[DataGrid] Remove stateId argument from GridApi getState method

[flaviendelangle]

@dtassone, we only have 2 calls of getState with a stateId, both in the same hook (useGridFilter)
Is this something we want to keep ?
95% of the time, the code written is const page = apiRef.current.getState().page instead of const page = apiRef.current.getState('page') so I'm not sure the overhead is worth it here.

But with this PR you can use the stateId with the correct typing without specifying it as a generic.

@m4theushw the doc generation does not like the generic, I'm loosing the StateId definition here. Same thing for the type returned.
It could be interesting to handle it. But I would more be in favor of droping the stateId param on getState (see above)

[oliviertassinari]

95% of the time, the code written is

Then +1 for removing these 5% percent. For instance, it's not supported in https://react-redux.js.org/api/hooks#usestore.

-const filterState = apiRef.current.getState('filter');
+const filterState = apiRef.current.getState().filter;

or even:

-const filterState = apiRef.current.getState('filter');
+const filterState = filterGridStateSelector(apiRef.current.getState());

[flaviendelangle]

In general, I'm in favor of avoiding methods with separate behaviors like this one.
If the stateId was something usefull, I would have prefered splitting the two :

getState: () => GridState
getSubState: <StateId extends keyof GridState>(stateId: StateId) => GridState[StateId]

[oliviertassinari]

The API is public, I have added a new label. If we could update the description of the PR to document it, it would be perfect. Actually, are we confortable with making the whole state public? If we are not, we can make the method private.

[m4theushw]

Actually, are we confortable with making the whole state public? If we are not, we can make the method private.

@oliviertassinari Yes, I would keep the state and this method public. For some parts of the state (e.g. focus) there's no alternative way to access them besides using getState.

[flaviendelangle]

For the stable version, we may want to make it private and only expose methods for the wanted substates.
That would allow us to consider the other sub states as internal and change their structure without any breaking change.

But for now I do agree that exposing it is simpler.

[oliviertassinari]

OK, I guess that once the component is stable and we need to change one part of the state, then we can change the public getState method to keep it unchanged for developers.