<?php

namespace GuzzleHttp\Promise;

use LogicException;
use RuntimeException;

/**
 * A promise represents the eventual result of an asynchronous operation.
 *
 * The primary way of interacting with a promise is through its then method,
 * which registers callbacks to receive either a promise’s eventual value or
 * the reason why the promise cannot be fulfilled.
 *
 * @link https://promisesaplus.com/
 */
interface PromiseInterface {
	const PENDING = 'pending';
	const FULFILLED = 'fulfilled';
	const REJECTED = 'rejected';

	/**
	 * Appends fulfillment and rejection handlers to the promise, and returns
	 * a new promise resolving to the return value of the called handler.
	 *
	 * @param callable $onFulfilled Invoked when the promise fulfills.
	 * @param callable $onRejected Invoked when the promise is rejected.
	 *
	 * @return PromiseInterface
	 */
	public function then(
		callable $onFulfilled = null,
		callable $onRejected = null
	);

	/**
	 * Appends a rejection handler callback to the promise, and returns a new
	 * promise resolving to the return value of the callback if it is called,
	 * or to its original fulfillment value if the promise is instead
	 * fulfilled.
	 *
	 * @param callable $onRejected Invoked when the promise is rejected.
	 *
	 * @return PromiseInterface
	 */
	public function otherwise( callable $onRejected );

	/**
	 * Get the state of the promise ("pending", "rejected", or "fulfilled").
	 *
	 * The three states can be checked against the constants defined on
	 * PromiseInterface: PENDING, FULFILLED, and REJECTED.
	 *
	 * @return string
	 */
	public function getState();

	/**
	 * Resolve the promise with the given value.
	 *
	 * @param mixed $value
	 *
	 * @throws RuntimeException if the promise is already resolved.
	 */
	public function resolve( $value );

	/**
	 * Reject the promise with the given reason.
	 *
	 * @param mixed $reason
	 *
	 * @throws RuntimeException if the promise is already resolved.
	 */
	public function reject( $reason );

	/**
	 * Cancels the promise if possible.
	 *
	 * @link https://github.com/promises-aplus/cancellation-spec/issues/7
	 */
	public function cancel();

	/**
	 * Waits until the promise completes if possible.
	 *
	 * Pass $unwrap as true to unwrap the result of the promise, either
	 * returning the resolved value or throwing the rejected exception.
	 *
	 * If the promise cannot be waited on, then the promise will be rejected.
	 *
	 * @param bool $unwrap
	 *
	 * @return mixed
	 * @throws LogicException if the promise has no wait function or if the
	 *                         promise does not settle after waiting.
	 */
	public function wait( $unwrap = true );
}
