React Redux

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React Redux

Why isn't my component re-rendering, or my mapStateToProps running?

Accidentally mutating or modifying your state directly is by far the most common reason why components do not re-render after an action has been dispatched. Redux expects that your reducers will update their state “immutably”, which effectively means always making copies of your data, and applying your changes to the copies. If you return the same object from a reducer, Redux assumes that nothing has been changed, even if you made changes to its contents. Similarly, React Redux tries to improve performance by doing shallow equality reference checks on incoming props in shouldComponentUpdate, and if all references are the same, shouldComponentUpdate returns false to skip actually updating your original component.

It's important to remember that whenever you update a nested value, you must also return new copies of anything above it in your state tree. If you have state.a.b.c.d, and you want to make an update to d, you would also need to return new copies of c, b, a, and state. This state tree mutation diagram demonstrates how a change deep in a tree requires changes all the way up.

Note that “updating data immutably” does not mean that you must use Immutable.js, although that is certainly an option. You can do immutable updates to plain JS objects and arrays using several different approaches:

  • Copying objects using functions like Object.assign() or _.extend(), and array functions such as slice() and concat()

  • The array spread operator in ES6, and the similar object spread operator that is proposed for a future version of JavaScript

  • Utility libraries that wrap immutable update logic into simpler functions

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Why is my component re-rendering too often?

React Redux implements several optimizations to ensure your actual component only re-renders when actually necessary. One of those is a shallow equality check on the combined props object generated by the mapStateToProps and mapDispatchToProps arguments passed to connect. Unfortunately, shallow equality does not help in cases where new array or object instances are created each time mapStateToProps is called. A typical example might be mapping over an array of IDs and returning the matching object references, such as:

const mapStateToProps = state => {
  return {
    objects: state.objectIds.map(id => state.objects[id])
  }
}

Even though the array might contain the exact same object references each time, the array itself is a different reference, so the shallow equality check fails and React Redux would re-render the wrapped component.

The extra re-renders could be resolved by saving the array of objects into the state using a reducer, caching the mapped array using Reselect, or implementing shouldComponentUpdate in the component by hand and doing a more in-depth props comparison using a function such as _.isEqual. Be careful to not make your custom shouldComponentUpdate() more expensive than the rendering itself! Always use a profiler to check your assumptions about performance.

For non-connected components, you may want to check what props are being passed in. A common issue is having a parent component re-bind a callback inside its render function, like <Child onClick={this.handleClick.bind(this)} />. That creates a new function reference every time the parent re-renders. It's generally good practice to only bind callbacks once in the parent component's constructor.

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How can I speed up my mapStateToProps?

While React Redux does work to minimize the number of times that your mapStateToProps function is called, it's still a good idea to ensure that your mapStateToProps runs quickly and also minimizes the amount of work it does. The common recommended approach is to create memoized “selector” functions using Reselect. These selectors can be combined and composed together, and selectors later in a pipeline will only run if their inputs have changed. This means you can create selectors that do things like filtering or sorting, and ensure that the real work only happens if needed.

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Why don't I have this.props.dispatch available in my connected component?

The connect() function takes two primary arguments, both optional. The first, mapStateToProps, is a function you provide to pull data from the store when it changes, and pass those values as props to your component. The second, mapDispatchToProps, is a function you provide to make use of the store's dispatch function, usually by creating pre-bound versions of action creators that will automatically dispatch their actions as soon as they are called.

If you do not provide your own mapDispatchToProps function when calling connect(), React Redux will provide a default version, which simply returns the dispatch function as a prop. That means that if you do provide your own function, dispatch is not automatically provided. If you still want it available as a prop, you need to explicitly return it yourself in your mapDispatchToProps implementation.

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Should I only connect my top component, or can I connect multiple components in my tree?

Early Redux documentation advised that you should only have a few connected components near the top of your component tree. However, time and experience has shown that such a component architecture generally requires a few components to know too much about the data requirements of all their descendants, and forces them to pass down a confusing number of props.

The current suggested best practice is to categorize your components as “presentational” or “container” components, and extract a connected container component wherever it makes sense:

Emphasizing “one container component at the top” in Redux examples was a mistake. Don't take this as a maxim. Try to keep your presentation components separate. Create container components by connecting them when it's convenient. Whenever you feel like you're duplicating code in parent components to provide data for same kinds of children, time to extract a container. Generally as soon as you feel a parent knows too much about “personal” data or actions of its children, time to extract a container.

In fact, benchmarks have shown that more connected components generally leads to better performance than fewer connected components.

In general, try to find a balance between understandable data flow and areas of responsibility with your components.

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