[DataGrid] Group events into a single enum

[flaviendelangle]

Breaking changes

• [XGrid] Replace event constants with the GridEvents enum ([DataGrid] Group events into a single enum #2279) @flaviendelangle

-import { GRID_CELL_EDIT_START } from '@material-ui/x-grid';
-apiRef.current.subscribeEvent(GRID_CELL_EDIT_START, (params, event) => { ... });
+import { GridEvents } from '@material-ui/x-grid';
+apiRef.current.subscribeEvent(GridEvents.cellEditStart, (params, event) => { ... });

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Part of #2227

With #2278 there is no change in the documentation output.

Several benefits here :

• Improve the clarity of the public API by gathering dozens of string variables
• Simplify the imports in our codebase
• Help the future typing of publishEvent / subscribeEvent (they will be able to import the enum instead of having floating strings as keys

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Do we want the user to use directly the string (like in https://github.com/mui-org/material-ui-x/blob/master/docs/src/pages/components/data-grid/events/SubscribeToEvents.tsx#L16) or do we want them to import the constant (like in https://github.com/mui-org/material-ui-x/blob/master/docs/src/pages/components/data-grid/editing/CatchEditingEventsGrid.tsx#L25)

If we want him to import the enum, could we type publishEvent / subscribeEvent first param accordingly ?

[oliviertassinari]

It looks like a great idea. I have merged HEAD into te PR to leverage #2278, showing the events.json has no diffs.

[oliviertassinari]

It should have been:

Suggested change

	apiRef.current.publishEvent(GridEvents.cellNavigationKeyDown, params, event);
	apiRef.current.publishEvent(GridEvents.columnHeaderNavigationKeyDown, params, event);

The keyboard navigation is no longer working on the header https://codesandbox.io/s/material-demo-forked-git2x