React/Promise
A lightweight implementation of CommonJS Promises/A for PHP.
Table of Contents
- Introduction
- Concepts
- API
- Examples
- Credits
- License
Introduction
React/Promise is a library implementing CommonJS Promises/A for PHP.
It also provides several other useful promise-related concepts, such as joining multiple promises and mapping and reducing collections of promises.
If you've never heard about promises before, read this first.
Concepts
Deferred
A Deferred represents a computation or unit of work that may not have completed yet. Typically (but not always), that computation will be something that executes asynchronously and completes at some point in the future.
Promise
While a deferred represents the computation itself, a Promise represents the result of that computation. Thus, each deferred has a promise that acts as a placeholder for its actual result.
API
Deferred
A deferred represents an operation whose resolution is pending. It has separate promise and resolver parts.
$deferred = new React\Promise\Deferred();
$promise = $deferred->promise();
$deferred->resolve(mixed $value = null);
$deferred->reject(mixed $reason = null);
$deferred->notify(mixed $update = null);
The promise
method returns the promise of the deferred.
The resolve
and reject
methods control the state of the deferred.
The notify
method is for progress notification.
The constructor of the Deferred
accepts an optional $canceller
argument.
See Promise for more information.
Deferred::promise()
$promise = $deferred->promise();
Returns the promise of the deferred, which you can hand out to others while keeping the authority to modify its state to yourself.
Deferred::resolve()
$deferred->resolve(mixed $value = null);
Resolves the promise returned by promise()
. All consumers are notified by
having $onFulfilled
(which they registered via $promise->then()
) called with
$value
.
If $value
itself is a promise, the promise will transition to the state of
this promise once it is resolved.
Deferred::reject()
$deferred->reject(mixed $reason = null);
Rejects the promise returned by promise()
, signalling that the deferred's
computation failed.
All consumers are notified by having $onRejected
(which they registered via
$promise->then()
) called with $reason
.
If $reason
itself is a promise, the promise will be rejected with the outcome
of this promise regardless whether it fulfills or rejects.
Deferred::notify()
$deferred->notify(mixed $update = null);
Triggers progress notifications, to indicate to consumers that the computation is making progress toward its result.
All consumers are notified by having $onProgress
(which they registered via
$promise->then()
) called with $update
.
PromiseInterface
The promise interface provides the common interface for all promise implementations.
A promise represents an eventual outcome, which is either fulfillment (success) and an associated value, or rejection (failure) and an associated reason.
Once in the fulfilled or rejected state, a promise becomes immutable. Neither its state nor its result (or error) can be modified.
Implementations
PromiseInterface::then()
$transformedPromise = $promise->then(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);
Transforms a promise's value by applying a function to the promise's fulfillment or rejection value. Returns a new promise for the transformed result.
The then()
method registers new fulfilled, rejection and progress handlers
with a promise (all parameters are optional):
$onFulfilled
will be invoked once the promise is fulfilled and passed the result as the first argument.$onRejected
will be invoked once the promise is rejected and passed the reason as the first argument.$onProgress
will be invoked whenever the producer of the promise triggers progress notifications and passed a single argument (whatever it wants) to indicate progress.
It returns a new promise that will fulfill with the return value of either
$onFulfilled
or $onRejected
, whichever is called, or will reject with
the thrown exception if either throws.
A promise makes the following guarantees about handlers registered in
the same call to then()
:
- Only one of
$onFulfilled
or$onRejected
will be called, never both. $onFulfilled
and$onRejected
will never be called more than once.$onProgress
may be called multiple times.
See also
- resolve() - Creating a resolved promise
- reject() - Creating a rejected promise
- ExtendedPromiseInterface::done()
- done() vs. then()
ExtendedPromiseInterface
The ExtendedPromiseInterface extends the PromiseInterface with useful shortcut and utility methods which are not part of the Promises/A specification.
Implementations
ExtendedPromiseInterface::done()
$promise->done(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);
Consumes the promise's ultimate value if the promise fulfills, or handles the ultimate error.
It will cause a fatal error if either $onFulfilled
or $onRejected
throw or
return a rejected promise.
Since the purpose of done()
is consumption rather than transformation,
done()
always returns null
.
See also
ExtendedPromiseInterface::otherwise()
$promise->otherwise(callable $onRejected);
Registers a rejection handler for promise. It is a shortcut for:
$promise->then(null, $onRejected);
Additionally, you can type hint the $reason
argument of $onRejected
to catch
only specific errors.
$promise
->otherwise(function (\RuntimeException $reason) {
// Only catch \RuntimeException instances
// All other types of errors will propagate automatically
})
->otherwise(function ($reason) {
// Catch other errors
)};
ExtendedPromiseInterface::always()
$newPromise = $promise->always(callable $onFulfilledOrRejected);
Allows you to execute "cleanup" type tasks in a promise chain.
It arranges for $onFulfilledOrRejected
to be called, with no arguments,
when the promise is either fulfilled or rejected.
- If
$promise
fulfills, and$onFulfilledOrRejected
returns successfully,$newPromise
will fulfill with the same value as$promise
. - If
$promise
fulfills, and$onFulfilledOrRejected
throws or returns a rejected promise,$newPromise
will reject with the thrown exception or rejected promise's reason. - If
$promise
rejects, and$onFulfilledOrRejected
returns successfully,$newPromise
will reject with the same reason as$promise
. - If
$promise
rejects, and$onFulfilledOrRejected
throws or returns a rejected promise,$newPromise
will reject with the thrown exception or rejected promise's reason.
always()
behaves similarly to the synchronous finally statement. When combined
with otherwise()
, always()
allows you to write code that is similar to the familiar
synchronous catch/finally pair.
Consider the following synchronous code:
try {
return doSomething();
} catch(\Exception $e) {
return handleError($e);
} finally {
cleanup();
}
Similar asynchronous code (with doSomething()
that returns a promise) can be
written:
return doSomething()
->otherwise('handleError')
->always('cleanup');
ExtendedPromiseInterface::progress()
$promise->progress(callable $onProgress);
Registers a handler for progress updates from promise. It is a shortcut for:
$promise->then(null, null, $onProgress);
CancellablePromiseInterface
A cancellable promise provides a mechanism for consumers to notify the creator of the promise that they are not longer interested in the result of an operation.
CancellablePromiseInterface::cancel()
$promise->cancel();
The cancel()
method notifies the creator of the promise that there is no
further interest in the results of the operation.
Once a promise is settled (either fulfilled or rejected), calling cancel()
on
a promise has no effect.
Implementations
Promise
Creates a promise whose state is controlled by the functions passed to
$resolver
.
$resolver = function (callable $resolve, callable $reject, callable $notify) {
// Do some work, possibly asynchronously, and then
// resolve or reject. You can notify of progress events
// along the way if you want/need.
$resolve($awesomeResult);
// or $resolve($anotherPromise);
// or $reject($nastyError);
// or $notify($progressNotification);
};
$canceller = function (callable $resolve, callable $reject, callable $progress) {
// Cancel/abort any running operations like network connections, streams etc.
$reject(new \Exception('Promise cancelled'));
};
$promise = new React\Promise\Promise($resolver, $canceller);
The promise constructor receives a resolver function and an optional canceller function which both will be called with 3 arguments:
$resolve($value)
- Primary function that seals the fate of the returned promise. Accepts either a non-promise value, or another promise. When called with a non-promise value, fulfills promise with that value. When called with another promise, e.g.$resolve($otherPromise)
, promise's fate will be equivalent to that of$otherPromise
.$reject($reason)
- Function that rejects the promise.$notify($update)
- Function that issues progress events for the promise.
If the resolver or canceller throw an exception, the promise will be rejected with that thrown exception as the rejection reason.
The resolver function will be called immediately, the canceller function only
once all consumers called the cancel()
method of the promise.
FulfilledPromise
Creates a already fulfilled promise.
$promise = React\Promise\FulfilledPromise($value);
Note, that $value
cannot be a promise. It's recommended to use
resolve() for creating resolved promises.
RejectedPromise
Creates a already rejected promise.
$promise = React\Promise\RejectedPromise($reason);
Note, that $reason
cannot be a promise. It's recommended to use
reject() for creating rejected promises.
LazyPromise
Creates a promise which will be lazily initialized by $factory
once a consumer
calls the then()
method.
$factory = function () {
$deferred = new React\Promise\Deferred();
// Do some heavy stuff here and resolve the deferred once completed
return $deferred->promise();
};
$promise = React\Promise\LazyPromise($factory);
// $factory will only be executed once we call then()
$promise->then(function ($value) {
});
Functions
Useful functions for creating, joining, mapping and reducing collections of promises.
resolve()
$promise = React\Promise\resolve(mixed $promiseOrValue);
Creates a promise for the supplied $promiseOrValue
.
If $promiseOrValue
is a value, it will be the resolution value of the
returned promise.
If $promiseOrValue
is a promise, it will simply be returned.
Note: The promise returned is always a promise implementing
ExtendedPromiseInterface. If you pass in a custom
promise which only implements PromiseInterface, this
promise will be assimilated to a extended promise following $promiseOrValue
.
reject()
$promise = React\Promise\reject(mixed $promiseOrValue);
Creates a rejected promise for the supplied $promiseOrValue
.
If $promiseOrValue
is a value, it will be the rejection value of the
returned promise.
If $promiseOrValue
is a promise, its completion value will be the rejected
value of the returned promise.
This can be useful in situations where you need to reject a promise without throwing an exception. For example, it allows you to propagate a rejection with the value of another promise.
all()
$promise = React\Promise\all(array|React\Promise\PromiseInterface $promisesOrValues);
Returns a promise that will resolve only once all the items in
$promisesOrValues
have resolved. The resolution value of the returned promise
will be an array containing the resolution values of each of the items in
$promisesOrValues
.
race()
$promise = React\Promise\race(array|React\Promise\PromiseInterface $promisesOrValues);
Initiates a competitive race that allows one winner. Returns a promise which is resolved in the same way the first settled promise resolves.
any()
$promise = React\Promise\any(array|React\Promise\PromiseInterface $promisesOrValues);
Returns a promise that will resolve when any one of the items in
$promisesOrValues
resolves. The resolution value of the returned promise
will be the resolution value of the triggering item.
The returned promise will only reject if all items in $promisesOrValues
are
rejected. The rejection value will be an array of all rejection reasons.
some()
$promise = React\Promise\some(array|React\Promise\PromiseInterface $promisesOrValues, integer $howMany);
Returns a promise that will resolve when $howMany
of the supplied items in
$promisesOrValues
resolve. The resolution value of the returned promise
will be an array of length $howMany
containing the resolution values of the
triggering items.
The returned promise will reject if it becomes impossible for $howMany
items
to resolve (that is, when (count($promisesOrValues) - $howMany) + 1
items
reject). The rejection value will be an array of
(count($promisesOrValues) - $howMany) + 1
rejection reasons.
map()
$promise = React\Promise\map(array|React\Promise\PromiseInterface $promisesOrValues, callable $mapFunc);
Traditional map function, similar to array_map()
, but allows input to contain
promises and/or values, and $mapFunc
may return either a value or a promise.
The map function receives each item as argument, where item is a fully resolved
value of a promise or value in $promisesOrValues
.
reduce()
$promise = React\Promise\reduce(array|React\Promise\PromiseInterface $promisesOrValues, callable $reduceFunc , $initialValue = null);
Traditional reduce function, similar to array_reduce()
, but input may contain
promises and/or values, and $reduceFunc
may return either a value or a
promise, and $initialValue
may be a promise or a value for the starting
value.
PromisorInterface
The React\Promise\PromisorInterface
provides a common interface for objects
that provide a promise. React\Promise\Deferred
implements it, but since it
is part of the public API anyone can implement it.
Examples
How to use Deferred
function getAwesomeResultPromise()
{
$deferred = new React\Promise\Deferred();
// Execute a Node.js-style function using the callback pattern
computeAwesomeResultAsynchronously(function ($error, $result) use ($deferred) {
if ($error) {
$deferred->reject($error);
} else {
$deferred->resolve($result);
}
});
// Return the promise
return $deferred->promise();
}
getAwesomeResultPromise()
->then(
function ($value) {
// Deferred resolved, do something with $value
},
function ($reason) {
// Deferred rejected, do something with $reason
},
function ($update) {
// Progress notification triggered, do something with $update
}
);
How promise forwarding works
A few simple examples to show how the mechanics of Promises/A forwarding works. These examples are contrived, of course, and in real usage, promise chains will typically be spread across several function calls, or even several levels of your application architecture.
Resolution forwarding
Resolved promises forward resolution values to the next promise.
The first promise, $deferred->promise()
, will resolve with the value passed
to $deferred->resolve()
below.
Each call to then()
returns a new promise that will resolve with the return
value of the previous handler. This creates a promise "pipeline".
$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($x) {
// $x will be the value passed to $deferred->resolve() below
// and returns a *new promise* for $x + 1
return $x + 1;
})
->then(function ($x) {
// $x === 2
// This handler receives the return value of the
// previous handler.
return $x + 1;
})
->then(function ($x) {
// $x === 3
// This handler receives the return value of the
// previous handler.
return $x + 1;
})
->then(function ($x) {
// $x === 4
// This handler receives the return value of the
// previous handler.
echo 'Resolve ' . $x;
});
$deferred->resolve(1); // Prints "Resolve 4"
Rejection forwarding
Rejected promises behave similarly, and also work similarly to try/catch: When you catch an exception, you must rethrow for it to propagate.
Similarly, when you handle a rejected promise, to propagate the rejection, "rethrow" it by either returning a rejected promise, or actually throwing (since promise translates thrown exceptions into rejections)
$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($x) {
throw new \Exception($x + 1);
})
->then(null, function (\Exception $x) {
// Propagate the rejection
throw $x;
})
->then(null, function (\Exception $x) {
// Can also propagate by returning another rejection
return React\Promise\reject((integer) $x->getMessage() + 1);
})
->then(null, function ($x) {
echo 'Reject ' . $x; // 3
});
$deferred->resolve(1); // Prints "Reject 3"
Mixed resolution and rejection forwarding
Just like try/catch, you can choose to propagate or not. Mixing resolutions and rejections will still forward handler results in a predictable way.
$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($x) {
return $x + 1;
})
->then(function ($x) {
throw \Exception($x + 1);
})
->then(null, function (\Exception $x) {
// Handle the rejection, and don't propagate.
// This is like catch without a rethrow
return (integer) $x->getMessage() + 1;
})
->then(function ($x) {
echo 'Mixed ' . $x; // 4
});
$deferred->resolve(1); // Prints "Mixed 4"
Progress event forwarding
In the same way as resolution and rejection handlers, your progress handler
MUST return a progress event to be propagated to the next link in the chain.
If you return nothing, null
will be propagated.
Also in the same way as resolutions and rejections, if you don't register a progress handler, the update will be propagated through.
If your progress handler throws an exception, the exception will be propagated to the next link in the chain. The best thing to do is to ensure your progress handlers do not throw exceptions.
This gives you the opportunity to transform progress events at each step in the chain so that they are meaningful to the next step. It also allows you to choose not to transform them, and simply let them propagate untransformed, by not registering a progress handler.
$deferred = new React\Promise\Deferred();
$deferred->promise()
->progress(function ($update) {
return $update + 1;
})
->progress(function ($update) {
echo 'Progress ' . $update; // 2
});
$deferred->notify(1); // Prints "Progress 2"
done() vs. then()
The golden rule is:
Either return your promise, or call done() on it.
At a first glance, then()
and done()
seem very similar. However, there are
important distinctions.
The intent of then()
is to transform a promise's value and to pass or return
a new promise for the transformed value along to other parts of your code.
The intent of done()
is to consume a promise's value, transferring
responsibility for the value to your code.
In addition to transforming a value, then()
allows you to recover from, or
propagate intermediate errors. Any errors that are not handled will be caught
by the promise machinery and used to reject the promise returned by then()
.
Calling done()
transfers all responsibility for errors to your code. If an
error (either a thrown exception or returned rejection) escapes the
$onFulfilled
or $onRejected
callbacks you provide to done, it will be
rethrown in an uncatchable way causing a fatal error.
function getJsonResult()
{
return queryApi()
->then(
// Transform API results to an object
function ($jsonResultString) {
return json_decode($jsonResultString);
},
// Transform API errors to an exception
function ($jsonErrorString) {
$object = json_decode($jsonErrorString);
throw new ApiErrorException($object->errorMessage);
}
);
}
// Here we provide no rejection handler.
// If the promise returned has been rejected,
// a React\Promise\UnhandledRejectionException will be thrown
getJsonResult()
->done(
// Consume transformed object
function ($jsonResultObject) {
// Do something with $jsonObject
}
);
// Here we provide a rejection handler which will either throw while debugging
// or log the exception.
getJsonResult()
->done(
function ($jsonObject) {
// Do something with $jsonObject
},
function (ApiErrorException $exception) {
if (isDebug()) {
throw $e;
} else {
logException($exception);
}
}
);
Note that if a rejection value is not an instance of \Exception
, it will be
wrapped in an exception of the type React\Promise\UnhandledRejectionException
.
You can get the original rejection reason by calling $exception->getReason()
.
Credits
React/Promise is a port of when.js by Brian Cavalier.
Also, large parts of the documentation have been ported from the when.js Wiki and the API docs.
License
React/Promise is released under the MIT license.