Subscribe to single property change in store in Redux

Question

In Redux I can easily subscribe to store changes with

store.subscribe(() => my handler goes here)

But what if my store is full of differ

Answer 1

In addition to what Andy Noelker said, mapStateToProps not only passes part of the state properly down your component tree, it also subscribes to changes made directly in these subscribed portions of the state.

It is true that every mapStateToProp function you bind to the store gets called each time any part of the state is changed, but the result of the call gets shallow compared to the previous call - if top level keys you subscribed onto did not change (the reference stays the same). Then mapStateToProps would not call re-render. So if you want the concept to work, you have to keep mapStateToProps simple, no merging, type changing or anything, they should simply pass down parts of the state.

If you want to reduce the data from the state when subscribing, for example you had list data in the state and you want to convert it to object with ids as keys, or you want to join multiple states into data structures, you should combine mapStateToProps with createSelector from reselect library, by doing all these modifications inside selector. Selectors are pure functions that reduce and cache state chunks passed in as input and if input did not change - they return exactly the same reference they did on the last call - without performing the reduction.