<?php
declare(strict_types=1);
namespace Sabre\Event;
use Exception;
use Throwable;
/**
* An implementation of the Promise pattern.
*
* A promise represents the result of an asynchronous operation.
* At any given point a promise can be in one of three states:
*
* 1. Pending (the promise does not have a result yet).
* 2. Fulfilled (the asynchronous operation has completed with a result).
* 3. Rejected (the asynchronous operation has completed with an error).
*
* To get a callback when the operation has finished, use the `then` method.
*
* @copyright Copyright (C) fruux GmbH (https://fruux.com/)
* @author Evert Pot (http://evertpot.com/)
* @license http://sabre.io/license/ Modified BSD License
*/
class Promise
{
/**
* The asynchronous operation is pending.
*/
const PENDING = 0;
/**
* The asynchronous operation has completed, and has a result.
*/
const FULFILLED = 1;
/**
* The asynchronous operation has completed with an error.
*/
const REJECTED = 2;
/**
* The current state of this promise.
*
* @var int
*/
public $state = self::PENDING;
/**
* Creates the promise.
*
* The passed argument is the executor. The executor is automatically
* called with two arguments.
*
* Each are callbacks that map to $this->fulfill and $this->reject.
* Using the executor is optional.
*/
public function __construct(callable $executor = null)
{
if ($executor) {
$executor(
[$this, 'fulfill'],
[$this, 'reject']
);
}
}
/**
* This method allows you to specify the callback that will be called after
* the promise has been fulfilled or rejected.
*
* Both arguments are optional.
*
* This method returns a new promise, which can be used for chaining.
* If either the onFulfilled or onRejected callback is called, you may
* return a result from this callback.
*
* If the result of this callback is yet another promise, the result of
* _that_ promise will be used to set the result of the returned promise.
*
* If either of the callbacks return any other value, the returned promise
* is automatically fulfilled with that value.
*
* If either of the callbacks throw an exception, the returned promise will
* be rejected and the exception will be passed back.
*/
public function then(callable $onFulfilled = null, callable $onRejected = null): Promise
{
// This new subPromise will be returned from this function, and will
// be fulfilled with the result of the onFulfilled or onRejected event
// handlers.
$subPromise = new self();
switch ($this->state) {
case self::PENDING:
// The operation is pending, so we keep a reference to the
// event handlers so we can call them later.
$this->subscribers[] = [$subPromise, $onFulfilled, $onRejected];
break;
case self::FULFILLED:
// The async operation is already fulfilled, so we trigger the
// onFulfilled callback asap.
$this->invokeCallback($subPromise, $onFulfilled);
break;
case self::REJECTED:
// The async operation failed, so we call the onRejected
// callback asap.
$this->invokeCallback($subPromise, $onRejected);
break;
}
return $subPromise;
}
/**
* Add a callback for when this promise is rejected.
*
* Its usage is identical to then(). However, the otherwise() function is
* preferred.
*/
public function otherwise(callable $onRejected): Promise
{
return $this->then(null, $onRejected);
}
/**
* Marks this promise as fulfilled and sets its return value.
*
* @param mixed $value
*/
public function fulfill($value = null)
{
if (self::PENDING !== $this->state) {
throw new PromiseAlreadyResolvedException('This promise is already resolved, and you\'re not allowed to resolve a promise more than once');
}
$this->state = self::FULFILLED;
$this->value = $value;
foreach ($this->subscribers as $subscriber) {
$this->invokeCallback($subscriber[0], $subscriber[1]);
}
}
/**
* Marks this promise as rejected, and set it's rejection reason.
*/
public function reject(Throwable $reason)
{
if (self::PENDING !== $this->state) {
throw new PromiseAlreadyResolvedException('This promise is already resolved, and you\'re not allowed to resolve a promise more than once');
}
$this->state = self::REJECTED;
$this->value = $reason;
foreach ($this->subscribers as $subscriber) {
$this->invokeCallback($subscriber[0], $subscriber[2]);
}
}
/**
* Stops execution until this promise is resolved.
*
* This method stops execution completely. If the promise is successful with
* a value, this method will return this value. If the promise was
* rejected, this method will throw an exception.
*
* This effectively turns the asynchronous operation into a synchronous
* one. In PHP it might be useful to call this on the last promise in a
* chain.
*
* @return mixed
*/
public function wait()
{
$hasEvents = true;
while (self::PENDING === $this->state) {
if (!$hasEvents) {
throw new \LogicException('There were no more events in the loop. This promise will never be fulfilled.');
}
// As long as the promise is not fulfilled, we tell the event loop
// to handle events, and to block.
$hasEvents = Loop\tick(true);
}
if (self::FULFILLED === $this->state) {
// If the state of this promise is fulfilled, we can return the value.
return $this->value;
} else {
// If we got here, it means that the asynchronous operation
// errored. Therefore we need to throw an exception.
throw $this->value;
}
}
/**
* A list of subscribers. Subscribers are the callbacks that want us to let
* them know if the callback was fulfilled or rejected.
*
* @var array
*/
protected $subscribers = [];
/**
* The result of the promise.
*
* If the promise was fulfilled, this will be the result value. If the
* promise was rejected, this property hold the rejection reason.
*
* @var mixed
*/
protected $value = null;
/**
* This method is used to call either an onFulfilled or onRejected callback.
*
* This method makes sure that the result of these callbacks are handled
* correctly, and any chained promises are also correctly fulfilled or
* rejected.
*
* @param callable $callBack
*/
private function invokeCallback(Promise $subPromise, callable $callBack = null)
{
// We use 'nextTick' to ensure that the event handlers are always
// triggered outside of the calling stack in which they were originally
// passed to 'then'.
//
// This makes the order of execution more predictable.
Loop\nextTick(function () use ($callBack, $subPromise) {
if (is_callable($callBack)) {
try {
$result = $callBack($this->value);
if ($result instanceof self) {
// If the callback (onRejected or onFulfilled)
// returned a promise, we only fulfill or reject the
// chained promise once that promise has also been
// resolved.
$result->then([$subPromise, 'fulfill'], [$subPromise, 'reject']);
} else {
// If the callback returned any other value, we
// immediately fulfill the chained promise.
$subPromise->fulfill($result);
}
} catch (Throwable $e) {
// If the event handler threw an exception, we need to make sure that
// the chained promise is rejected as well.
$subPromise->reject($e);
}
} else {
if (self::FULFILLED === $this->state) {
$subPromise->fulfill($this->value);
} else {
$subPromise->reject($this->value);
}
}
});
}
}
