React/Promise

A lightweight implementation of CommonJS Promises/A for PHP.

Table of Contents

1. Introduction
2. Concepts
   • Deferred
   • Promise
3. API
   • Deferred
     • Deferred::promise()
     • Deferred::resolve()
     • Deferred::reject()
     • Deferred::progress()
   • PromiseInterface
     • PromiseInterface::then()
   • CancellablePromiseInterface
     • CancellablePromiseInterface::cancel()
   • Promise
   • FulfilledPromise
   • RejectedPromise
   • LazyPromise
   • Functions
     • resolve()
     • reject()
     • all()
     • race()
     • any()
     • some()
     • map()
     • reduce()
   • PromisorInterface
4. Examples
   • How to use Deferred
   • How promise forwarding works
     • Resolution forwarding
     • Rejection forwarding
     • Mixed resolution and rejection forwarding
     • Progress event forwarding
5. Credits
6. 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->progress(mixed $update = null);

The promise method returns the promise of the deferred.

The resolve and reject methods control the state of the deferred.

The progress 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::progress()

$deferred->progress(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.

PromiseInterface::then()

$newPromise = $promise->then(callable $onFulfilled = null, callable $onRejected = null, callable $onProgress = null);

The then() method registers new fulfilled, rejection and progress handlers with this 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():

1. Only one of $onFulfilled or $onRejected will be called, never both.
2. $onFulfilled and $onRejected will never be called more than once.
3. $onProgress may be called multiple times.

Implementations

• Promise
• FulfilledPromise
• RejectedPromise
• LazyPromise

See also

• resolve() - Creating a resolved promise
• reject() - Creating a rejected promise

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
• FulfilledPromise
• RejectedPromise
• LazyPromise

Promise

Creates a promise whose state is controlled by the functions passed to $resolver.

$resolver = function (callable $resolve, callable $reject, callable $progress) {
    // 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 $progress($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.
• $progress($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 resolved 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.

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();

$deferred->promise()
    ->then(null, null, function ($update) {
        return $update + 1;
    })
    ->then(null, null, function ($update) {
        echo 'Progress ' . $update; // 2
    });

$deferred->progress(1);  // Prints "Progress 2"

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.