Systems

Systems = Functions in a Dojo contract

TL;DR

Systems are Dojo contract functions.
Systems can access to the world using the self.world(<NAMESPACE>) function.
Systems engage the world contract to alter models' state.
Systems ought to be concise and specific.
In most scenarios, systems are stateless.

What are systems?

Within Dojo we define systems as functions within a Dojo contract that act on the world.

Systems play a pivotal role in your world's logic, directly mutating its component states. It's important to understand that to enact these mutations, a system needs explicit permission from the models owner.

Permissions

In order to write data to the world, a system needs explicit permission from the models owner.

A simple way to think about system design for permissions:

System Permissions

Dojo interface

In a Dojo contract, you must first define a Dojo interface to declare the systems that your contract will expose.

#[starknet::interface]
trait IActions<T> {
    fn spawn(ref self: T);
    fn move(ref self: T, direction: Direction);
}

System implementation

To implement the code related to the system, you must be placed inside a #[dojo::contract] and implement the interface you've defined.

#[dojo::contract]
mod actions {
    use super::IActions;
 
    #[abi(embed_v0)]
    impl ActionsImpl of IActions<ContractState> {
        fn spawn(ref self: ContractState) {
            // Get the default world.
            let mut world = self.world(@"dojo_starter");
 
            // Get the address of the current caller, possibly
            // the player's address.
            let player = get_caller_address();
 
            // Retrieve the player's current position from the world.
            let mut position: Position = world.read_model(player);
 
            // Update the world state with the new data.
 
            // 1. Move the player's position 10 units in both
            // the x and y direction.
            let new_position = Position {
                player,
                vec: Vec2 { 
                    x: position.vec.x + 10,
                    y: position.vec.y + 10
                }
            };
 
            // Write the new position to the world.
            world.write_model(@new_position);
 
            // 2. Set the player's remaining moves to 100.
            let moves = Moves {
                player, remaining: 100,
                last_direction: Direction::None, can_move: true
            };
 
            // Write the new moves to the world.
            world.write_model(@moves);
        }
 
        fn move(ref self: ContractState, direction: Direction) {
 
            // Get the default world.
            let mut world = self.world(@"dojo_starter");
 
            // Get the address of the current caller, possibly
            // the player's address.
            let player = get_caller_address();
 
            // Retrieve the player's current position and moves data
            // from the world.
            let mut position: Position = world.read_model(player);
            let mut moves: Moves = world.read_model(player);
 
            // Deduct one from the player's remaining moves.
            moves.remaining -= 1;
 
            // Update the last direction the player moved in.
            moves.last_direction = direction;
 
            // Calculate the player's next position based on
            // the provided direction.
            let next = next_position(position, direction);
 
            // Write the new position to the world.
            world.write_model(@next);
 
            // Write the new moves to the world.
            world.write_model(@moves);
 
            // Emit an event to the world to notify about
            // the player's move.
            world.emit_event(@Moved { player, direction });
        }
    }
}

Inside the system's implementation, you can use the Dojo api to easily interact with the world.