Coroutine.php

<?php

declare (strict_types=1);
namespace GuzzleHttp\Promise;

use Generator;
use Throwable;

/**
 * Creates a promise that is resolved using a generator that yields values or
 * promises (somewhat similar to C#'s async keyword).
 *
 * When called, the Coroutine::of method will start an instance of the generator
 * and returns a promise that is fulfilled with its final yielded value.
 *
 * Control is returned back to the generator when the yielded promise settles.
 * This can lead to less verbose code when doing lots of sequential async calls
 * with minimal processing in between.
 *
 *     use GuzzleHttp\Promise;
 *
 *     function createPromise($value) {
 *         return new Promise\FulfilledPromise($value);
 *     }
 *
 *     $promise = Promise\Coroutine::of(function () {
 *         $value = (yield createPromise('a'));
 *         try {
 *             $value = (yield createPromise($value . 'b'));
 *         } catch (\Throwable $e) {
 *             // The promise was rejected.
 *         }
 *         yield $value . 'c';
 *     });
 *
 *     // Outputs "abc"
 *     $promise->then(function ($v) { echo $v; });
 *
 * @param callable $generatorFn Generator function to wrap into a promise.
 *
 * @return Promise
 *
 * @see https://github.com/petkaantonov/bluebird/blob/master/API.md#generators inspiration
 */
final class Coroutine implements PromiseInterface {
    
    /**
     * @var PromiseInterface|null
     */
    private $currentPromise;
    
    /**
     * @var Generator
     */
    private $generator;
    
    /**
     * @var Promise
     */
    private $result;
    public function __construct(callable $generatorFn) {
        $this->generator = $generatorFn();
        $this->result = new Promise(function () : void {
            while (isset($this->currentPromise)) {
                $this->currentPromise
                    ->wait();
            }
        });
        try {
            $this->nextCoroutine($this->generator
                ->current());
        } catch (Throwable $throwable) {
            $this->result
                ->reject($throwable);
        }
    }
    
    /**
     * Create a new coroutine.
     */
    public static function of(callable $generatorFn) : self {
        return new self($generatorFn);
    }
    public function then(?callable $onFulfilled = null, ?callable $onRejected = null) : PromiseInterface {
        return $this->result
            ->then($onFulfilled, $onRejected);
    }
    public function otherwise(callable $onRejected) : PromiseInterface {
        return $this->result
            ->otherwise($onRejected);
    }
    public function wait(bool $unwrap = true) {
        return $this->result
            ->wait($unwrap);
    }
    public function getState() : string {
        return $this->result
            ->getState();
    }
    public function resolve($value) : void {
        $this->result
            ->resolve($value);
    }
    public function reject($reason) : void {
        $this->result
            ->reject($reason);
    }
    public function cancel() : void {
        $this->currentPromise
            ->cancel();
        $this->result
            ->cancel();
    }
    private function nextCoroutine($yielded) : void {
        $this->currentPromise = Create::promiseFor($yielded)->then([
            $this,
            '_handleSuccess',
        ], [
            $this,
            '_handleFailure',
        ]);
    }
    
    /**
     * @internal
     */
    public function _handleSuccess($value) : void {
        unset($this->currentPromise);
        try {
            $next = $this->generator
                ->send($value);
            if ($this->generator
                ->valid()) {
                $this->nextCoroutine($next);
            }
            else {
                $this->result
                    ->resolve($value);
            }
        } catch (Throwable $throwable) {
            $this->result
                ->reject($throwable);
        }
    }
    
    /**
     * @internal
     */
    public function _handleFailure($reason) : void {
        unset($this->currentPromise);
        try {
            $nextYield = $this->generator
                ->throw(Create::exceptionFor($reason));
            // The throw was caught, so keep iterating on the coroutine
            $this->nextCoroutine($nextYield);
        } catch (Throwable $throwable) {
            $this->result
                ->reject($throwable);
        }
    }

}