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What Even Is A Dispatch Function?

Dustin Myers on February 15, 2020

Learning redux? Or useReducer? Then chances are you've been frustrated by the black magic that is the dispatch function 🧙‍♂️! Well, luckily you hav...

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Habib Ur Rehman • May 12 '21

The disptach function calls a reducer function, but i came to know that the control flows in this way.
dispatch -> store -> combineReducer -> reducer
I am not understanding that disptach function is calling the exact reducer function but why the the data is passed to store first then to combineReducer and then to reducer.\
Please can you explain this

KelvinIOT • Oct 26 '21 • Edited

Hello Habib,

So i understand that Store holds the whole state tree of your application.
Then you pass to the store, your combineReducer which combines all your separate reducers in an object.
When the dispatch function calls the store, it actually tries all your Reducers.
But the Reducers listen for events checking for a matching action-type, so if the action-type passed to the dispatch finds a match in a reducer, that particular reducer runs.

Dustin Myers • May 2 '22

This is absolutely right! Sorry I was late on the reply. Hopefully kelvin's response helped you understand how this system works with combined reducers. Feel free to reach out if you have any other questions! And thank you Kelvin. That was a great explanation 👍👍

Habib Ur Rehman • Oct 28 '21

Thank you very much for understanding.

KeithArnold • Jul 11 '21

Thank you Dustin, I've been spinning my wheels for more than a week to understand useReducer and at last it makes sense!

Dustin Myers • May 2 '22

Seriously the best kind of reply! Thank you, and I'm glad it was helpful!

Zalyotov Alexey • Mar 17 '22

Thanks a lot! It's pretty clear and helpful

Dustin Myers • May 2 '22

Thank you! I'm really glad this helped!

John Keers • Apr 29 '20 • Edited

If you have multiple reducers, can you tell me how the dispatch function knows which reducer to call?
Is there a switch statement internally with action type as the parameter?

Update: Looking at the source code for createStore it looks like it only ever takes a single reducer? Does that mean you simply use combineReducer() and createStore will choose the correct reducer to use?

Dustin Myers • May 24 '20 • Edited

Actually when you dispatch an action, it will go through all your reducers that you've "combined". It's the action type that's the magic here. You probably only have one reducer that handles that specific action type. All the other reducers will look at the action type, and return their slice of the state untouched.

gaddopur • Jul 18 '20

Thank you very much. It helped very much. Before reading this I was in confusion but you clear all of my doubts once again thank you very much.

Dustin Myers • Jul 24 '20

Awesome!! So glad it helped 😊

Sodj • Jan 12 '23 • Edited

I came here trying to understand why the actionCreator function can't call the dispatch directly ... why do we have to do dispatch(actionCreator()); instead of just actionCreator();