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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()
   • PromiseInterface
     • PromiseInterface::then()
     • PromiseInterface::catch()
     • PromiseInterface::finally()
     • PromiseInterface::cancel()
     • PromiseInterface::otherwise()
     • PromiseInterface::always()
   • Promise
   • Functions
     • resolve()
     • reject()
     • all()
     • race()
     • any()
     • set_rejection_handler()
4. Examples
   • How to use Deferred
   • How promise forwarding works
     • Resolution forwarding
     • Rejection forwarding
     • Mixed resolution and rejection forwarding
5. Install
6. Tests
7. Credits
8. 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.

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()

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()

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.

See also the resolve() function.

Deferred::reject()

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.

See also the reject() function.

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()

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::catch()

Registers a rejection handler for promise. It is a shortcut for:

Additionally, you can type hint the $reason argument of $onRejected to catch only specific errors.

PromiseInterface::finally()

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.

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.

Consider the following synchronous code:

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

PromiseInterface::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.

PromiseInterface::otherwise()

Deprecated since v3.0.0, see catch() 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:

PromiseInterface::always()

Deprecated since v3.0.0, see finally() instead.

The always() method allows you to execute "cleanup" type tasks in a promise chain.

This method continues to exist only for BC reasons and to ease upgrading between versions. It is an alias for:

Promise

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

The promise constructor receives a resolver function and an optional canceller function which both will be called with two 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 and joining collections of promises.

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.

resolve()

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.

The resulting $promise implements the PromiseInterface and can be consumed like any other promise:

reject()

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.

The resulting $promise implements the PromiseInterface and can be consumed like any other promise:

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:

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.

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).

See also the set_rejection_handler() function.

all()

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()

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()

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.

set_rejection_handler()

Sets the global rejection handler for unhandled promise rejections.

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.

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.

The function returns the previous rejection handler or null if using the default promise rejection handler.

The default promise rejection handler will log an error message plus its stack trace:

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.

Examples

How to use Deferred

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".

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)

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.

Install

The recommended way to install this library is through Composer. New to Composer?

This project follows SemVer. This will install the latest supported version from this branch:

See also the CHANGELOG for details about version upgrades.

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.

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:

Tests

To run the test suite, you first need to clone this repo and then install all dependencies through Composer:

To run the test suite, go to the project root and run:

On top of this, we use PHPStan on max level to ensure type safety across the project:

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.