# create-subscription
`create-subscription` is a utility for subscribing to external data sources inside React components. It is officially supported and maintained by the React team.
## When should you NOT use this?
This utility should be used for subscriptions to a single value that are typically only read in one place and may update frequently (e.g. a component that subscribes to a geolocation API to show a dot on a map).
Other cases have **better long-term solutions**:
* Redux/Flux stores should use the [context API](https://reactjs.org/docs/context.html) instead.
* I/O subscriptions (e.g. notifications) that update infrequently should use [`react-cache`](https://github.com/facebook/react/blob/master/packages/react-cache/README.md) instead.
* Complex libraries like Relay/Apollo should manage subscriptions manually with the same techniques which this library uses under the hood (as referenced [here](https://gist.github.com/bvaughn/d569177d70b50b58bff69c3c4a5353f3)) in a way that is most optimized for their library usage.
## Limitations in async mode
The main motivation for `create-subscription` is to provide a way for library authors to ensure compatibility with React's upcoming asynchronous rendering mode. `create-subscription` guarantees correctness in async mode, accounting for the subtle bugs and edge cases that a library author might otherwise miss.
However, [it achieves correctness by sometimes de-opting to synchronous mode](https://github.com/facebook/react/issues/13186#issuecomment-403959161), obviating the benefits of async rendering. This is an inherent limitation of storing state outside of React's managed state queue and rendering in response to a change event.
The effect of de-opting to sync mode is that the main thread may periodically be blocked (in the case of CPU-bound work), and placeholders may appear earlier than desired (in the case of IO-bound work).
For **full compatibility** with asynchronous rendering, including both **time-slicing** and **React Suspense**, the suggested longer-term solution is to move to one of the patterns described in the previous section.
## What types of subscriptions can this support?
This abstraction can handle a variety of subscription types, including:
* Event dispatchers like `HTMLInputElement`.
* Custom pub/sub components like Relay's `FragmentSpecResolver`.
* Observable types like RxJS `BehaviorSubject` and `ReplaySubject`. (Types like RxJS `Subject` or `Observable` are not supported, because they provide no way to read the "current" value after it has been emitted.)
* Native Promises.
# Installation
```sh
# Yarn
yarn add create-subscription
# NPM
npm install create-subscription --save
```
# Usage
To configure a subscription, you must provide two methods: `getCurrentValue` and `subscribe`.
```js
import { createSubscription } from "create-subscription";
const Subscription = createSubscription({
getCurrentValue(source) {
// Return the current value of the subscription (source),
// or `undefined` if the value can't be read synchronously (e.g. native Promises).
},
subscribe(source, callback) {
// Subscribe (e.g. add an event listener) to the subscription (source).
// Call callback(newValue) whenever a subscription changes.
// Return an unsubscribe method,
// Or a no-op if unsubscribe is not supported (e.g. native Promises).
}
});
```
To use the `Subscription` component, pass the subscribable property (e.g. an event dispatcher, observable) as the `source` property and use a [render prop](https://reactjs.org/docs/render-props.html), `children`, to handle the subscribed value when it changes:
```js
<Subscription source={eventDispatcher}>
{value => <AnotherComponent value={value} />}
</Subscription>
```
# Examples
This API can be used to subscribe to a variety of "subscribable" sources, from event dispatchers to RxJS observables. Below are a few examples of how to subscribe to common types.
## Subscribing to event dispatchers
Below is an example showing how `create-subscription` can be used to subscribe to event dispatchers such as DOM elements.
```js
import React from "react";
import { createSubscription } from "create-subscription";
// Start with a simple component.
// In this case, it's a functional component, but it could have been a class.
function FollowerComponent({ followersCount }) {
return <div>You have {followersCount} followers!</div>;
}
// Create a wrapper component to manage the subscription.
const EventHandlerSubscription = createSubscription({
getCurrentValue: eventDispatcher => eventDispatcher.value,
subscribe: (eventDispatcher, callback) => {
const onChange = event => callback(eventDispatcher.value);
eventDispatcher.addEventListener("change", onChange);
return () => eventDispatcher.removeEventListener("change", onChange);
}
});
// Your component can now be used as shown below.
// In this example, 'eventDispatcher' represents a generic event dispatcher.
<EventHandlerSubscription source={eventDispatcher}>
{value => <FollowerComponent followersCount={value} />}
</EventHandlerSubscription>;
```
## Subscribing to observables
Below are examples showing how `create-subscription` can be used to subscribe to certain types of observables (e.g. RxJS `BehaviorSubject` and `ReplaySubject`).
**Note** that it is not possible to support all observable types (e.g. RxJS `Subject` or `Observable`) because some provide no way to read the "current" value after it has been emitted.
### `BehaviorSubject`
```js
const BehaviorSubscription = createSubscription({
getCurrentValue: behaviorSubject => behaviorSubject.getValue(),
subscribe: (behaviorSubject, callback) => {
const subscription = behaviorSubject.subscribe(callback);
return () => subscription.unsubscribe();
}
});
```
### `ReplaySubject`
```js
const ReplaySubscription = createSubscription({
getCurrentValue: replaySubject => {
let currentValue;
// ReplaySubject does not have a sync data getter,
// So we need to temporarily subscribe to retrieve the most recent value.
replaySubject
.subscribe(value => {
currentValue = value;
})
.unsubscribe();
return currentValue;
},
subscribe: (replaySubject, callback) => {
const subscription = replaySubject.subscribe(callback);
return () => subscription.unsubscribe();
}
});
```
## Subscribing to a Promise
Below is an example showing how `create-subscription` can be used with native Promises.
**Note** that an initial render value of `undefined` is unavoidable due to the fact that Promises provide no way to synchronously read their current value.
**Note** the lack of a way to "unsubscribe" from a Promise can result in memory leaks as long as something has a reference to the Promise. This should be taken into consideration when determining whether Promises are appropriate to use in this way within your application.
```js
import React from "react";
import { createSubscription } from "create-subscription";
// Start with a simple component.
function LoadingComponent({ loadingStatus }) {
if (loadingStatus === undefined) {
// Loading
} else if (loadingStatus === null) {
// Error
} else {
// Success
}
}
// Wrap the functional component with a subscriber HOC.
// This HOC will manage subscriptions and pass values to the decorated component.
// It will add and remove subscriptions in an async-safe way when props change.
const PromiseSubscription = createSubscription({
getCurrentValue: promise => {
// There is no way to synchronously read a Promise's value,
// So this method should return undefined.
return undefined;
},
subscribe: (promise, callback) => {
promise.then(
// Success
value => callback(value),
// Failure
() => callback(null)
);
// There is no way to "unsubscribe" from a Promise.
// create-subscription will still prevent stale values from rendering.
return () => {};
}
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