Components: Under the Hood

Components provide powerful modularity to Starknet contracts. But how does this magic actually happen behind the scenes?

This chapter will dive deep into the compiler internals to explain the mechanisms that enable component composability.

A Primer on Embeddable Impls

Before digging into components, we need to understand embeddable impls.

An impl of a Starknet interface trait (marked with #[starknet::interface]) can be made embeddable. Embeddable impls can be injected into any contract, adding new entry points and modifying the ABI of the contract.

Let's look at an example to see this in action:

#[starknet::interface]
trait SimpleTrait<TContractState> {
    fn ret_4(self: @TContractState) -> u8;
}

#[starknet::embeddable]
impl SimpleImpl<TContractState> of SimpleTrait<TContractState> {
    fn ret_4(self: @TContractState) -> u8 {
        4
    }
}

#[starknet::contract]
mod simple_contract {
    #[storage]
    struct Storage {}

    #[abi(embed_v0)]
    impl MySimpleImpl = super::SimpleImpl<ContractState>;
}

By embedding SimpleImpl, we externally expose ret4 in the contract's ABI.

Now that we’re more familiar with the embedding mechanism, we can now see how components build on this.

Inside Components: Generic Impls

Recall the impl block syntax used in components:

    #[embeddable_as(OwnableImpl)]
    impl Ownable<
        TContractState, +HasComponent<TContractState>,
    > of super::IOwnable<ComponentState<TContractState>> {

The key points:

The generic impl Ownable requires the implementation of the HasComponent<TContractState> trait by the underlying contract, which is automatically generated with the component!() macro when using a component inside a contract.

The compiler will generate an embeddable impl that wraps any function in Ownable, replacing the self: ComponentState<TContractState> argument with self: TContractState, where access to the component state is made via the get_component function in the HasComponent<TContractState> trait.

For each component, the compiler generates a HasComponent trait. This trait defines the interface to bridge between the actual TContractState of a generic contract, and ComponentState<TContractState>.
```
// generated per component
trait HasComponent<TContractState> {
    fn get_component(self: @TContractState) -> @ComponentState<TContractState>;
    fn get_component_mut(ref self: TContractState) -> ComponentState<TContractState>;
    fn get_contract(self: @ComponentState<TContractState>) -> @TContractState;
    fn get_contract_mut(ref self: ComponentState<TContractState>) -> TContractState;
    fn emit<S, impl IntoImp: traits::Into<S, Event>>(ref self: ComponentState<TContractState>, event: S);
}
```
In our context ComponentState<TContractState> is a type specific to the ownable component, i.e. it has members based on the storage variables defined in ownable_component::Storage. Moving from the generic TContractState to ComponentState<TContractState> will allow us to embed OwnableImpl in any contract that wants to use it. The opposite direction (ComponentState<TContractState> to ContractState) is useful for dependencies (see the Upgradeable component depending on an IOwnable implementation example in the Components dependencies section).

To put it briefly, one should think of an implementation of the above HasComponent<T> as saying: “Contract whose state T has the upgradeable component”.
Ownable is annotated with the embeddable_as(<name>) attribute:

embeddable_as is similar to embeddable; it only applies to impls of starknet::interface traits and allows embedding this impl in a contract module. That said, embeddable_as(<name>) has another role in the context of components. Eventually, when embedding OwnableImpl in some contract, we expect to get an impl with the following functions:
```
    fn owner(self: @TContractState) -> ContractAddress;
  fn transfer_ownership(ref self: TContractState, new_owner: ContractAddress);
  fn renounce_ownership(ref self: TContractState);
```
Note that while starting with a function receiving the generic type ComponentState<TContractState>, we want to end up with a function receiving ContractState. This is where embeddable_as(<name>) comes in. To see the full picture, we need to see what is the impl generated by the compiler due to the embeddable_as(OwnableImpl) annotation:

#[starknet::embeddable]
impl OwnableImpl<
    TContractState, +HasComponent<TContractState>, impl TContractStateDrop: Drop<TContractState>,
> of super::IOwnable<TContractState> {
    fn owner(self: @TContractState) -> ContractAddress {
        let component = HasComponent::get_component(self);
        Ownable::owner(component)
    }

    fn transfer_ownership(ref self: TContractState, new_owner: ContractAddress) {
        let mut component = HasComponent::get_component_mut(ref self);
        Ownable::transfer_ownership(ref component, new_owner)
    }

    fn renounce_ownership(ref self: TContractState) {
        let mut component = HasComponent::get_component_mut(ref self);
        Ownable::renounce_ownership(ref component)
    }
}

Note that thanks to having an impl of HasComponent<TContractState>, the compiler was able to wrap our functions in a new impl that doesn’t directly know about the ComponentState type. OwnableImpl, whose name we chose when writing embeddable_as(OwnableImpl), is the impl that we will embed in a contract that wants ownership.

Contract Integration

We've seen how generic impls enable component reusability. Next let's see how a contract integrates a component.

The contract uses an impl alias to instantiate the component's generic impl with the concrete ContractState of the contract.

    #[abi(embed_v0)]
    impl OwnableImpl = OwnableComponent::OwnableImpl<ContractState>;

    impl OwnableInternalImpl = OwnableComponent::InternalImpl<ContractState>;

The above lines use the Cairo impl embedding mechanism alongside the impl alias syntax. We’re instantiating the embeddable generated impl OwnableImpl<TContractState> with the concrete type ContractState. Recall that the generic impl is Ownable<TContractState>, and the component! macro provides HasComponent<TContractState> so the wrapper can delegate to it.

Note that only the using contract could have implemented this trait since only it knows about both the contract state and the component state.

This glues everything together to inject the component logic into the contract.

Key Takeaways

Embeddable impls allow injecting components logic into contracts by adding entry points and modifying the contract ABI.
The compiler automatically generates a HasComponent trait implementation when a component is used in a contract. This creates a bridge between the contract's state and the component's state, enabling interaction between the two.
Components encapsulate reusable logic in a generic, contract-agnostic way. Contracts integrate components through impl aliases and access them via the generated HasComponent trait.
Components build on embeddable impls by defining generic component logic that can be integrated into any contract wanting to use that component. Impl aliases instantiate these generic impls with the contract's concrete storage types.

The Cairo Programming Language

Components: Under the Hood

A Primer on Embeddable Impls

Inside Components: Generic Impls

Contract Integration

Key Takeaways