Promise
A lightweight implementation of
CommonJS Promises/A for PHP., (*1)
, (*2)
Table of Contents
- Introduction
-
Concepts
-
API
-
Examples
- Install
- Tests
- Credits
- License
Introduction
Promise is a library implementing
CommonJS Promises/A for PHP., (*3)
It also provides several other useful promise-related concepts, such as joining
multiple promises and mapping and reducing collections of promises., (*4)
If you've never heard about promises before,
read this first., (*5)
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., (*6)
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., (*7)
API
Deferred
A deferred represents an operation whose resolution is pending. It has separate
promise and resolver parts., (*8)
$deferred = new React\Promise\Deferred();
$promise = $deferred->promise();
$deferred->resolve(mixed $value);
$deferred->reject(\Throwable $reason);
The promise method returns the promise of the deferred., (*9)
The resolve and reject methods control the state of the deferred., (*10)
The constructor of the Deferred accepts an optional $canceller argument.
See Promise for more information., (*11)
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., (*12)
Deferred::resolve()
$deferred->resolve(mixed $value);
Resolves the promise returned by promise(). All consumers are notified by
having $onFulfilled (which they registered via $promise->then()) called with
$value., (*13)
If $value itself is a promise, the promise will transition to the state of
this promise once it is resolved., (*14)
See also the resolve() function., (*15)
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., (*16)
See also the reject() function., (*17)
PromiseInterface
The promise interface provides the common interface for all promise
implementations.
See Promise for the only public implementation exposed by this
package., (*18)
A promise represents an eventual outcome, which is either fulfillment (success)
and an associated value, or rejection (failure) and an associated reason., (*19)
Once in the fulfilled or rejected state, a promise becomes immutable.
Neither its state nor its result (or error) can be modified., (*20)
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., (*21)
The then() method registers new fulfilled and rejection handlers with a promise
(all parameters are optional):, (*22)
-
$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., (*23)
A promise makes the following guarantees about handlers registered in
the same call to then():, (*24)
- Only one of
$onFulfilled or $onRejected will be called,
never both.
-
$onFulfilled and $onRejected will never be called more
than once.
See also
PromiseInterface::catch()
$promise->catch(callable $onRejected);
Registers a rejection handler for promise. It is a shortcut for:, (*25)
$promise->then(null, $onRejected);
Additionally, you can type hint the $reason argument of $onRejected to catch
only specific errors., (*26)
$promise
->catch(function (\RuntimeException $reason) {
// Only catch \RuntimeException instances
// All other types of errors will propagate automatically
})
->catch(function (\Throwable $reason) {
// Catch other errors
});
PromiseInterface::finally()
$newPromise = $promise->finally(callable $onFulfilledOrRejected);
Allows you to execute "cleanup" type tasks in a promise chain., (*27)
It arranges for $onFulfilledOrRejected to be called, with no arguments,
when the promise is either fulfilled or rejected., (*28)
- 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.
finally() behaves similarly to the synchronous finally statement. When combined
with catch(), finally() allows you to write code that is similar to the familiar
synchronous catch/finally pair., (*29)
Consider the following synchronous code:, (*30)
try {
return doSomething();
} catch (\Throwable $e) {
return handleError($e);
} finally {
cleanup();
}
Similar asynchronous code (with doSomething() that returns a promise) can be
written:, (*31)
return doSomething()
->catch('handleError')
->finally('cleanup');
PromiseInterface::cancel()
``` php
$promise->cancel();, (*32)
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.
#### ~~PromiseInterface::otherwise()~~
> Deprecated since v3.0.0, see [`catch()`](#promiseinterfacecatch) instead.
The `otherwise()` method registers a rejection handler for a promise.
This method continues to exist only for BC reasons and to ease upgrading
between versions. It is an alias for:
```php
$promise->catch($onRejected);
~~PromiseInterface::always()~~
Deprecated since v3.0.0, see finally() instead., (*33)
The always() method allows you to execute "cleanup" type tasks in a promise chain., (*34)
This method continues to exist only for BC reasons and to ease upgrading
between versions. It is an alias for:, (*35)
$promise->finally($onFulfilledOrRejected);
Promise
Creates a promise whose state is controlled by the functions passed to
$resolver., (*36)
$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 two arguments:, (*37)
-
$resolve($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., (*38)
The resolver function will be called immediately, the canceller function only
once all consumers called the cancel() method of the promise., (*39)
Functions
Useful functions for creating and joining collections of promises., (*40)
All functions working on promise collections (like all(), race(),
etc.) support cancellation. This means, if you call cancel() on the returned
promise, all promises in the collection are cancelled., (*41)
resolve()
$promise = React\Promise\resolve(mixed $promiseOrValue);
Creates a promise for the supplied $promiseOrValue., (*42)
If $promiseOrValue is a value, it will be the resolution value of the
returned promise., (*43)
If $promiseOrValue is a thenable (any object that provides a then() method),
a trusted promise that follows the state of the thenable is returned., (*44)
If $promiseOrValue is a promise, it will be returned as is., (*45)
The resulting $promise implements the PromiseInterface
and can be consumed like any other promise:, (*46)
$promise = React\Promise\resolve(42);
$promise->then(function (int $result): void {
var_dump($result);
}, function (\Throwable $e): void {
echo 'Error: ' . $e->getMessage() . PHP_EOL;
});
reject()
$promise = React\Promise\reject(\Throwable $reason);
Creates a rejected promise for the supplied $reason., (*47)
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., (*48)
The resulting $promise implements the PromiseInterface
and can be consumed like any other promise:, (*49)
$promise = React\Promise\reject(new RuntimeException('Request failed'));
$promise->then(function (int $result): void {
var_dump($result);
}, function (\Throwable $e): void {
echo 'Error: ' . $e->getMessage() . PHP_EOL;
});
Note that rejected promises should always be handled similar to how any
exceptions should always be caught in a try + catch block. If you remove the
last reference to a rejected promise that has not been handled, it will
report an unhandled promise rejection:, (*50)
function incorrect(): int
{
$promise = React\Promise\reject(new RuntimeException('Request failed'));
// Commented out: No rejection handler registered here.
// $promise->then(null, function (\Throwable $e): void { /* ignore */ });
// Returning from a function will remove all local variable references, hence why
// this will report an unhandled promise rejection here.
return 42;
}
// Calling this function will log an error message plus its stack trace:
// Unhandled promise rejection with RuntimeException: Request failed in example.php:10
incorrect();
A rejected promise will be considered "handled" if you catch the rejection
reason with either the then() method, the
catch() method, or the
finally() method. Note that each of these methods
return a new promise that may again be rejected if you re-throw an exception., (*51)
A rejected promise will also be considered "handled" if you abort the operation
with the cancel() method (which in turn would
usually reject the promise if it is still pending)., (*52)
See also the set_rejection_handler() function., (*53)
all()
$promise = React\Promise\all(iterable $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., (*54)
race()
$promise = React\Promise\race(iterable $promisesOrValues);
Initiates a competitive race that allows one winner. Returns a promise which is
resolved in the same way the first settled promise resolves., (*55)
The returned promise will become infinitely pending if $promisesOrValues
contains 0 items., (*56)
any()
$promise = React\Promise\any(iterable $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., (*57)
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()., (*58)
The returned promise will also reject with a React\Promise\Exception\LengthException
if $promisesOrValues contains 0 items., (*59)
set_rejection_handler()
React\Promise\set_rejection_handler(?callable $callback): ?callable;
Sets the global rejection handler for unhandled promise rejections., (*60)
Note that rejected promises should always be handled similar to how any
exceptions should always be caught in a try + catch block. If you remove
the last reference to a rejected promise that has not been handled, it will
report an unhandled promise rejection. See also the reject() function
for more details., (*61)
The ?callable $callback argument MUST be a valid callback function that
accepts a single Throwable argument or a null value to restore the
default promise rejection handler. The return value of the callback function
will be ignored and has no effect, so you SHOULD return a void value. The
callback function MUST NOT throw or the program will be terminated with a
fatal error., (*62)
The function returns the previous rejection handler or null if using the
default promise rejection handler., (*63)
The default promise rejection handler will log an error message plus its stack
trace:, (*64)
// Unhandled promise rejection with RuntimeException: Unhandled in example.php:2
React\Promise\reject(new RuntimeException('Unhandled'));
The promise rejection handler may be used to use customize the log message or
write to custom log targets. As a rule of thumb, this function should only be
used as a last resort and promise rejections are best handled with either the
then() method, the
catch() method, or the
finally() method.
See also the reject() function for more details., (*65)
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., (*66)
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., (*67)
Each call to then() returns a new promise that will resolve with the return
value of the previous handler. This creates a promise "pipeline"., (*68)
$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., (*69)
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), (*70)
$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($x) {
throw new \Exception($x + 1);
})
->catch(function (\Exception $x) {
// Propagate the rejection
throw $x;
})
->catch(function (\Exception $x) {
// Can also propagate by returning another rejection
return React\Promise\reject(
new \Exception($x->getMessage() + 1)
);
})
->catch(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., (*71)
$deferred = new React\Promise\Deferred();
$deferred->promise()
->then(function ($x) {
return $x + 1;
})
->then(function ($x) {
throw new \Exception($x + 1);
})
->catch(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"
Install
The recommended way to install this library is through Composer.
New to Composer?, (*72)
This project follows SemVer.
This will install the latest supported version from this branch:, (*73)
composer require react/promise:^3.2
See also the CHANGELOG for details about version upgrades., (*74)
This project aims to run on any platform and thus does not require any PHP
extensions and supports running on PHP 7.1 through current PHP 8+.
It's highly recommended to use the latest supported PHP version for this project., (*75)
We're committed to providing long-term support (LTS) options and to provide a
smooth upgrade path. If you're using an older PHP version, you may use the
2.x branch (PHP 5.4+) or
1.x branch (PHP 5.3+) which both
provide a compatible API but do not take advantage of newer language features.
You may target multiple versions at the same time to support a wider range of
PHP versions like this:, (*76)
composer require "react/promise:^3 || ^2 || ^1"
Tests
To run the test suite, you first need to clone this repo and then install all
dependencies through Composer:, (*77)
composer install
To run the test suite, go to the project root and run:, (*78)
vendor/bin/phpunit
On top of this, we use PHPStan on max level to ensure type safety across the project:, (*79)
vendor/bin/phpstan
Credits
Promise is a port of when.js
by Brian Cavalier., (*80)
Also, large parts of the documentation have been ported from the when.js
Wiki and the
API docs., (*81)
License
Released under the MIT license., (*82)
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