yew/functional/hooks/

use_reducer.rs

use std::cell::RefCell;
use std::fmt;
use std::marker::PhantomData;
use std::ops::Deref;
use std::rc::Rc;

use implicit_clone::ImplicitClone;

use crate::functional::{hook, Hook, HookContext};
use crate::html::IntoPropValue;
use crate::Callback;

type DispatchFn<T> = Rc<dyn Fn(<T as Reducible>::Action)>;

/// A trait that implements a reducer function of a type.
pub trait Reducible {
    /// The action type of the reducer.
    type Action;

    /// The reducer function.
    fn reduce(self: Rc<Self>, action: Self::Action) -> Rc<Self>;
}

struct UseReducer<T>
where
    T: Reducible,
{
    current_state: Rc<RefCell<Rc<T>>>,

    dispatch: DispatchFn<T>,
}

/// State handle for [`use_reducer`] and [`use_reducer_eq`] hook
pub struct UseReducerHandle<T>
where
    T: Reducible,
{
    /// Shared source of truth, updated synchronously by dispatch.
    current_state: Rc<RefCell<Rc<T>>>,
    /// Accumulates `Rc<T>` clones returned by [`Deref::deref`] so that references
    /// remain valid for the lifetime of this handle. Reset on each re-render when
    /// a new handle is created.
    deref_history: RefCell<Vec<Rc<T>>>,
    dispatch: DispatchFn<T>,
}

impl<T> UseReducerHandle<T>
where
    T: Reducible,
{
    /// Dispatch the given action to the reducer.
    pub fn dispatch(&self, value: T::Action) {
        (self.dispatch)(value)
    }

    /// Returns the dispatcher of the current state.
    pub fn dispatcher(&self) -> UseReducerDispatcher<T> {
        UseReducerDispatcher {
            dispatch: self.dispatch.clone(),
        }
    }
}

impl<T> Deref for UseReducerHandle<T>
where
    T: Reducible,
{
    type Target = T;

    fn deref(&self) -> &Self::Target {
        let rc = match self.current_state.try_borrow() {
            Ok(shared) => Rc::clone(&*shared),
            Err(_) => {
                // RefCell is mutably borrowed (during dispatch). Use the last
                // value we successfully read.
                let history = self.deref_history.borrow();
                Rc::clone(history.last().expect("deref_history is never empty"))
            }
        };

        let ptr: *const T = Rc::as_ptr(&rc);

        // Only store a new entry when the Rc allocation differs from the most
        // recent one, avoiding unbounded growth from repeated reads of the same
        // state.
        {
            let mut history = self.deref_history.borrow_mut();
            if !Rc::ptr_eq(history.last().expect("deref_history is never empty"), &rc) {
                history.push(rc);
            }
        }

        // SAFETY: `ptr` points into the heap allocation of an `Rc<T>`. That Rc
        // is kept alive in `self.deref_history` (either the entry we just pushed,
        // or a previous entry with the same allocation). `deref_history` lives as
        // long as `self`, and `Rc` guarantees its heap allocation stays live while
        // any clone exists. Therefore `ptr` is valid for the lifetime of `&self`.
        unsafe { &*ptr }
    }
}

impl<T> Clone for UseReducerHandle<T>
where
    T: Reducible,
{
    fn clone(&self) -> Self {
        // Take a fresh snapshot so the clone's deref_history is never empty.
        let snapshot = match self.current_state.try_borrow() {
            Ok(shared) => Rc::clone(&*shared),
            Err(_) => {
                let history = self.deref_history.borrow();
                Rc::clone(history.last().expect("deref_history is never empty"))
            }
        };
        Self {
            current_state: Rc::clone(&self.current_state),
            deref_history: RefCell::new(vec![snapshot]),
            dispatch: Rc::clone(&self.dispatch),
        }
    }
}

impl<T> fmt::Debug for UseReducerHandle<T>
where
    T: Reducible + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let value = if let Ok(rc_ref) = self.current_state.try_borrow() {
            format!("{:?}", *rc_ref)
        } else {
            let history = self.deref_history.borrow();
            format!(
                "{:?}",
                **history.last().expect("deref_history is never empty")
            )
        };
        f.debug_struct("UseReducerHandle")
            .field("value", &value)
            .finish()
    }
}

impl<T> PartialEq for UseReducerHandle<T>
where
    T: Reducible + PartialEq,
{
    fn eq(&self, rhs: &Self) -> bool {
        let self_snapshot = self.deref_history.borrow();
        let rhs_snapshot = rhs.deref_history.borrow();
        *self_snapshot[0] == *rhs_snapshot[0]
    }
}

impl<T> ImplicitClone for UseReducerHandle<T> where T: Reducible {}

/// Dispatcher handle for [`use_reducer`] and [`use_reducer_eq`] hook
pub struct UseReducerDispatcher<T>
where
    T: Reducible,
{
    dispatch: DispatchFn<T>,
}

impl<T> Clone for UseReducerDispatcher<T>
where
    T: Reducible,
{
    fn clone(&self) -> Self {
        Self {
            dispatch: Rc::clone(&self.dispatch),
        }
    }
}

impl<T> fmt::Debug for UseReducerDispatcher<T>
where
    T: Reducible + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("UseReducerDispatcher").finish()
    }
}

impl<T> PartialEq for UseReducerDispatcher<T>
where
    T: Reducible,
{
    fn eq(&self, rhs: &Self) -> bool {
        // We are okay with comparisons from different compilation units to result in false
        // not-equal results. This should only lead in the worst-case to some unneeded
        // re-renders.
        #[allow(ambiguous_wide_pointer_comparisons)]
        Rc::ptr_eq(&self.dispatch, &rhs.dispatch)
    }
}

impl<T> ImplicitClone for UseReducerDispatcher<T> where T: Reducible {}

impl<T> From<UseReducerDispatcher<T>> for Callback<<T as Reducible>::Action>
where
    T: Reducible,
{
    fn from(val: UseReducerDispatcher<T>) -> Self {
        Callback { cb: val.dispatch }
    }
}

impl<T> IntoPropValue<Callback<<T as Reducible>::Action>> for UseReducerDispatcher<T>
where
    T: Reducible,
{
    fn into_prop_value(self) -> Callback<<T as Reducible>::Action> {
        Callback { cb: self.dispatch }
    }
}

impl<T> UseReducerDispatcher<T>
where
    T: Reducible,
{
    /// Dispatch the given action to the reducer.
    pub fn dispatch(&self, value: T::Action) {
        (self.dispatch)(value)
    }

    /// Get a callback, invoking which is equivalent to calling `dispatch()`
    /// on this same dispatcher.
    pub fn to_callback(&self) -> Callback<<T as Reducible>::Action> {
        Callback {
            cb: self.dispatch.clone(),
        }
    }
}

/// The base function of [`use_reducer`] and [`use_reducer_eq`]
fn use_reducer_base<'hook, T>(
    init_fn: impl 'hook + FnOnce() -> T,
    should_render_fn: fn(&T, &T) -> bool,
) -> impl 'hook + Hook<Output = UseReducerHandle<T>>
where
    T: Reducible + 'static,
{
    struct HookProvider<'hook, T, F>
    where
        T: Reducible + 'static,
        F: 'hook + FnOnce() -> T,
    {
        _marker: PhantomData<&'hook ()>,

        init_fn: F,
        should_render_fn: fn(&T, &T) -> bool,
    }

    impl<'hook, T, F> Hook for HookProvider<'hook, T, F>
    where
        T: Reducible + 'static,
        F: 'hook + FnOnce() -> T,
    {
        type Output = UseReducerHandle<T>;

        fn run(self, ctx: &mut HookContext) -> Self::Output {
            let Self {
                init_fn,
                should_render_fn,
                ..
            } = self;

            let state = ctx.next_state(move |re_render| {
                let val = Rc::new(RefCell::new(Rc::new(init_fn())));
                let should_render_fn = Rc::new(should_render_fn);

                UseReducer {
                    current_state: val.clone(),
                    dispatch: Rc::new(move |action: T::Action| {
                        let should_render = {
                            let should_render_fn = should_render_fn.clone();
                            let mut val = val.borrow_mut();
                            let next_val = (*val).clone().reduce(action);
                            let should_render = should_render_fn(&next_val, &val);
                            *val = next_val;

                            should_render
                        };

                        // Currently, this triggers a render immediately, so we need to release the
                        // borrowed reference first.
                        if should_render {
                            re_render()
                        }
                    }),
                }
            });

            let current_state = state.current_state.clone();
            let snapshot = state.current_state.borrow().clone();
            let dispatch = state.dispatch.clone();

            UseReducerHandle {
                current_state,
                deref_history: RefCell::new(vec![snapshot]),
                dispatch,
            }
        }
    }

    HookProvider {
        _marker: PhantomData,
        init_fn,
        should_render_fn,
    }
}

/// This hook is an alternative to [`use_state`](super::use_state()).
/// It is used to handle component's state and is used when complex actions needs to be performed on
/// said state.
///
/// The state is expected to implement the [`Reducible`] trait which provides an `Action` type and a
/// reducer function.
///
/// The state object returned by the initial state function is required to
/// implement a `Reducible` trait which defines the associated `Action` type and a
/// reducer function.
///
/// This hook will trigger a re-render whenever the reducer function produces a new `Rc` value upon
/// receiving an action. If the reducer function simply returns the original `Rc` then the component
/// will not re-render. See [`use_reducer_eq`] if you want the component to first compare the old
/// and new state and only re-render when the state actually changes.
///
/// To cause a re-render even if the reducer function returns the same `Rc`, take a look at
/// [`use_force_update`].
///
/// # Example
/// ```rust
/// # use yew::prelude::*;
/// # use std::rc::Rc;
/// #
///
/// /// reducer's Action
/// enum CounterAction {
///     Double,
///     Square,
/// }
///
/// /// reducer's State
/// struct CounterState {
///     counter: i32,
/// }
///
/// impl Default for CounterState {
///     fn default() -> Self {
///         Self { counter: 1 }
///     }
/// }
///
/// impl Reducible for CounterState {
///     /// Reducer Action Type
///     type Action = CounterAction;
///
///     /// Reducer Function
///     fn reduce(self: Rc<Self>, action: Self::Action) -> Rc<Self> {
///         let next_ctr = match action {
///             CounterAction::Double => self.counter * 2,
///             CounterAction::Square => self.counter.pow(2),
///         };
///
///         Self { counter: next_ctr }.into()
///     }
/// }
///
/// #[component(UseReducer)]
/// fn reducer() -> Html {
///     // The use_reducer hook takes an initialization function which will be called only once.
///     let counter = use_reducer(CounterState::default);
///
///     let double_onclick = {
///         let counter = counter.clone();
///         Callback::from(move |_| counter.dispatch(CounterAction::Double))
///     };
///     let square_onclick = {
///         let counter = counter.clone();
///         Callback::from(move |_| counter.dispatch(CounterAction::Square))
///     };
///
///     html! {
///         <>
///             <div id="result">{ counter.counter }</div>
///
///             <button onclick={double_onclick}>{ "Double" }</button>
///             <button onclick={square_onclick}>{ "Square" }</button>
///         </>
///     }
/// }
/// ```
///
/// # Tip
///
/// The dispatch function is guaranteed to be the same across the entire
/// component lifecycle. You can safely omit the `UseReducerHandle` from the
/// dependents of `use_effect_with` if you only intend to dispatch
/// values from within the hooks.
///
/// # Caution
///
/// The value held in the handle will reflect the value of at the time the
/// handle is returned by the `use_reducer`. It is possible that the handle does
/// not dereference to an up to date value if you are moving it into a
/// `use_effect_with` hook. You can register the
/// state to the dependents so the hook can be updated when the value changes.
#[hook]
pub fn use_reducer<T, F>(init_fn: F) -> UseReducerHandle<T>
where
    T: Reducible + 'static,
    F: FnOnce() -> T,
{
    use_reducer_base(init_fn, |a, b| !address_eq(a, b))
}

/// [`use_reducer`] but only re-renders when `prev_state != next_state`.
///
/// This requires the state to implement [`PartialEq`] in addition to the [`Reducible`] trait
/// required by [`use_reducer`].
#[hook]
pub fn use_reducer_eq<T, F>(init_fn: F) -> UseReducerHandle<T>
where
    T: Reducible + PartialEq + 'static,
    F: FnOnce() -> T,
{
    use_reducer_base(init_fn, |a, b| !address_eq(a, b) && a != b)
}

/// Check if two references point to the same address.
fn address_eq<T>(a: &T, b: &T) -> bool {
    std::ptr::eq(a as *const T, b as *const T)
}