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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 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 (ie widget1 dispatch an action that targets widget2), widget1 only dispatch that its button was clicked. Then the saga listen for this button click and then update widget2 by dispatching a new event that widget2 is aware of.

This adds a level of indirection that is unnecessary for simple apps, but make it more easy to scale complex applications. You can now publish widget1 and widget2 to different npm repositories so that they never have to know about each others, without having them to share a global registry of actions. The 2 widgets are now bounded contexts that can live separately. They do not need each others 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 of this quote in its entirety!

To learn more about this amazing way to handle concurrent flows, start with the official documentation and explore some examples! (read this comparison 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:

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:

import injectSaga from 'utils/injectSaga';
import { RESTART_ON_REMOUNT } from 'utils/constants';
import saga from './saga';

// ...

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

export default compose(
  withSaga,
)(YourComponent);

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

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