We use Redux to manage application state. The docs do a good job explaining the concepts, so go read them.

Quick Intro

Just like the React introduction, this is a quick introduction to redux, focusing on how it fits into React and why we chose it.

One of the core problems that React does not address is managing state. In the React intro, we talked about data flowing down and events flowing up. Conceptually this is nice, but you quickly run into awkward situations in large apps.

Let's look at an example. Say you have a page with a tabbed interface. Here, Tab1 is managing a list of items, so naturally it uses local state. Tab2 renders different stuff.

const Tab1 = React.createClass({
  getInitialState: function() {
    return { items: [] };
  },

  handleAddItem: function(item) {
    this.setState({ items: [...this.state.items, item]});
  },

  render: function() {
    /* ... Renders the items and button to add new item ... */
  }
});

const Tab2 = React.createClass({
  render: function() {
    /* ... Renders other data ... */
  }
});

// Assume `Tab1` and `Tab2` are wrapped with a factory when importing
const Tabs = React.createClass({
  render: function() {
    return div(
      { className: 'tabs' },
      // ... Render the tab buttons ...
      Tab1(),
      Tab2()
    );
  }
});

What happens when Tab2 needs the list of items though? This scenario comes up all time: components that aren't directly related need access to the same state. A small change would be to move the items state up to the Tabs component, and pass it down to both Tab1 and Tab2.

But now Tabs has to implement the handleAddItem method to add an item because it's managing that state. This quickly gets ugly as the end result is the root component ends up with a ton of state and methods to manage it: a god component is born.

Additionally, how do we know what data each tab needs? We end up passing all the state down because we don't know. This is not a modular solution: one object managing the state and every component receiving the entire state is like using tons of global variables.

Evolution of Flux

Facebook addressed this with the flux architecture, which takes the state out of the components and into a "store". Redux is the latest evolution of this idea and solves a lot of problems previous flux libraries had (read it's documentation for more info).

Because the state exists outside the component tree, any component can read from it. Additionally, state is updated with actions that any component can fire. We have guidelines for where to read/write state, but it completely solves the problem described above. Both Tab1 and Tab2 would be listening for changes in the item state, and Tab1 would fire actions to change it.

With redux, state is managed modularly with reducers but tied together into a single object. This means a single JS object represents most* of your state. It may sound crazy at first, but think of it as an object with references to many pieces of state; that's all it is.

This makes it very easy to test, debug, and generally think about. You can log your entire state to the console and inspect it. You can even dump in old states and "replay" to see how the UI changed over time.

I said "most*" because it's perfectly fine to use both component local state and redux. Be aware that any debugging tools will not see local state at all though. It should only be used for transient state; we'll talk more about that in the guidelines.

Immutability

Another important concept is immutability. In large apps, mutating state makes it very hard to track what changed when. It's very easy to run into situations where something changes out from under you, and the UI is rendered with invalid data.

Redux enforces the state to be updated immutably. That means you always return new state. It doesn't mean you do a deep copy of the state each time: when you need to change some part of the tree you only need to create new objects to replace the ones your changing (and walk up to the root to create a new root). Unchanged subtrees will reference the same objects.

This removes a whole class of errors, almost like Rust removing a whole class of memory errors by enforcing ownership.

Order of Execution

One of best things about React is that rendering is synchronous. That means when you render a component, given some data, it will fully render in the same tick. If you want the UI to change over time, you have to change the data and rerender, instead of arbitrary UI mutations.

The reason this is desired is because if you build the UI around promises or event emitters, updating the UI becomes very brittle because anything can happen at any time. The state might be updated in the middle of rendering it, maybe because you resolved a few promises which made your rendering code run a few ticks later.

Redux embraces the synchronous execution semantics as well. What this means is that everything happens in a very controlled way. When updating state through an action, all reducers are run and a new state is synchronously generated. At that point, the new state is handed off to React and synchronously rendered.

Updating and rendering happen in two phases, so the UI will always represent consistent state. The state can never be in the middle of updating when rendering.

What about asynchronous work? That's where middleware come in. At this point you should probably go study our code, but middleware allows you to dispatch special actions that indicate asynchronous work. The middleware will catch these actions and do something async, dispatching "raw" actions along the way (it's common to emit a START, DONE, and ERROR action).

Ultimately there are 3 "phases" or level of abstraction: the async layer talks to the network and may dispatch actions, actions are synchronously pumped through reducers to generate state, and state is rendered with react.

Next

Read the Redux Guidelines next to learn how to write React code specifically for the devtools.

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