Add method to check if MMKV instance with given ID exists

[DorianMazur]

Description:

Currently, there's no straightforward way to check if an MMKV instance with a specific ID already exists. This functionality would be useful for managing multiple MMKV instances and avoiding conflicts or unnecessary instance creation.

Feature request:

Add a static method to the MMKV class that allows checking for the existence of an instance with a given ID.

Proposed usage:

import { MMKV } from 'react-native-mmkv'

const instanceExists = MMKV.exists('some-id')
console.log(instanceExists) // Should return true or false

Benefits:

• Simplifies instance management in apps using multiple MMKV stores
• Helps prevent accidental overwriting of existing instances

[maintenance-hans[bot]]

Guten Tag, Hans here.

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[mrousavy]

Well you can just load the MMKV instance and check if there are any keys in it, no?

const instance = new MMKV({ id: 'some-id' })
const instanceExists = instance.getAllKeys().length > 0
console.log(instanceExists) // Should return true or false

[mrousavy]

Or maybe instance.size > 0, this is size in bytes

[DorianMazur]

I have encrypted instance, I need to check if instance exists before encrypting it. Initializing without encryption key will erase all data.

[mrousavy]

Initializing without encryption key will erase all data.

This is also a bit weird imo - that should throw an error instead of erasing all data..