Need an example to show how to use trait-contract as a storage in contract.

[atenjin]

Is there a way to load a contract from an AccountId directly?

Describe the details
Example delegator and multisig_plain show how to do a contract call another contract. However I'm interesting in that whether ink! allow to load a contract from an AccountId?
In example delegator, the contract call should pass the contract type, and in example multisig_plain, the contract allow to pass an AccountId, but call some function need to pass the selector and encoded params.

So is there a way to load a contract from an AccountId? I look though the code and find the trait:

pub trait FromAccountId<T>
where
    T: Environment,
{
    /// Creates the contract instance from the account ID of the already instantiated contract.
    fn from_account_id(account_id: <T as Environment>::AccountId) -> Self;
}

But there is no example to show how to use this trait. And in expanded code for a contract, this trait is not implemented for the contract.
Thus, I don't know whether this thing "load a contract from an AccountId" is allowed?
If this thing is allowed, then, how can I use it?
(
Sorry for I don't understand the doc comments in create_builder.rs file:
https://github.com/paritytech/ink/blob/3803a2662e89dfa97b6f8b17e87c0cce2d873f48/crates/env/src/call/create_builder.rs#L146-L164
)

In my mind, what I expect is like:

// load a contract from an AccountId
let erc20_instance: Erc20 = Erc20::from_account_id(account_id);

And it seems that the trait-* design like the example "trait-erc20", "trait-flipper" is a way to solve this situation? May be we need more example to show how to use trait-* contract to design a contract specification.

[atenjin]

for example the trait-contract may could be used like(I guess):

    #[ink(storage)]
    pub struct ExampleContract {
        holder: Lazy<dyn BaseErc20>,
    }

to receive different erc20 implementation. (But I can't compile it now).

Thus the final thing what I expect is like the interface in solidity. The caller contract could load an Address to an interface and call the real contract.

But now I don't know how to use from_account_id to those things, and there is no example to show how to use trait-contract in a contract.

[atenjin]

I think I figure out how to use FromAccountId::from_account_id.
This method is only used in the condition like delegator example.

contract A crate is depended by contract B crate, like accumulator is depended by delegator in delegator example. In B cargo.toml, the A must open feature ink-as-dependency, then in conditional compilation for A, the macro would generate the implementation FromAccountId::from_account_id for struct A, and then, in contract B, could use

            use ink_env::call::FromAccountId;
            let instance: A = A::from_account_id(address);

to load contract A from an address/account_id.

But now I still don't know how to use trait-contract as a storage.

[Robbepop]

for example the trait-contract may could be used like(I guess):

    #[ink(storage)]
    pub struct ExampleContract {
        holder: Lazy<dyn BaseErc20>,
    }

to receive different erc20 implementation. (But I can't compile it now).

Thus the final thing what I expect is like the interface in solidity. The caller contract could load an Address to an interface and call the real contract.

But now I don't know how to use from_account_id to those things, and there is no example to show how to use trait-contract in a contract.

This has not yet been implemented for ink! but we are aware that this is needed and are already working out some possible designs for this feature.

[Robbepop]

Closed since superseeded by #631.
Please continue discussion in the linked issue.