Promise

A lightweight implementation of CommonJS Promises/A for PHP.

The master branch contains the code for the upcoming 3.0 release. For the code of the current stable 2.x release, checkout the 2.x branch.

The upcoming 3.0 release will be the way forward for this package. However we will still actively support 2.0 and 1.0 for those not yet on PHP 7+.

Table of Contents

1. Introduction
2. Concepts
   • Deferred
   • Promise
3. API
   • Deferred
     • Deferred::promise()
     • Deferred::resolve()
     • Deferred::reject()
   • PromiseInterface
     • PromiseInterface::then()
     • PromiseInterface::done()
     • PromiseInterface::otherwise()
     • PromiseInterface::always()
     • PromiseInterface::cancel()
   • Promise
   • 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
   • done() vs. then()
5. Credits
6. License

Introduction

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(\Throwable $reason);

The promise method returns the promise of the deferred.

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

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(\Throwable $reason);

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.

PromiseInterface

The promise interface provides the common interface for all promise implementations. See Promise for the only public implementation exposed by this package.

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()

$transformedPromise = $promise->then(callable $onFulfilled = null, callable $onRejected = 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 and rejection 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.

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.

See also

• resolve() - Creating a resolved promise
• reject() - Creating a rejected promise
• PromiseInterface::done()
• done() vs. then()

PromiseInterface::done()

$promise->done(callable $onFulfilled = null, callable $onRejected = null);

Consumes the promise's ultimate value if the promise fulfills, or handles the ultimate error.

It will cause a fatal error (E_USER_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

• PromiseInterface::then()
• done() vs. then()

PromiseInterface::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 (\Throwable $reason) {
        // Catch other errors
    )};

PromiseInterface::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 (\Throwable $e) {
    return handleError($e);
} finally {
    cleanup();
}

Similar asynchronous code (with doSomething() that returns a promise) can be written:

return doSomething()
    ->otherwise('handleError')
    ->always('cleanup');

PromiseInterface::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.

Promise

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

$resolver = function (callable $resolve, callable $reject) {
    // Do some work, possibly asynchronously, and then
    // resolve or reject.

    $resolve($awesomeResult);
    // or throw new Exception('Promise rejected');
    // or $resolve($anotherPromise);
    // or $reject($nastyError);
};

$canceller = function () {
    // Cancel/abort any running operations like network connections, streams etc.

    // Reject promise by throwing an exception
    throw 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. It is recommended to just throw an exception instead of using $reject().

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.

Functions

Useful functions for creating, joining, mapping and reducing collections of promises.

All functions working on promise collections (like all(), race(), some() etc.) support cancellation. This means, if you call cancel() on the returned promise, all promises in the collection are cancelled.

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 thenable (any object that provides a then() method), a trusted promise that follows the state of the thenable is returned.

If $promiseOrValue is a promise, it will be returned as is.

reject()

$promise = React\Promise\reject(\Throwable $reason);

Creates a rejected promise for the supplied $reason.

Note that the \Throwable interface introduced in PHP 7 covers both user land \Exception's and \Error internal PHP errors. By enforcing \Throwable as reason to reject a promise, any language error or user land exception can be used to reject a promise.

all()

$promise = React\Promise\all(array $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 $promisesOrValues);

Initiates a competitive race that allows one winner. Returns a promise which is resolved in the same way the first settled promise resolves.

The returned promise will become infinitely pending if $promisesOrValues contains 0 items.

any()

$promise = React\Promise\any(array $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 a React\Promise\Exception\CompositeException which holds all rejection reasons. The rejection reasons can be obtained with CompositeException::getThrowables().

The returned promise will also reject with a React\Promise\Exception\LengthException if $promisesOrValues contains 0 items.

some()

$promise = React\Promise\some(array $promisesOrValues, integer $howMany);

Returns a promise that will resolve when at least $howMany of the supplied items in $promisesOrValues fulfill. The resolution value of the returned promise will be an array of length $howMany containing the resolution values of $howMany fulfilled promises that were resolved first.

The returned promise will reject if it becomes impossible for $howMany items to resolve (that is, when (count($promisesOrValues) - $howMany) + 1 items reject). The rejection value will be a React\Promise\Exception\CompositeException which holds (count($promisesOrValues) - $howMany) + 1 rejection reasons. The rejection reasons can be obtained with CompositeException::getExceptions().

The returned promise will also reject with a React\Promise\Exception\LengthException if $promisesOrValues contains less items than $howMany.

map()

$promise = React\Promise\map(array $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 $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 (\Throwable $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 (\Throwable $reason) {
            // Deferred rejected, do something with $reason
        }
    );

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);
    })
    ->otherwise(function (\Exception $x) {
        // Propagate the rejection
        throw $x;
    })
    ->otherwise(function (\Exception $x) {
        // Can also propagate by returning another rejection
        return React\Promise\reject(
            new \Exception($x->getMessage() + 1)
        );
    })
    ->otherwise(function ($x) {
        echo 'Reject ' . $x->getMessage(); // 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 new \Exception($x + 1);
    })
    ->otherwise(function (\Exception $x) {
        // Handle the rejection, and don't propagate.
        // This is like catch without a rethrow
        return $x->getMessage() + 1;
    })
    ->then(function ($x) {
        echo 'Mixed ' . $x; // 4
    });

$deferred->resolve(1);  // Prints "Mixed 4"

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 cause 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, the ApiErrorException will be thrown
getJsonResult()
    ->done(
        // Consume transformed object
        function ($jsonResultObject) {
            // Do something with $jsonResultObject
        }
    );

// Here we provide a rejection handler which will either throw while debugging
// or log the exception
getJsonResult()
    ->done(
        function ($jsonResultObject) {
            // Do something with $jsonResultObject
        },
        function (ApiErrorException $exception) {
            if (isDebug()) {
                throw $exception;
            } else {
                logException($exception);
            }
        }
    );

Credits

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

Released under the MIT license.