vendor/guzzlehttp/promises/src/Promise.php

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<?php

namespace GuzzleHttp\Promise;

/**
 * Promises/A+ implementation that avoids recursion when possible.
 *
 * @link https://promisesaplus.com/
 */
class Promise implements PromiseInterface
{
    private $state = self::PENDING;
    private $result;
    private $cancelFn;
    private $waitFn;
    private $waitList;
    private $handlers = [];

    /**
     * @param callable $waitFn   Fn that when invoked resolves the promise.
     * @param callable $cancelFn Fn that when invoked cancels the promise.
     */
    public function __construct(
        callable $waitFn = null,
        callable $cancelFn = null
    ) {
        $this->waitFn = $waitFn;
        $this->cancelFn = $cancelFn;
    }

    public function then(
        callable $onFulfilled = null,
        callable $onRejected = null
    ) {
        if ($this->state === self::PENDING) {
            $p = new Promise(null, [$this, 'cancel']);
            $this->handlers[] = [$p, $onFulfilled, $onRejected];
            $p->waitList = $this->waitList;
            $p->waitList[] = $this;
            return $p;
        }

        // Return a fulfilled promise and immediately invoke any callbacks.
        if ($this->state === self::FULFILLED) {
            $promise = Create::promiseFor($this->result);
            return $onFulfilled ? $promise->then($onFulfilled) : $promise;
        }

        // It's either cancelled or rejected, so return a rejected promise
        // and immediately invoke any callbacks.
        $rejection = Create::rejectionFor($this->result);
        return $onRejected ? $rejection->then(null, $onRejected) : $rejection;
    }

    public function otherwise(callable $onRejected)
    {
        return $this->then(null, $onRejected);
    }

    public function wait($unwrap = true)