Add hooks usage section

[andrew-fleming]

It might be helpful for users not familiar with hooks to have a more formal description and example. This probably belongs in /components.adoc#customization

[o-tsaruk]

Hey, does someone have a simple example of ERC20HooksTrait usage?

[ericnordelo]

We will include a guide for them, but in the meantime we will be updating the wizard probably this week with the ERC20Votes extension, and that extension uses hooks.

[andrew-fleming]

Hey, @o-tsaruk. In the mean time, you can also take a look at our ERC20Votes mock for a usage example: https://github.com/OpenZeppelin/cairo-contracts/blob/main/src/tests/mocks/erc20_votes_mocks.cairo#L63-L86

[o-tsaruk]

Thank you, friends! Do you know if can we use the storage variable inside ERC20HooksTrait implementation? Or it's possible only for ERC-20 storage variables?

#[storage]
struct Storage {
    #[substorage(v0)]
    erc20: ERC20Component::Storage,
    some_variable: u256,
 }

[andrew-fleming]

Sorry for the delay @o-tsaruk . We can definitely use a contract's storage variables inside the hooks trait. It's a different approach than with the mock example.

First, we need to remove the generic syntax of the implementation and instead pass in the contract's state (ContractState). And then we can access the state with get_contract from the HasComponent trait.

Here's an example with your struct:

impl ERC20HooksImpl of ERC20Component::ERC20HooksTrait<ContractState> {
    fn before_update(
        ref self: ERC20Component::ComponentState<ContractState>,
        from: ContractAddress,
        recipient: ContractAddress,
        amount: u256
    ) {
        let mut contract_state = ERC20Component::HasComponent::get_contract_mut(ref self);
        contract_state.some_variable.write(12345);
    }
    
    (...)
}

S/o to @ericnordelo for the solution