dioxus/packages/core/src/scopes.rs

use futures_channel::mpsc::UnboundedSender;
use fxhash::{FxHashMap, FxHashSet};
use slab::Slab;
use std::{
    any::{Any, TypeId},
    borrow::Borrow,
    cell::{Cell, RefCell},
    collections::HashMap,
    future::Future,
    pin::Pin,
    rc::Rc,
};

use crate::{innerlude::*, unsafe_utils::extend_vnode};
use bumpalo::{boxed::Box as BumpBox, Bump};

pub(crate) type FcSlot = *const ();

pub(crate) struct Heuristic {
    hook_arena_size: usize,
    node_arena_size: usize,
}

// a slab-like arena with stable references even when new scopes are allocated
// uses a bump arena as a backing
//
// has an internal heuristics engine to pre-allocate arenas to the right size
pub(crate) struct ScopeArena {
    pub pending_futures: RefCell<FxHashSet<ScopeId>>,
    pub scope_counter: Cell<usize>,
    pub sender: UnboundedSender<SchedulerMsg>,
    pub bump: Bump,
    pub scopes: RefCell<FxHashMap<ScopeId, *mut ScopeState>>,
    pub heuristics: RefCell<FxHashMap<FcSlot, Heuristic>>,
    pub free_scopes: RefCell<Vec<*mut ScopeState>>,
    pub nodes: RefCell<Slab<*const VNode<'static>>>,
}

impl ScopeArena {
    pub(crate) fn new(sender: UnboundedSender<SchedulerMsg>) -> Self {
        let bump = Bump::new();

        // allocate a container for the root element
        // this will *never* show up in the diffing process
        // todo: figure out why this is necessary. i forgot. whoops.
        let el = bump.alloc(VElement {
            tag: "root",
            namespace: None,
            key: None,
            id: Cell::new(Some(ElementId(0))),
            parent: Default::default(),
            listeners: &[],
            attributes: &[],
            children: &[],
        });

        let node = bump.alloc(VNode::Element(el));
        let mut nodes = Slab::new();
        let root_id = nodes.insert(unsafe { std::mem::transmute(node as *const _) });

        debug_assert_eq!(root_id, 0);

        Self {
            scope_counter: Cell::new(0),
            bump,
            pending_futures: RefCell::new(FxHashSet::default()),
            scopes: RefCell::new(FxHashMap::default()),
            heuristics: RefCell::new(FxHashMap::default()),
            free_scopes: RefCell::new(Vec::new()),
            nodes: RefCell::new(nodes),
            sender,
        }
    }

    /// Safety:
    /// - Obtaining a mutable refernece to any Scope is unsafe
    /// - Scopes use interior mutability when sharing data into components
    pub(crate) fn get_scope(&self, id: ScopeId) -> Option<&ScopeState> {
        unsafe { self.scopes.borrow().get(&id).map(|f| &**f) }
    }

    pub(crate) fn get_scope_raw(&self, id: ScopeId) -> Option<*mut ScopeState> {
        self.scopes.borrow().get(&id).copied()
    }

    pub(crate) fn new_with_key(
        &self,
        fc_ptr: *const (),
        vcomp: Box<dyn AnyProps>,
        parent_scope: Option<ScopeId>,
        container: ElementId,
        subtree: u32,
    ) -> ScopeId {
        // Increment the ScopeId system. ScopeIDs are never reused
        let new_scope_id = ScopeId(self.scope_counter.get());
        self.scope_counter.set(self.scope_counter.get() + 1);

        // Get the height of the scope
        let height = parent_scope
            .map(|id| self.get_scope(id).map(|scope| scope.height))
            .flatten()
            .unwrap_or_default();

        let parent_scope = parent_scope.map(|f| self.get_scope_raw(f)).flatten();

        /*
        This scopearena aggressively reuse old scopes when possible.
        We try to minimize the new allocations for props/arenas.

        However, this will probably lead to some sort of fragmentation.
        I'm not exactly sure how to improve this today.
        */
        if let Some(old_scope) = self.free_scopes.borrow_mut().pop() {
            // reuse the old scope
            let scope = unsafe { &mut *old_scope };
            scope.props.get_mut().replace(vcomp);
            scope.parent_scope = parent_scope;
            scope.height = height;
            scope.subtree.set(subtree);
            scope.our_arena_idx = new_scope_id;
            scope.container = container;
            let any_item = self.scopes.borrow_mut().insert(new_scope_id, scope);
            debug_assert!(any_item.is_none());
        } else {
            // else create a new scope
            let (node_capacity, hook_capacity) = self
                .heuristics
                .borrow()
                .get(&fc_ptr)
                .map(|h| (h.node_arena_size, h.hook_arena_size))
                .unwrap_or_default();

            self.scopes.borrow_mut().insert(
                new_scope_id,
                self.bump.alloc(ScopeState::new(
                    height,
                    container,
                    new_scope_id,
                    self.sender.clone(),
                    parent_scope,
                    vcomp,
                    node_capacity,
                    hook_capacity,
                )),
            );
        }

        new_scope_id
    }

    // Removes a scope and its descendents from the arena
    pub fn try_remove(&self, id: ScopeId) -> Option<()> {
        self.ensure_drop_safety(id);

        // Safety:
        // - ensure_drop_safety ensures that no references to this scope are in use
        // - this raw pointer is removed from the map
        let scope = unsafe { &mut *self.scopes.borrow_mut().remove(&id).unwrap() };
        scope.reset();

        self.free_scopes.borrow_mut().push(scope);

        Some(())
    }

    pub fn reserve_node<'a>(&self, node: &'a VNode<'a>) -> ElementId {
        let mut els = self.nodes.borrow_mut();
        let entry = els.vacant_entry();
        let key = entry.key();
        let id = ElementId(key);
        let node = unsafe { extend_vnode(node) };
        entry.insert(node as *const _);
        id
    }

    pub fn update_node<'a>(&self, node: &'a VNode<'a>, id: ElementId) {
        let node = unsafe { extend_vnode(node) };
        *self.nodes.borrow_mut().get_mut(id.0).unwrap() = node;
    }

    pub fn collect_garbage(&self, id: ElementId) {
        self.nodes.borrow_mut().remove(id.0);
    }

    /// This method cleans up any references to data held within our hook list. This prevents mutable aliasing from
    /// causing UB in our tree.
    ///
    /// This works by cleaning up our references from the bottom of the tree to the top. The directed graph of components
    /// essentially forms a dependency tree that we can traverse from the bottom to the top. As we traverse, we remove
    /// any possible references to the data in the hook list.
    ///
    /// References to hook data can only be stored in listeners and component props. During diffing, we make sure to log
    /// all listeners and borrowed props so we can clear them here.
    ///
    /// This also makes sure that drop order is consistent and predictable. All resources that rely on being dropped will
    /// be dropped.
    pub(crate) fn ensure_drop_safety(&self, scope_id: ScopeId) {
        if let Some(scope) = self.get_scope(scope_id) {
            let mut items = scope.items.borrow_mut();

            // make sure we drop all borrowed props manually to guarantee that their drop implementation is called before we
            // run the hooks (which hold an &mut Reference)
            // recursively call ensure_drop_safety on all children
            items.borrowed_props.drain(..).for_each(|comp| {
                let scope_id = comp
                    .scope
                    .get()
                    .expect("VComponents should be associated with a valid Scope");

                self.ensure_drop_safety(scope_id);

                drop(comp.props.take());
            });

            // Now that all the references are gone, we can safely drop our own references in our listeners.
            items
                .listeners
                .drain(..)
                .for_each(|listener| drop(listener.callback.callback.borrow_mut().take()));
        }
    }

    pub(crate) fn run_scope(&self, id: ScopeId) {
        // Cycle to the next frame and then reset it
        // This breaks any latent references, invalidating every pointer referencing into it.
        // Remove all the outdated listeners
        self.ensure_drop_safety(id);

        // todo: we *know* that this is aliased by the contents of the scope itself
        let scope = unsafe { &mut *self.get_scope_raw(id).expect("could not find scope") };

        // Safety:
        // - We dropped the listeners, so no more &mut T can be used while these are held
        // - All children nodes that rely on &mut T are replaced with a new reference
        scope.hook_idx.set(0);

        // book keeping to ensure safety around the borrowed data
        {
            // Safety:
            // - We've dropped all references to the wip bump frame with "ensure_drop_safety"
            unsafe { scope.reset_wip_frame() };

            let mut items = scope.items.borrow_mut();

            // just forget about our suspended nodes while we're at it
            items.tasks.clear();

            // guarantee that we haven't screwed up - there should be no latent references anywhere
            debug_assert!(items.listeners.is_empty());
            debug_assert!(items.borrowed_props.is_empty());
            debug_assert!(items.tasks.is_empty());
        }

        // safety: this is definitely not dropped

        /*
        If the component returns None, then we fill in a placeholder node. This will wipe what was there.
        An alternate approach is to leave the Real Dom the same, but that can lead to safety issues and a lot more checks.

        Instead, we just treat the `None` as a shortcut to placeholder.
        If the developer wants to prevent a scope from updating, they should control its memoization instead.

        Also, the way we implement hooks allows us to cut rendering short before the next hook is recalled.
        I'm not sure if React lets you abort the component early, but we let you do that.
        */

        let props = scope.props.borrow();
        let render = props.as_ref().unwrap();
        if let Some(node) = render.render(scope) {
            if !scope.items.borrow().tasks.is_empty() {
                self.pending_futures.borrow_mut().insert(id);
            }

            let frame = scope.wip_frame();
            let node = frame.bump.alloc(node);
            frame.node.set(unsafe { extend_vnode(node) });
        } else {
            let frame = scope.wip_frame();
            let node = frame
                .bump
                .alloc(VNode::Placeholder(frame.bump.alloc(VPlaceholder {
                    id: Default::default(),
                })));
            frame.node.set(unsafe { extend_vnode(node) });
        }

        // make the "wip frame" contents the "finished frame"
        // any future dipping into completed nodes after "render" will go through "fin head"
        scope.cycle_frame();
    }

    pub fn call_listener_with_bubbling(&self, event: UserEvent, element: ElementId) {
        let nodes = self.nodes.borrow();
        let mut cur_el = Some(element);

        while let Some(id) = cur_el.take() {
            if let Some(el) = nodes.get(id.0) {
                let real_el = unsafe { &**el };
                if let VNode::Element(real_el) = real_el {
                    for listener in real_el.listeners.borrow().iter() {
                        if listener.event == event.name {
                            let mut cb = listener.callback.callback.borrow_mut();
                            if let Some(cb) = cb.as_mut() {
                                // todo: arcs are pretty heavy to clone
                                // we really want to convert arc to rc
                                // unfortunately, the SchedulerMsg must be send/sync to be sent across threads
                                // we could convert arc to rc internally or something
                                (cb)(event.data.clone());
                            }
                        }
                    }

                    cur_el = real_el.parent.get();
                }
            }
        }
    }

    // The head of the bumpframe is the first linked NodeLink
    pub fn wip_head(&self, id: ScopeId) -> &VNode {
        let scope = self.get_scope(id).unwrap();
        let frame = scope.wip_frame();
        let node = unsafe { &*frame.node.get() };
        unsafe { extend_vnode(node) }
    }

    // The head of the bumpframe is the first linked NodeLink
    pub fn fin_head(&self, id: ScopeId) -> &VNode {
        let scope = self.get_scope(id).unwrap();
        let frame = scope.fin_frame();
        let node = unsafe { &*frame.node.get() };
        unsafe { extend_vnode(node) }
    }

    pub fn root_node(&self, id: ScopeId) -> &VNode {
        self.fin_head(id)
    }
}

/// Components in Dioxus use the "Context" object to interact with their lifecycle.
///
/// This lets components access props, schedule updates, integrate hooks, and expose shared state.
///
/// For the most part, the only method you should be using regularly is `render`.
///
/// ## Example
///
/// ```ignore
/// #[derive(Props)]
/// struct ExampleProps {
///     name: String
/// }
///
/// fn Example(cx: Scope<ExampleProps>) -> Element {
///     cx.render(rsx!{ div {"Hello, {cx.props.name}"} })
/// }
/// ```
pub struct Scope<'a, P> {
    pub scope: &'a ScopeState,
    pub props: &'a P,
}

impl<P> Copy for Scope<'_, P> {}
impl<P> Clone for Scope<'_, P> {
    fn clone(&self) -> Self {
        Self {
            scope: self.scope,
            props: self.props,
        }
    }
}

impl<'a, P> std::ops::Deref for Scope<'a, P> {
    // rust will auto deref again to the original 'a lifetime at the call site
    type Target = &'a ScopeState;
    fn deref(&self) -> &Self::Target {
        &self.scope
    }
}

/// A component's unique identifier.
///
/// `ScopeId` is a `usize` that is unique across the entire VirtualDOM and across time. ScopeIDs will never be reused
/// once a component has been unmounted.
#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
pub struct ScopeId(pub usize);

/// Every component in Dioxus is represented by a `ScopeState`.
///
/// Scopes contain the state for hooks, the component's props, and other lifecycle information.
///
/// Scopes are allocated in a generational arena. As components are mounted/unmounted, they will replace slots of dead components.
/// The actual contents of the hooks, though, will be allocated with the standard allocator. These should not allocate as frequently.
///
/// We expose the `Scope` type so downstream users can traverse the Dioxus VirtualDOM for whatever
/// use case they might have.
pub struct ScopeState {
    pub(crate) parent_scope: Option<*mut ScopeState>,
    pub(crate) container: ElementId,
    pub(crate) our_arena_idx: ScopeId,
    pub(crate) height: u32,
    pub(crate) sender: UnboundedSender<SchedulerMsg>,

    // todo: subtrees
    pub(crate) is_subtree_root: Cell<bool>,
    pub(crate) subtree: Cell<u32>,

    pub(crate) props: RefCell<Option<Box<dyn AnyProps>>>,

    // nodes, items
    pub(crate) frames: [BumpFrame; 2],
    pub(crate) generation: Cell<u32>,
    pub(crate) items: RefCell<SelfReferentialItems<'static>>,

    // hooks
    pub(crate) hook_arena: Bump,
    pub(crate) hook_vals: RefCell<Vec<*mut dyn Any>>,
    pub(crate) hook_idx: Cell<usize>,

    // shared state -> todo: move this out of scopestate
    pub(crate) shared_contexts: RefCell<HashMap<TypeId, Rc<dyn Any>>>,
}

pub struct SelfReferentialItems<'a> {
    pub(crate) listeners: Vec<&'a Listener<'a>>,
    pub(crate) borrowed_props: Vec<&'a VComponent<'a>>,
    pub(crate) tasks: Vec<Pin<BumpBox<'a, dyn Future<Output = ()>>>>,
}

// Public methods exposed to libraries and components
impl ScopeState {
    fn new(
        height: u32,
        container: ElementId,
        our_arena_idx: ScopeId,
        sender: UnboundedSender<SchedulerMsg>,
        parent_scope: Option<*mut ScopeState>,
        vcomp: Box<dyn AnyProps>,
        node_capacity: usize,
        hook_capacity: usize,
    ) -> Self {
        let frames = [BumpFrame::new(node_capacity), BumpFrame::new(node_capacity)];

        ScopeState {
            sender,
            container,
            our_arena_idx,
            parent_scope,
            props: RefCell::new(Some(vcomp)),
            height,
            frames,

            // todo: subtrees
            subtree: Cell::new(0),
            is_subtree_root: Cell::new(false),

            generation: 0.into(),

            shared_contexts: Default::default(),

            items: RefCell::new(SelfReferentialItems {
                listeners: Default::default(),
                borrowed_props: Default::default(),
                tasks: Default::default(),
            }),

            hook_arena: Bump::new(),
            hook_vals: RefCell::new(Vec::with_capacity(hook_capacity)),
            hook_idx: Default::default(),
        }
    }

    /// Get the subtree ID that this scope belongs to.
    ///
    /// Each component has its own subtree ID - the root subtree has an ID of 0. This ID is used by the renderer to route
    /// the mutations to the correct window/portal/subtree.
    ///
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// let mut dom = VirtualDom::new(|cx, props|cx.render(rsx!{ div {} }));
    /// dom.rebuild();
    ///
    /// let base = dom.base_scope();
    ///
    /// assert_eq!(base.subtree(), 0);
    /// ```
    ///
    /// todo: enable
    pub(crate) fn _subtree(&self) -> u32 {
        self.subtree.get()
    }

    /// Create a new subtree with this scope as the root of the subtree.
    ///
    /// Each component has its own subtree ID - the root subtree has an ID of 0. This ID is used by the renderer to route
    /// the mutations to the correct window/portal/subtree.
    ///
    /// This method
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// fn App(cx: Scope<()>) -> Element {
    ///     rsx!(cx, div { "Subtree {id}"})
    /// };
    /// ```
    ///
    /// todo: enable subtree
    pub(crate) fn _create_subtree(&self) -> Option<u32> {
        if self.is_subtree_root.get() {
            None
        } else {
            todo!()
        }
    }

    /// Get the height of this Scope - IE the number of scopes above it.
    ///
    /// A Scope with a height of `0` is the root scope - there are no other scopes above it.
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// let mut dom = VirtualDom::new(|cx, props| cx.render(rsx!{ div {} }));
    /// dom.rebuild();
    ///
    /// let base = dom.base_scope();
    ///
    /// assert_eq!(base.height(), 0);
    /// ```
    pub fn height(&self) -> u32 {
        self.height
    }

    /// Get the Parent of this Scope within this Dioxus VirtualDOM.
    ///
    /// This ID is not unique across Dioxus VirtualDOMs or across time. IDs will be reused when components are unmounted.
    ///
    /// The base component will not have a parent, and will return `None`.
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// let mut dom = VirtualDom::new(|cx, props| cx.render(rsx!{ div {} }));
    /// dom.rebuild();
    ///
    /// let base = dom.base_scope();
    ///
    /// assert_eq!(base.parent(), None);
    /// ```
    pub fn parent(&self) -> Option<ScopeId> {
        // safety: the pointer to our parent is *always* valid thanks to the bump arena
        self.parent_scope.map(|p| unsafe { &*p }.our_arena_idx)
    }

    /// Get the ID of this Scope within this Dioxus VirtualDOM.
    ///
    /// This ID is not unique across Dioxus VirtualDOMs or across time. IDs will be reused when components are unmounted.
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// let mut dom = VirtualDom::new(|cx, props| cx.render(rsx!{ div {} }));
    /// dom.rebuild();
    /// let base = dom.base_scope();
    ///
    /// assert_eq!(base.scope_id(), 0);
    /// ```
    pub fn scope_id(&self) -> ScopeId {
        self.our_arena_idx
    }

    /// Create a subscription that schedules a future render for the reference component
    ///
    /// ## Notice: you should prefer using prepare_update and get_scope_id
    pub fn schedule_update(&self) -> Rc<dyn Fn() + 'static> {
        let (chan, id) = (self.sender.clone(), self.scope_id());
        Rc::new(move || {
            let _ = chan.unbounded_send(SchedulerMsg::Immediate(id));
        })
    }

    /// Schedule an update for any component given its ScopeId.
    ///
    /// A component's ScopeId can be obtained from `use_hook` or the [`Context::scope_id`] method.
    ///
    /// This method should be used when you want to schedule an update for a component
    pub fn schedule_update_any(&self) -> Rc<dyn Fn(ScopeId)> {
        let chan = self.sender.clone();
        Rc::new(move |id| {
            let _ = chan.unbounded_send(SchedulerMsg::Immediate(id));
        })
    }

    /// Get the [`ScopeId`] of a mounted component.
    ///
    /// `ScopeId` is not unique for the lifetime of the VirtualDom - a ScopeId will be reused if a component is unmounted.
    pub fn needs_update(&self) {
        self.needs_update_any(self.scope_id())
    }

    /// Get the [`ScopeId`] of a mounted component.
    ///
    /// `ScopeId` is not unique for the lifetime of the VirtualDom - a ScopeId will be reused if a component is unmounted.
    pub fn needs_update_any(&self, id: ScopeId) {
        let _ = self.sender.unbounded_send(SchedulerMsg::Immediate(id));
    }

    /// Get the Root Node of this scope
    pub fn root_node(&self) -> &VNode {
        let node = unsafe { &*self.fin_frame().node.get() };
        unsafe { std::mem::transmute(node) }
    }

    /// This method enables the ability to expose state to children further down the VirtualDOM Tree.
    ///
    /// This is a "fundamental" operation and should only be called during initialization of a hook.
    ///
    /// For a hook that provides the same functionality, use `use_provide_state` and `use_consume_state` instead.
    ///
    /// When the component is dropped, so is the context. Be aware of this behavior when consuming
    /// the context via Rc/Weak.
    ///
    /// # Example
    ///
    /// ```rust, ignore
    /// struct SharedState(&'static str);
    ///
    /// static App: Component<()> = |cx, props|{
    ///     cx.use_hook(|_| cx.provide_state(SharedState("world")), |_| {}, |_| {});
    ///     rsx!(cx, Child {})
    /// }
    ///
    /// static Child: Component<()> = |cx| {
    ///     let state = cx.consume_state::<SharedState>();
    ///     rsx!(cx, div { "hello {state.0}" })
    /// }
    /// ```
    pub fn provide_state<T: 'static>(&self, value: T) {
        self.shared_contexts
            .borrow_mut()
            .insert(TypeId::of::<T>(), Rc::new(value))
            .map(|f| f.downcast::<T>().ok())
            .flatten();
    }

    /// Try to retrieve a SharedState with type T from the any parent Scope.
    pub fn consume_state<T: 'static>(&self) -> Option<Rc<T>> {
        if let Some(shared) = self.shared_contexts.borrow().get(&TypeId::of::<T>()) {
            Some(shared.clone().downcast::<T>().unwrap())
        } else {
            let mut search_parent = self.parent_scope;

            while let Some(parent_ptr) = search_parent {
                // safety: all parent pointers are valid thanks to the bump arena
                let parent = unsafe { &*parent_ptr };
                if let Some(shared) = parent.shared_contexts.borrow().get(&TypeId::of::<T>()) {
                    return Some(shared.clone().downcast::<T>().unwrap());
                }
                search_parent = parent.parent_scope;
            }
            None
        }
    }

    /// Pushes the future onto the poll queue to be polled after the component renders.
    ///
    /// The future is forcibly dropped if the component is not ready by the next render
    pub fn push_task<'src, F>(&'src self, fut: impl FnOnce() -> F + 'src) -> usize
    where
        F: Future<Output = ()>,
        F::Output: 'src,
        F: 'src,
    {
        self.sender
            .unbounded_send(SchedulerMsg::NewTask(self.our_arena_idx))
            .unwrap();

        // wrap it in a type that will actually drop the contents
        //
        // Safety: we just made the pointer above and will promise not to alias it!
        // The main reason we do this through from_raw is because Bumpalo's box does
        // not support unsized coercion
        let fut: &mut dyn Future<Output = ()> = self.bump().alloc(fut());
        let boxed_fut: BumpBox<dyn Future<Output = ()>> = unsafe { BumpBox::from_raw(fut) };
        let pinned_fut: Pin<BumpBox<_>> = boxed_fut.into();

        // erase the 'src lifetime for self-referential storage
        // todo: provide a miri test around this
        // concerned about segfaulting
        let self_ref_fut = unsafe { std::mem::transmute(pinned_fut) };

        // Push the future into the tasks
        let mut items = self.items.borrow_mut();
        items.tasks.push(self_ref_fut);
        items.tasks.len() - 1
    }

    /// Take a lazy VNode structure and actually build it with the context of the VDom's efficient VNode allocator.
    ///
    /// This function consumes the context and absorb the lifetime, so these VNodes *must* be returned.
    ///
    /// ## Example
    ///
    /// ```ignore
    /// fn Component(cx: Scope<Props>) -> Element {
    ///     // Lazy assemble the VNode tree
    ///     let lazy_nodes = rsx!("hello world");
    ///
    ///     // Actually build the tree and allocate it
    ///     cx.render(lazy_tree)
    /// }
    ///```
    pub fn render<'src>(&'src self, rsx: Option<LazyNodes<'src, '_>>) -> Option<VNode<'src>> {
        let fac = NodeFactory {
            bump: &self.wip_frame().bump,
        };
        match rsx {
            Some(s) => Some(s.call(fac)),
            None => todo!("oh no no nodes"),
        }
    }

    /// Store a value between renders
    ///
    /// This is *the* foundational hook for all other hooks.
    ///
    /// - Initializer: closure used to create the initial hook state
    /// - Runner: closure used to output a value every time the hook is used
    ///
    /// To "cleanup" the hook, implement `Drop` on the stored hook value. Whenever the component is dropped, the hook
    /// will be dropped as well.
    ///
    /// # Example
    ///
    /// ```ignore
    /// // use_ref is the simplest way of storing a value between renders
    /// fn use_ref<T: 'static>(initial_value: impl FnOnce() -> T) -> &RefCell<T> {
    ///     use_hook(
    ///         || Rc::new(RefCell::new(initial_value())),
    ///         |state| state,
    ///     )
    /// }
    /// ```
    pub fn use_hook<'src, State: 'static, Output: 'src>(
        &'src self,
        initializer: impl FnOnce(usize) -> State,
        runner: impl FnOnce(&'src mut State) -> Output,
    ) -> Output {
        let mut vals = self.hook_vals.borrow_mut();

        let hook_len = vals.len();
        let cur_idx = self.hook_idx.get();

        if cur_idx >= hook_len {
            vals.push(self.hook_arena.alloc(initializer(hook_len)));
        }

        let state = vals
            .get(cur_idx)
            .and_then(|inn| {
                self.hook_idx.set(cur_idx + 1);
                let raw_box = unsafe { &mut **inn };
                raw_box.downcast_mut::<State>()
            })
            .expect(
                r###"
                Unable to retrieve the hook that was initialized at this index.
                Consult the `rules of hooks` to understand how to use hooks properly.

                You likely used the hook in a conditional. Hooks rely on consistent ordering between renders.
                Functions prefixed with "use" should never be called conditionally.
                "###,
            );

        runner(state)
    }

    /// The "work in progress frame" represents the frame that is currently being worked on.
    pub(crate) fn wip_frame(&self) -> &BumpFrame {
        match self.generation.get() & 1 == 0 {
            true => &self.frames[0],
            false => &self.frames[1],
        }
    }

    /// Mutable access to the "work in progress frame" - used to clear it
    pub(crate) fn wip_frame_mut(&mut self) -> &mut BumpFrame {
        match self.generation.get() & 1 == 0 {
            true => &mut self.frames[0],
            false => &mut self.frames[1],
        }
    }

    /// Access to the frame where finalized nodes existed
    pub(crate) fn fin_frame(&self) -> &BumpFrame {
        match self.generation.get() & 1 == 1 {
            true => &self.frames[0],
            false => &self.frames[1],
        }
    }

    /// Reset this component's frame
    ///
    /// # Safety:
    ///
    /// This method breaks every reference of VNodes in the current frame.
    ///
    /// Calling reset itself is not usually a big deal, but we consider it important
    /// due to the complex safety guarantees we need to uphold.
    pub(crate) unsafe fn reset_wip_frame(&mut self) {
        self.wip_frame_mut().bump.reset();
    }

    /// Cycle to the next generation
    pub(crate) fn cycle_frame(&self) {
        self.generation.set(self.generation.get() + 1);
    }

    /// Get the [`Bump`] of the WIP frame.
    pub(crate) fn bump(&self) -> &Bump {
        &self.wip_frame().bump
    }

    // todo: disable bookkeeping on drop (unncessary)
    pub(crate) fn reset(&mut self) {
        // first: book keaping
        self.hook_idx.set(0);
        self.parent_scope = None;
        self.generation.set(0);
        self.is_subtree_root.set(false);
        self.subtree.set(0);

        // next: shared context data
        self.shared_contexts.get_mut().clear();

        // next: reset the node data
        let SelfReferentialItems {
            borrowed_props,
            listeners,
            tasks,
        } = self.items.get_mut();
        borrowed_props.clear();
        listeners.clear();
        tasks.clear();
        self.frames[0].reset();
        self.frames[1].reset();

        // Finally, free up the hook values
        self.hook_arena.reset();
        self.hook_vals.get_mut().drain(..).for_each(|state| {
            let as_mut = unsafe { &mut *state };
            let boxed = unsafe { bumpalo::boxed::Box::from_raw(as_mut) };
            drop(boxed);
        });
    }
}

pub(crate) struct BumpFrame {
    pub bump: Bump,
    pub node: Cell<*const VNode<'static>>,
}
impl BumpFrame {
    pub(crate) fn new(capacity: usize) -> Self {
        let bump = Bump::with_capacity(capacity);

        let node = &*bump.alloc(VText {
            text: "placeholdertext",
            id: Default::default(),
            is_static: false,
        });
        let node = bump.alloc(VNode::Text(unsafe { std::mem::transmute(node) }));
        let nodes = Cell::new(node as *const _);
        Self { bump, node: nodes }
    }

    pub(crate) fn reset(&mut self) {
        self.bump.reset();
        let node = self.bump.alloc(VText {
            text: "placeholdertext",
            id: Default::default(),
            is_static: false,
        });
        let node = self
            .bump
            .alloc(VNode::Text(unsafe { std::mem::transmute(node) }));
        self.node.set(node as *const _);
    }
}

#[test]
fn sizeof() {
    dbg!(std::mem::size_of::<ScopeState>());
}