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<?php
namespace Sabre\Event;
use Exception;
/**
* An implementation of the Promise pattern.
*
* Promises basically allow you to avoid what is commonly called 'callback
* hell'. It allows for easily chaining of asynchronous operations.
*
* @copyright Copyright (C) 2013-2015 fruux GmbH (https://fruux.com/).
* @author Evert Pot (http://evertpot.com/)
* @license http://sabre.io/license/ Modified BSD License
*/
class Promise {
/**
* Pending promise. No result yet.
*/
const PENDING = 0;
/**
* The promise has been fulfilled. It was successful.
*/
const FULFILLED = 1;
/**
* The promise was rejected. The operation failed.
*/
const REJECTED = 2;
/**
* The current state of this promise.
*
* @var int
*/
protected $state = self::PENDING;
/**
* 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 is most commonly an exception.
*
* @var mixed
*/
protected $value = null;
/**
* 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.
*
* @param callable $executor
*/
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.
*
* @param callable $onFulfilled
* @param callable $onRejected
* @return Promise
*/
function then(callable $onFulfilled = null, callable $onRejected = null) {
$subPromise = new self();
switch ($this->state) {
case self::PENDING :
$this->subscribers[] = [$subPromise, $onFulfilled, $onRejected];
break;
case self::FULFILLED :
$this->invokeCallback($subPromise, $onFulfilled);
break;
case self::REJECTED :
$this->invokeCallback($subPromise, $onRejected);
break;
}
return $subPromise;
}
/**
* Add a callback for when this promise is rejected.
*
* I would have used the word 'catch', but it's a reserved word in PHP, so
* we're not allowed to call our function that.
*
* @param callable $onRejected
* @return Promise
*/
function error(callable $onRejected) {
return $this->then(null, $onRejected);
}
/**
* Marks this promise as fulfilled and sets its return value.
*
* @param mixed $value
* @return void
*/
function fulfill($value = null) {
if ($this->state !== self::PENDING) {
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.
*
* @param mixed $reason
* @return void
*/
function reject($reason = null) {
if ($this->state !== self::PENDING) {
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]);
}
}
/**
* It's possible to send an array of promises to the all method. This
* method returns a promise that will be fulfilled, only if all the passed
* promises are fulfilled.
*
* @param Promise[] $promises
* @return Promise
*/
static function all(array $promises) {
return new self(function($success, $fail) use ($promises) {
$successCount = 0;
$completeResult = [];
foreach ($promises as $promiseIndex => $subPromise) {
$subPromise->then(
function($result) use ($promiseIndex, &$completeResult, &$successCount, $success, $promises) {
$completeResult[$promiseIndex] = $result;
$successCount++;
if ($successCount === count($promises)) {
$success($completeResult);
}
return $result;
}
)->error(
function($reason) use ($fail) {
$fail($reason);
}
);
}
});
}
/**
* 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 Promise $subPromise
* @param callable $callBack
* @return void
*/
protected function invokeCallback(Promise $subPromise, callable $callBack = null) {
if (is_callable($callBack)) {
try {
$result = $callBack($this->value);
if ($result instanceof self) {
$result->then([$subPromise, 'fulfill'], [$subPromise, 'reject']);
} else {
$subPromise->fulfill($result);
}
} catch (Exception $e) {
$subPromise->reject($e);
}
} else {
if ($this->state === self::FULFILLED) {
$subPromise->fulfill($this->value);
} else {
$subPromise->reject($this->value);
}
}
}
}

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