react-boilerplate/docs/js/redux-saga.md

# `redux-saga`

`redux-saga` is a library to manage side effects in your application. It works
beautifully for data fetching, concurrent computations and a lot more.
[Sebastien Lorber](https://twitter.com/sebastienlorber) put it best:

> Imagine there is widget1 and widget2. When some button on widget1 is clicked,
> then it should have an effect on widget2. Instead of coupling the 2 widgets
> together (i.e. widget1 dispatches an action that targets widget2), widget1 only
> dispatches that its button was clicked. Then the saga listens for this button
> click and updates widget2 by dispatching a new event that widget2 is aware of.
>
> This adds a level of indirection that is unnecessary for simple apps, but makes
> it easier to scale complex applications. You can now publish widget1 and
> widget2 to different npm repositories so that they never have to know about
> each other, without having them share a global registry of actions. The 2
> widgets are now bounded by contexts that can live separately. They don't need
> each other to be consistent and can be reused in other apps as well. **The saga
> is the coupling point between the two widgets that coordinate them in a
> meaningful way for your business.**

_Note: It is well worth reading the [source](https://stackoverflow.com/questions/34570758/why-do-we-need-middleware-for-async-flow-in-redux/34623840#34623840)
of this quote in its entirety!_

To learn more about this amazing way to handle concurrent flows, start with the
[official documentation](https://redux-saga.github.io/redux-saga) and explore
some examples! (Read [this comparison](https://stackoverflow.com/questions/34930735/pros-cons-of-using-redux-saga-with-es6-generators-vs-redux-thunk-with-es7-async/34933395) if you're used to `redux-thunk`)

## Usage

Sagas are associated with a container, just like actions, constants, selectors
and reducers. If your container already has a `saga.js` file, simply add your
saga to that. If your container does not yet have a `saga.js` file, add one with
this boilerplate structure:

```JS
import { takeLatest, call, put, select } from 'redux-saga/effects';

// Root saga
export default function* rootSaga() {
  // if necessary, start multiple sagas at once with `all`
  yield [
    takeLatest(LOAD_REPOS, getRepos),
    takeLatest(LOAD_USERS, getUsers),
  ];
}
```

Then, in your `index.js`, use a decorator to inject the root saga:

```JS
import injectSaga from 'utils/injectSaga';
import { DAEMON } from 'utils/constants';
import saga from './saga';

// ...

// `mode` is an optional argument, default value is `DAEMON`
const withSaga = injectSaga({ key: 'yourcomponent', saga, mode: DAEMON });

export default compose(
  withSaga,
)(YourComponent);
```

A `mode` argument can be one of three constants (import them from `utils/constants`):

- `DAEMON` (default value) — starts a saga on component mount and never cancels it or starts again;
- `RESTART_ON_REMOUNT` — starts a saga when a component is being mounted
  and cancels with `task.cancel()` on component un-mount for improved performance;
- `ONCE_TILL_UNMOUNT` — behaves like `RESTART_ON_REMOUNT` but never runs the saga again.

Now add as many sagas to your `saga.js` file as you want!

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