Support of types keys for storage instance

[ipakhomov]

Hey there, thanks for amazing library!

Currently, keys for values in a storage aren't typed, they are just strings.

Having ability to pass an optional generic type when initializing a new mmkv instance could improve developer experience.
This will allow to specify which values are usable with particular storage instance. Example:

type StorableValues = 'token' | 'userId';

export const storage = new MMKV<StorableValues>({ ... });
...
storage.getString('foo'); // Typescript will report this error
storage.getString('token'); // All good

Of course, this can be achieved using a separate enum for keys in the app, but having generic support is more elegant and easier in use.

Please let me know if this feature is desirable, and you'd like me to submit a PR with these changes!

[maintenance-hans]

Guten Tag, Hans here! 🍻

Thanks for your suggestion! It’s a well-written request for a feature that could indeed improve developer experience. Mrousavy appreciates feedback like zis!

Feel free to go ahead and submit a PR with your changes if you’re willing. It’s always helpful when contributors pitch in! If you have any questions or need help with zis, just let us know.

Note: If you think I made a mistake, please ping @mrousavy to take a look.

[ipakhomov]

@mrousavy I'm looking forward for your thoughts on this

[ipakhomov]

@mrousavy I decided to go further and added support of a generic type allows to describe not only keys, but also values for a storage instance. Example:

type StorageValues = {
  userToken: string;
  sessionTTL: number;
  isPremiumUser: boolean;
  cachedData: ArrayBuffer;
};
// Create a Typed MMKV Instance:
const storage = new MMKV<StorageValues>();

storage.set('userToken', 'abc123');  // Ok
storage.set('userToken', false); // TS error because value of 'userToken' is 'string'
...
storage.getString('userToken'); // All good as well
storage.getString('sessionTTL'); // TS error because 'sessionTTL' is not a 'string'

Created a PR where I updated all methods and hooks to support this: #826

Please take a look when you have time :)

FYI @Zcating