freya_core/lifecycle/

readable.rs

//! Type-erased readable state that hides generic type parameters.

use std::{
    cell::Ref,
    rc::Rc,
};

use crate::prelude::*;

/// A type-erased readable state that only exposes the value type `T`.
///
/// This abstraction allows components to accept read-only state from any source without
/// knowing whether it comes from local state ([`use_state`]) or
/// global state (Freya Radio). Unlike [`Writable`], this only
/// provides read access to the underlying value.
///
/// # Sources
///
/// `Readable` can be created from:
/// - [`State<T>`] via [`From`] or [`IntoReadable`]
/// - `RadioSlice` via `IntoReadable`
/// - [`Writable<T>`] via [`From<Writable<T>>`]
///
/// # Example
///
/// ```rust, ignore
/// #[derive(PartialEq)]
/// struct Counter {
///     count: Readable<i32>,
/// }
///
/// impl Component for Counter {
///     fn render(&self) -> impl IntoElement {
///         // The component only reads the value, never modifies it
///         format!("Count: {}", self.count.read())
///     }
/// }
///
/// fn app() -> impl IntoElement {
///     let local = use_state(|| 0);
///     let radio = use_radio(AppChannel::Count);
///     let slice = radio.slice_current(|s| &s.count);
///
///     rect()
///         // Pass local state as Readable
///         .child(Counter { count: local.into_readable() })
///         // Pass global radio slice as Readable
///         .child(Counter { count: slice.into_readable() })
/// }
/// ```
pub struct Readable<T: 'static> {
    pub(crate) read_fn: Rc<dyn Fn() -> ReadableRef<T>>,
    pub(crate) peek_fn: Rc<dyn Fn() -> ReadableRef<T>>,
    pub(crate) equal_fn: Rc<dyn Fn(&T) -> bool>,
}

impl<T: 'static> Clone for Readable<T> {
    fn clone(&self) -> Self {
        Self {
            read_fn: self.read_fn.clone(),
            peek_fn: self.peek_fn.clone(),
            equal_fn: self.equal_fn.clone(),
        }
    }
}

impl<T: 'static> PartialEq for Readable<T> {
    fn eq(&self, other: &Self) -> bool {
        (self.equal_fn)(&*other.peek())
    }
}

impl<T: PartialEq> From<T> for Readable<T> {
    fn from(value: T) -> Self {
        Readable::from_value(value)
    }
}

impl<T: 'static + PartialEq> Readable<T> {
    /// Create from an owned value.
    pub fn from_value(value: T) -> Self {
        let value = Rc::new(value);
        Self {
            read_fn: Rc::new({
                let value = value.clone();
                move || ReadableRef::Borrowed(value.clone())
            }),
            peek_fn: Rc::new({
                let value = value.clone();
                move || ReadableRef::Borrowed(value.clone())
            }),
            equal_fn: Rc::new(move |other| other == &*value),
        }
    }
}
impl<T: 'static> Readable<T> {
    /// Create from local `State<T>`.
    pub fn from_state(state: State<T>) -> Self {
        Self {
            read_fn: Rc::new(move || ReadableRef::Ref(state.read())),
            peek_fn: Rc::new(move || ReadableRef::Ref(state.peek())),
            equal_fn: Rc::new(move |_| true),
        }
    }

    /// Create a new `Readable` with custom read and peek functions.
    pub fn new(
        read_fn: impl Fn() -> ReadableRef<T> + 'static,
        peek_fn: impl Fn() -> ReadableRef<T> + 'static,
        equal_fn: impl Fn(&T) -> bool + 'static,
    ) -> Self {
        Self {
            read_fn: Rc::new(read_fn),
            peek_fn: Rc::new(peek_fn),
            equal_fn: Rc::new(equal_fn),
        }
    }

    /// Read the value, subscribing to changes if the underlying source supports it.
    ///
    /// Whether this actually subscribes depends on how the `Readable` was created:
    /// - From [`State<T>`]: subscribes to state changes.
    /// - From a `RadioSlice`: subscribes to radio channel changes.
    /// - From a plain value ([`from_value`](Self::from_value)): no subscription, just returns the value.
    pub fn read(&self) -> ReadableRef<T> {
        (self.read_fn)()
    }

    /// Read the value without subscribing to changes.
    pub fn peek(&self) -> ReadableRef<T> {
        (self.peek_fn)()
    }

    /// Derive a new `Readable` that exposes only a part of the value.
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// let user = use_state(|| (String::from("Alice"), 30));
    /// let user: Readable<(String, u32)> = user.into_readable();
    ///
    /// let name = user.map(|user| &user.0, |name| name == "Alice");
    /// ```
    pub fn map<O>(
        &self,
        map_fn: impl Fn(&T) -> &O + 'static,
        equal_fn: impl Fn(&O) -> bool + 'static,
    ) -> Readable<O> {
        let readable = self.clone();
        let map_fn = Rc::new(map_fn);
        Readable {
            read_fn: Rc::new({
                let map_fn = map_fn.clone();
                let readable = readable.clone();
                move || {
                    let ReadableRef::Ref(r) = readable.read() else {
                        unreachable!("Unsupported")
                    };

                    ReadableRef::Ref(r.map(|r| Ref::map(r, |v| (map_fn)(v))))
                }
            }),
            peek_fn: Rc::new(move || {
                let ReadableRef::Ref(r) = readable.peek() else {
                    unreachable!("Unsupported")
                };

                ReadableRef::Ref(r.map(|r| Ref::map(r, |v| (map_fn)(v))))
            }),
            equal_fn: Rc::new(equal_fn),
        }
    }
}

pub trait IntoReadable<T: 'static> {
    fn into_readable(self) -> Readable<T>;
}

impl<T: 'static> IntoReadable<T> for State<T> {
    fn into_readable(self) -> Readable<T> {
        Readable::from_state(self)
    }
}

impl<T: 'static + PartialEq> IntoReadable<T> for T {
    fn into_readable(self) -> Readable<T> {
        Readable::from_value(self)
    }
}

impl<T: 'static> IntoReadable<T> for Memo<T> {
    fn into_readable(self) -> Readable<T> {
        Readable::from_state(self.state)
    }
}

impl<T> From<State<T>> for Readable<T> {
    fn from(value: State<T>) -> Self {
        value.into_readable()
    }
}

impl<T> From<Memo<T>> for Readable<T> {
    fn from(value: Memo<T>) -> Self {
        value.into_readable()
    }
}