/*
 * 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&lt;String&gt; foo = foo();
 * foo.onResolve(new Runnable() {
 *   public void run() {
 *     System.out.println(foo.getValue());
 *   }
 * });
 * </pre>
 * 
 * Example chaining usage;
 * 
 * <pre>
 * Success&lt;String,String&gt; doubler = new Success&lt;String,String&gt;() {
 *   public Promise&lt;String&gt; call(Promise&lt;String&gt; p) throws Exception {
 *     return Promises.resolved(p.getValue()+p.getValue());
 *   }
 * };
 * final Promise&lt;String&gt; 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: 8a24f66339967ecdd03def57f31ac8c371a7f4cd $
 */
@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 will
	 * 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 will be failed with the exception.
	 * 
	 * <p>
	 * If this Promise is resolved with a failure, the returned Promise will 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 will 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 will be failed with the exception.
	 * 
	 * <p>
	 * If this Promise is resolved with a failure, the returned Promise will 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 will map the value of this Promise to the
	 *        value that will 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 will 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 will be failed with the exception.
	 * 
	 * <p>
	 * If this Promise is resolved with a failure, the returned Promise will 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 will flatMap the value of this Promise to
	 *        a Promise that will 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 will 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 will
	 * be resolved with the recovery value.</li>
	 * <li>If the recovery value is {@code null}, the returned Promise will be
	 * failed with the failure of this Promise.</li>
	 * <li>If the specified Function throws an exception, the returned Promise
	 * will 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 will 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
	 * will be resolved with the recovery Promise.</li>
	 * <li>If the recovery Promise is {@code null}, the returned Promise will be
	 * failed with the failure of this Promise.</li>
	 * <li>If the specified Function throws an exception, the returned Promise
	 * will 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 will 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 will
	 * 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 will 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);
}