Promise is resolved with a failure, the returned Promise must be
* failed with that failure.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param predicate The Predicate to evaluate the value of this Promise.
* Must not be {@code null}.
* @return A Promise that filters the value of this Promise.
*/
Promise<T> filter(Predicate<? super T> predicate);
/**
* Map the value of this Promise.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must be
* resolved with the value of specified Function as applied to the value of
* this Promise. If the specified Function throws an exception, the returned
* Promise must be failed with the exception.
*
* <p>
* If this Promise is resolved with a failure, the returned Promise must be
* failed with that failure.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param <R> The value type associated with the returned Promise.
* @param mapper The Function that must map the value of this Promise to the
* value that must be used to resolve the returned Promise. Must not
* be {@code null}.
* @return A Promise that returns the value of this Promise as mapped by the
* specified Function.
*/
<R> Promise<R> map(Function<? super T, ? extends R> mapper);
/**
* FlatMap the value of this Promise.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must be
* resolved with the Promise from the specified Function as applied to the
* value of this Promise. If the specified Function throws an exception, the
* returned Promise must be failed with the exception.
*
* <p>
* If this Promise is resolved with a failure, the returned Promise must be
* failed with that failure.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param <R> The value type associated with the returned Promise.
* @param mapper The Function that must flatMap the value of this Promise to
* a Promise that must be used to resolve the returned Promise. Must
* not be {@code null}.
* @return A Promise that returns the value of this Promise as mapped by the
* specified Function.
*/
<R> Promise<R> flatMap(Function<? super T, Promise<? extends R>> mapper);
/**
* Recover from a failure of this Promise with a recovery value.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must be
* resolved with the value of this Promise.
*
* <p>
* If this Promise is resolved with a failure, the specified Function is
* applied to this Promise to produce a recovery value.
* <ul>
* <li>If the recovery value is not {@code null}, the returned Promise must
* be resolved with the recovery value.</li>
* <li>If the recovery value is {@code null}, the returned Promise must be
* failed with the failure of this Promise.</li>
* <li>If the specified Function throws an exception, the returned Promise
* must be failed with that exception.</li>
* </ul>
*
* <p>
* To recover from a failure of this Promise with a recovery value of
* {@code null}, the {@link #recoverWith(Function)} method must be used. The
* specified Function for {@link #recoverWith(Function)} can return
* {@code Promises.resolved(null)} to supply the desired {@code null} value.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param recovery If this Promise resolves with a failure, the specified
* Function is called to produce a recovery value to be used to
* resolve the returned Promise. Must not be {@code null}.
* @return A Promise that resolves with the value of this Promise or
* recovers from the failure of this Promise.
*/
Promise<T> recover(Function<Promise<?>, ? extends T> recovery);
/**
* Recover from a failure of this Promise with a recovery Promise.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must be
* resolved with the value of this Promise.
*
* <p>
* If this Promise is resolved with a failure, the specified Function is
* applied to this Promise to produce a recovery Promise.
* <ul>
* <li>If the recovery Promise is not {@code null}, the returned Promise
* must be resolved with the recovery Promise.</li>
* <li>If the recovery Promise is {@code null}, the returned Promise must be
* failed with the failure of this Promise.</li>
* <li>If the specified Function throws an exception, the returned Promise
* must be failed with that exception.</li>
* </ul>
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param recovery If this Promise resolves with a failure, the specified
* Function is called to produce a recovery Promise to be used to
* resolve the returned Promise. Must not be {@code null}.
* @return A Promise that resolves with the value of this Promise or
* recovers from the failure of this Promise.
*/
Promise<T> recoverWith(Function<Promise<?>, Promise<? extends T>> recovery);
/**
* Fall back to the value of the specified Promise if this Promise fails.
*
* <p>
* If this Promise is successfully resolved, the returned Promise must be
* resolved with the value of this Promise.
*
* <p>
* If this Promise is resolved with a failure, the successful result of the
* specified Promise is used to resolve the returned Promise. If the
* specified Promise is resolved with a failure, the returned Promise must
* be failed with the failure of this Promise rather than the failure of the
* specified Promise.
*
* <p>
* This method may be called at any time including before and after this
* Promise has been resolved.
*
* @param fallback The Promise whose value must be used to resolve the
* returned Promise if this Promise resolves with a failure. Must not
* be {@code null}.
* @return A Promise that returns the value of this Promise or falls back to
* the value of the specified Promise.
*/
Promise<T> fallbackTo(Promise<? extends T> fallback);
}