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# Guzzle Promises [Promises/A+](https://promisesaplus.com/) implementation that handles promise chaining and resolution iteratively, allowing for "infinite" promise chaining while keeping the stack size constant. Read [this blog post](https://blog.domenic.me/youre-missing-the-point-of-promises/) for a general introduction to promises. - [Features](#features) - [Quick start](#quick-start) - [Synchronous wait](#synchronous-wait) - [Cancellation](#cancellation) - [API](#api) - [Promise](#promise) - [FulfilledPromise](#fulfilledpromise) - [RejectedPromise](#rejectedpromise) - [Promise interop](#promise-interop) - [Implementation notes](#implementation-notes) # Features - [Promises/A+](https://promisesaplus.com/) implementation. - Promise resolution and chaining is handled iteratively, allowing for "infinite" promise chaining. - Promises have a synchronous `wait` method. - Promises can be cancelled. - Works with any object that has a `then` function. - C# style async/await coroutine promises using `GuzzleHttp\Promise\coroutine()`. # Quick start A *promise* represents the eventual result of an asynchronous operation. The primary way of interacting with a promise is through its `then` method, which registers callbacks to receive either a promise's eventual value or the reason why the promise cannot be fulfilled. ## Callbacks Callbacks are registered with the `then` method by providing an optional `$onFulfilled` followed by an optional `$onRejected` function. ```php use GuzzleHttp\Promise\Promise; $promise = new Promise(); $promise->then( // $onFulfilled function ($value) { echo 'The promise was fulfilled.'; }, // $onRejected function ($reason) { echo 'The promise was rejected.'; } ); ``` *Resolving* a promise means that you either fulfill a promise with a *value* or reject a promise with a *reason*. Resolving a promises triggers callbacks registered with the promises's `then` method. These callbacks are triggered only once and in the order in which they were added. ## Resolving a promise Promises are fulfilled using the `resolve($value)` method. Resolving a promise with any value other than a `GuzzleHttp\Promise\RejectedPromise` will trigger all of the onFulfilled callbacks (resolving a promise with a rejected promise will reject the promise and trigger the `$onRejected` callbacks). ```php use GuzzleHttp\Promise\Promise; $promise = new Promise(); $promise ->then(function ($value) { // Return a value and don't break the chain return "Hello, " . $value; }) // This then is executed after the first then and receives the value // returned from the first then. ->then(function ($value) { echo $value; }); // Resolving the promise triggers the $onFulfilled callbacks and outputs // "Hello, reader". $promise->resolve('reader.'); ``` ## Promise forwarding Promises can be chained one after the other. Each then in the chain is a new promise. The return value of of a promise is what's forwarded to the next promise in the chain. Returning a promise in a `then` callback will cause the subsequent promises in the chain to only be fulfilled when the returned promise has been fulfilled. The next promise in the chain will be invoked with the resolved value of the promise. ```php use GuzzleHttp\Promise\Promise; $promise = new Promise(); $nextPromise = new Promise(); $promise ->then(function ($value) use ($nextPromise) { echo $value; return $nextPromise; }) ->then(function ($value) { echo $value; }); // Triggers the first callback and outputs "A" $promise->resolve('A'); // Triggers the second callback and outputs "B" $nextPromise->resolve('B'); ``` ## Promise rejection When a promise is rejected, the `$onRejected` callbacks are invoked with the rejection reason. ```php use GuzzleHttp\Promise\Promise; $promise = new Promise(); $promise->then(null, function ($reason) { echo $reason; }); $promise->reject('Error!'); // Outputs "Error!" ``` ## Rejection forwarding If an exception is thrown in an `$onRejected` callback, subsequent `$onRejected` callbacks are invoked with the thrown exception as the reason. ```php use GuzzleHttp\Promise\Promise; $promise = new Promise(); $promise->then(null, function ($reason) { throw new \Exception($reason); })->then(null, function ($reason) { assert($reason->getMessage() === 'Error!'); }); $promise->reject('Error!'); ``` You can also forward a rejection down the promise chain by returning a `GuzzleHttp\Promise\RejectedPromise` in either an `$onFulfilled` or `$onRejected` callback. ```php use GuzzleHttp\Promise\Promise; use GuzzleHttp\Promise\RejectedPromise; $promise = new Promise(); $promise->then(null, function ($reason) { return new RejectedPromise($reason); })->then(null, function ($reason) { assert($reason === 'Error!'); }); $promise->reject('Error!'); ``` If an exception is not thrown in a `$onRejected` callback and the callback does not return a rejected promise, downstream `$onFulfilled` callbacks are invoked using the value returned from the `$onRejected` callback. ```php use GuzzleHttp\Promise\Promise; use GuzzleHttp\Promise\RejectedPromise; $promise = new Promise(); $promise ->then(null, function ($reason) { return "It's ok"; }) ->then(function ($value) { assert($value === "It's ok"); }); $promise->reject('Error!'); ``` # Synchronous wait You can synchronously force promises to complete using a promise's `wait` method. When creating a promise, you can provide a wait function that is used to synchronously force a promise to complete. When a wait function is invoked it is expected to deliver a value to the promise or reject the promise. If the wait function does not deliver a value, then an exception is thrown. The wait function provided to a promise constructor is invoked when the `wait` function of the promise is called. ```php $promise = new Promise(function () use (&$promise) { $promise->resolve('foo'); }); // Calling wait will return the value of the promise. echo $promise->wait(); // outputs "foo" ``` If an exception is encountered while invoking the wait function of a promise, the promise is rejected with the exception and the exception is thrown. ```php $promise = new Promise(function () use (&$promise) { throw new \Exception('foo'); }); $promise->wait(); // throws the exception. ``` Calling `wait` on a promise that has been fulfilled will not trigger the wait function. It will simply return the previously resolved value. ```php $promise = new Promise(function () { die('this is not called!'); }); $promise->resolve('foo'); echo $promise->wait(); // outputs "foo" ``` Calling `wait` on a promise that has been rejected will throw an exception. If the rejection reason is an instance of `\Exception` the reason is thrown. Otherwise, a `GuzzleHttp\Promise\RejectionException` is thrown and the reason can be obtained by calling the `getReason` method of the exception. ```php $promise = new Promise(); $promise->reject('foo'); $promise->wait(); ``` > PHP Fatal error: Uncaught exception 'GuzzleHttp\Promise\RejectionException' with message 'The promise was rejected with value: foo' ## Unwrapping a promise When synchronously waiting on a promise, you are joining the state of the promise into the current state of execution (i.e., return the value of the promise if it was fulfilled or throw an exception if it was rejected). This is called "unwrapping" the promise. Waiting on a promise will by default unwrap the promise state. You can force a promise to resolve and *not* unwrap the state of the promise by passing `false` to the first argument of the `wait` function: ```php $promise = new Promise(); $promise->reject('foo'); // This will not throw an exception. It simply ensures the promise has // been resolved. $promise->wait(false); ``` When unwrapping a promise, the resolved value of the prom