[Feature Request] Generic Types for Box

[Afsar-Pasha]

It would be great if Box support generic types such as Box<K, V>.

[simc]

What exactly do you mean? You can store any object in a box.

[Afsar-Pasha]

Ex, if I want a box to store only String keys then, by using Hive.openBox<dynamc, String>('box') compiler would allow only to write String as value in Box.put, Box.putAt, etc.
This could reduce bugs, and also compiler can understand the return type of Box.get(key).

[simc]

This only works with values because the add() method implicitly stores an integer key.
So if you would open a box which only supports String keys and you use the add() method, there will be an exception.
I'm not sure if the users expect that...

[Afsar-Pasha]

Maybe Box<T> in which value is T and key is always dynamic?

[simc]

I'll implement that 👍

[simc]

I thought about this and (afaik) it is impossible to implement this. The best example are lists: You cannot cast List<dynamic> to List<SomeObject> so if you open a box of type Box<List<SomeObject>>, you expect to get a List<SomeObject> if you call box.get('myKey') but you actually get a List<dynamic>.

If you have a solution for this problem, I would happily implement this feature.

[Afsar-Pasha]

I think it can be implemented by using this approach but that would require a little bit of boilerplate and that would be hacky, so closing this issue. Would re-open if I got any neat solution.

[Afsar-Pasha]

Instead of that approach we can add an anonymous function to convert List and Map such as

Hive.openBox<List<int>>('box', castToList: (l)=>l.cast<int>().toList())

in BoxImpl

T Function(List<dynamic> l) castToList;
T Function(Map<dynamic, dynamic> m) castToMap;
BoxImpl(... , {this.castToList, this.castToMap} ...) //constructor
...
  T get(dynamic key, {T defaultValue}){
  ...
    if(frame.value is List)
      return castToList(frame.value);
    if(frame.value is Map)
      return castToMap(frame.value);

but still requires some boilerplate and a little bit confusing.

[simc]

Unfortunately cast() only works for "one dimension". List<List<dynamic>>.cast<List<List<int>>() will fail.

I think this is a major flaw of the language :/

[Afsar-Pasha]

I mean they can provide a function to convert List<dynamic> to List<T> for that, the function can be

(list) => (l.map((l) => l.cast<int>().toList()).toList().cast<List<int>>().toList())