src/Promise.php
public function then(callable $onFulfilled = null, callable $onRejected = null)
    {
        $newPromise = new self($this->loop);

        $onFulfilled = null !== $onFulfilled ? $onFulfilled : function (ResponseInterface $response) {
            return $response;
        };

        $onRejected = null !== $onRejected ? $onRejected : function (Exception $exception) {
            throw $exception;
        };

        $this->onFulfilled = function (ResponseInterface $response) use ($onFulfilled, $newPromise) {
            try {
                $return = $onFulfilled($response);

                $newPromise->resolve(null !== $return ? $return : $response);
            } catch (Exception $exception) {
                $newPromise->reject($exception);
            }
        };

        $this->onRejected = function (Exception $exception) use ($onRejected, $newPromise) {
            try {
                $newPromise->resolve($onRejected($exception));
            } catch (Exception $exception) {
                $newPromise->reject($exception);
            }
        };

        if (HttpPromise::FULFILLED === $this->state) {
            $this->doResolve($this->response);
        }

        if (HttpPromise::REJECTED === $this->state) {
            $this->doReject($this->exception);
        }

        return $newPromise;
    }
public function then(callable $onFulfilled = null, callable $onRejected = null)
    {
        $newPromise = new self($this->loop);

        $onFulfilled = null !== $onFulfilled ? $onFulfilled : function (ResponseInterface $response) {
            return $response;
        };

        $onRejected = null !== $onRejected ? $onRejected : function (Exception $exception) {
            throw $exception;
        };

        $this->onFulfilled = function (ResponseInterface $response) use ($onFulfilled, $newPromise) {
            try {
                $return = $onFulfilled($response);

                $newPromise->resolve(null !== $return ? $return : $response);
            } catch (Exception $exception) {
                $newPromise->reject($exception);
            }
        };

        $this->onRejected = function (Exception $exception) use ($onRejected, $newPromise) {
            try {
                $newPromise->resolve($onRejected($exception));
            } catch (Exception $exception) {
                $newPromise->reject($exception);
            }
        };

        if (HttpPromise::FULFILLED === $this->state) {
            $this->doResolve($this->response);
        }

        if (HttpPromise::REJECTED === $this->state) {
            $this->doReject($this->exception);
        }

        return $newPromise;
    }
Allow to apply callable when the promise resolve.

@param callable|null $onFulfilled
@param callable|null $onRejected

@return Promise