Trait wnf::AsState

source ·

pub trait AsState: Sealed {
    type Data: ?Sized;

    fn as_state(&self) -> BorrowedState<'_, Self::Data>;
}

Expand description

A trait for types that can be borrowed as a state

This is implemented for both OwnedState<T> and BorrowedState<'_, T>. There are two main use cases for it:

Functions that can accept either a reference to an owned or a borrowed state: Even though a BorrowedState<'_, T> plays the role of a reference to a state, it’s not technically a reference. As a consequence, there is no deref coercion for states, i.e. you cannot just pass an &'a OwnedState<T> where a BorrowedState<'a, T> is expected. In order to accept both types, functions can instead take a reference to a generic type implementing AsState:

fn add_to_state(state: &impl AsState<Data = u32>, delta: u32) -> io::Result<()> {
    let state = state.as_state();
    let value = state.get()?;
    let new_value = value + delta;
    state.set(&new_value)
}

Types that can contain either an owned or a borrowed state:

struct StateWrapper<S> {
    state: S,
}

impl<S> StateWrapper<S>
where
    S: AsState<Data = u32>,
{
    fn add(&self, delta: u32) -> io::Result<()> {
        let state = self.state.as_state();
        let value = state.get()?;
        let new_value = value + delta;
        state.set(&new_value)
    }
}