Redux FAQ: Store Setup

Table of Contents

Store Setup

Is it OK to have more than one middleware chain in my store enhancer? What is the difference between next and dispatch in a middleware function?

Redux middleware act like a linked list. Each middleware function can either call next(action) to pass an action along to the next middleware in line, call dispatch(action) to restart the processing at the beginning of the list, or do nothing at all to stop the action from being processed further.

This chain of middleware is defined by the arguments passed to the applyMiddleware function used when creating a store. Defining multiple chains will not work correctly, as they would have distinctly different dispatch references and the different chains would effectively be disconnected.

Further information

Documentation

Discussions

How do I subscribe to only a portion of the state? Can I get the dispatched action as part of the subscription?

Redux provides a single store.subscribe method for notifying listeners that the store has updated. Listener callbacks do not receive the current state as an argument—it is simply an indication that something has changed. The subscriber logic can then call getState() to get the current state value.

This API is intended as a low-level primitive with no dependencies or complications, and can be used to build higher-level subscription logic. UI bindings such as Presenter-middleware can create a subscription for each connected component. It is also possible to write functions that can intelligently compare the old state vs the new state, and execute additional logic if certain pieces have changed.

The new state is not passed to the listeners in order to simplify implementing store enhancers such as the Redux DevTools. In addition, subscribers are intended to react to the state value itself, not the action. Middleware can be used if the action is important and needs to be handled specifically.

Further information

Documentation

Discussions

Libraries

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