PLEASE, stop using

 useSelector(state => state.mainReducer);

it doesn't make any sense

there should be a simple state transformation (subselection)

const a = useSelector(state => state.a)

taken directly from redux docs:

const counter = useSelector(state => state.counter)  

update

you can see effect (from store change) with slightly changed component

function MyComponent(props) {
  const a = useSelector(state => state.a);
  const dispatch = useDispatch(); 

  console.log('render: ', a);

  useEffect(() => {
    console.log('use effect: ', a);
    dispatch({ type: 'A', payload: a });
  }, [a]) 

  useEffect(() => {
    console.log('did mount: ', a);
    dispatch({ type: 'A', payload: 1 })
  }, []); 

  return (<View style={styles.container}>
    <Text style={styles.text}>{a}</Text>
  </View>);
};

It should result in log:

• render: 0 // initial state
• use effect: 0 // first effect run
• // dispatch 0 ... processed in store by reducer but results in the same state ...
  // ... and in our rendering process we still working on an 'old' a readed from state on the beginning of render
• did mount: 0 // 'old' a
  // dispatch 1 ... changed state in redux store

• .... rendered text 0
  ...
  ...

• // useSelector forces rerendering - change detected

• render: 1 // latest dispatched value, processed by reducers into new state, rereaded by selector
• use effect: 1 // useEffect works AS EXPECTED as an effect of a change
• .... rendered text 1

...
...

• no more rerenderings - latest dispach not changed state

Of course dispatch from other component will force update in this component ... if value will be different.

When setState is called multiple times in React event handler or synchronous lifecycle method, it will be batched into a single update. That's why you are getting two logs, initial render causes first log and setState causes second log and when you called useState inside setTimeOut then it logged as you expected because setTimeOut is an asynchronous operation and it behaves like this with other asynchronous operations such as async/await, then/catch, fetch, etc. because Separate state updates will not be batched in asynchronous operations.