dioxus/packages/core/src/nodes.rs

//! Virtual Node Support
//!
//! VNodes represent lazily-constructed VDom trees that support diffing and event handlers. These VNodes should be *very*
//! cheap and *very* fast to construct - building a full tree should be quick.

use crate::{
    innerlude::{empty_cell, Context, Element, ElementId, Properties, Scope, ScopeId, ScopeState},
    lazynodes::LazyNodes,
};
use bumpalo::{boxed::Box as BumpBox, Bump};
use std::{
    any::Any,
    cell::{Cell, RefCell},
    fmt::{Arguments, Debug, Formatter},
};

/// A cached node is a "pointer" to a "rendered" node in a particular scope
///
/// It does not provide direct access to the node, so it doesn't carry any lifetime information with it
///
/// It is used during the diffing/rendering process as a runtime key into an existing set of nodes. The "render" key
/// is essentially a unique key to guarantee safe usage of the Node.
pub struct NodeLink {
    pub(crate) gen_id: u32,
    pub(crate) scope_id: ScopeId,
}

/// A composable "VirtualNode" to declare a User Interface in the Dioxus VirtualDOM.
///
/// VNodes are designed to be lightweight and used with with a bump allocator. To create a VNode, you can use either of:
/// - the [`rsx`] macro
/// - the [`html`] macro
/// - the [`NodeFactory`] API
pub enum VNode<'src> {
    /// Text VNodes simply bump-allocated (or static) string slices
    ///
    /// # Example
    ///
    /// ```
    /// let node = cx.render(rsx!{ "hello" }).unwrap();
    ///
    /// if let VNode::Text(vtext) = node {
    ///     assert_eq!(vtext.text, "hello");
    ///     assert_eq!(vtext.dom_id.get(), None);
    ///     assert_eq!(vtext.is_static, true);
    /// }
    /// ```
    Text(&'src VText<'src>),

    /// Element VNodes are VNodes that may contain attributes, listeners, a key, a tag, and children.
    ///
    /// # Example
    ///
    /// ```rust
    /// let node = cx.render(rsx!{
    ///     div {
    ///         key: "a",
    ///         onclick: |e| log::info!("clicked"),
    ///         hidden: "true",
    ///         style: { background_color: "red" }
    ///         "hello"
    ///     }
    /// }).unwrap();
    /// if let VNode::Element(velement) = node {
    ///     assert_eq!(velement.tag_name, "div");
    ///     assert_eq!(velement.namespace, None);
    ///     assert_eq!(velement.key, Some("a));
    /// }
    /// ```
    Element(&'src VElement<'src>),

    /// Fragment nodes may contain many VNodes without a single root.
    ///
    /// # Example
    ///
    /// ```rust
    /// rsx!{
    ///     a {}
    ///     link {}
    ///     style {}
    ///     "asd"
    ///     Example {}
    /// }
    /// ```
    Fragment(VFragment<'src>),

    /// Component nodes represent a mounted component with props, children, and a key.
    ///
    /// # Example
    ///
    /// ```rust
    /// fn Example(cx: Context<()>) -> DomTree {
    ///     todo!()
    /// }
    ///
    /// let node = cx.render(rsx!{
    ///     Example {}
    /// }).unwrap();
    ///
    /// if let VNode::Component(vcomp) = node {
    ///     assert_eq!(vcomp.user_fc, Example as *const ());
    /// }
    /// ```
    Component(&'src VComponent<'src>),

    /// Suspended VNodes represent chunks of the UI tree that are not yet ready to be displayed.
    ///
    /// These nodes currently can only be constructed via the [`use_suspense`] hook.
    ///
    /// # Example
    ///
    /// ```rust
    /// rsx!{
    /// }
    /// ```
    Suspended(&'src VSuspended<'src>),

    /// Anchors are a type of placeholder VNode used when fragments don't contain any children.
    ///
    /// Anchors cannot be directly constructed via public APIs.
    ///
    /// # Example
    ///
    /// ```rust
    /// let node = cx.render(rsx! ( Fragment {} )).unwrap();
    /// if let VNode::Fragment(frag) = node {
    ///     let root = &frag.children[0];
    ///     assert_eq!(root, VNode::Anchor);
    /// }
    /// ```
    Anchor(&'src VAnchor),

    /// A type of node that links this node to another scope or render cycle
    ///
    /// Is essentially a "pointer" to a "rendered" node in a particular scope
    ///
    /// Used in portals
    Linked(NodeLink),
}

impl<'src> VNode<'src> {
    /// Get the VNode's "key" used in the keyed diffing algorithm.
    pub fn key(&self) -> Option<&'src str> {
        match &self {
            VNode::Element(el) => el.key,
            VNode::Component(c) => c.key,
            VNode::Fragment(f) => f.key,

            VNode::Text(_t) => None,
            VNode::Suspended(_s) => None,
            VNode::Anchor(_f) => None,
            VNode::Linked(_c) => None,
        }
    }

    /// Get the ElementID of the mounted VNode.
    ///
    /// Panics if the mounted ID is None or if the VNode is not represented by a single Element.
    pub fn mounted_id(&self) -> ElementId {
        self.try_mounted_id().unwrap()
    }

    /// Try to get the ElementID of the mounted VNode.
    ///
    /// Returns None if the VNode is not mounted, or if the VNode cannot be presented by a mounted ID (Fragment/Component)
    pub fn try_mounted_id(&self) -> Option<ElementId> {
        match &self {
            VNode::Text(el) => el.dom_id.get(),
            VNode::Element(el) => el.dom_id.get(),
            VNode::Anchor(el) => el.dom_id.get(),
            VNode::Suspended(el) => el.dom_id.get(),
            VNode::Linked(_) => None,
            VNode::Fragment(_) => None,
            VNode::Component(_) => None,
        }
    }

    pub fn children(&self) -> &[VNode<'src>] {
        match &self {
            VNode::Fragment(f) => &f.children,
            VNode::Component(c) => todo!("children are not accessible through this"),
            _ => &[],
        }
    }

    // Create an "owned" version of the vnode.
    pub fn decouple(&self) -> VNode<'src> {
        match self {
            VNode::Text(t) => VNode::Text(*t),
            VNode::Element(e) => VNode::Element(*e),
            VNode::Component(c) => VNode::Component(*c),
            VNode::Suspended(s) => VNode::Suspended(*s),
            VNode::Anchor(a) => VNode::Anchor(*a),
            VNode::Fragment(f) => VNode::Fragment(VFragment {
                children: f.children,
                key: f.key,
            }),
            VNode::Linked(c) => VNode::Linked(NodeLink {
                gen_id: c.gen_id,
                scope_id: c.scope_id,
            }),
        }
    }
}

impl Debug for VNode<'_> {
    fn fmt(&self, s: &mut Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
        match &self {
            VNode::Element(el) => s
                .debug_struct("VElement")
                .field("name", &el.tag_name)
                .field("key", &el.key)
                .finish(),

            VNode::Text(t) => write!(s, "VText {{ text: {} }}", t.text),
            VNode::Anchor(_) => write!(s, "VAnchor"),

            VNode::Fragment(frag) => write!(s, "VFragment {{ children: {:?} }}", frag.children),
            VNode::Suspended { .. } => write!(s, "VSuspended"),
            VNode::Component(comp) => write!(s, "VComponent {{ fc: {:?}}}", comp.user_fc),
            VNode::Linked(c) => write!(
                s,
                "VCached {{ gen_id: {}, scope_id: {:?} }}",
                c.gen_id, c.scope_id
            ),
        }
    }
}

/// A placeholder node only generated when Fragments don't have any children.
pub struct VAnchor {
    pub dom_id: Cell<Option<ElementId>>,
}

/// A bump-allocated string slice and metadata.
pub struct VText<'src> {
    pub text: &'src str,

    pub dom_id: Cell<Option<ElementId>>,

    pub is_static: bool,
}

/// A list of VNodes with no single root.
pub struct VFragment<'src> {
    pub key: Option<&'src str>,

    pub children: &'src [VNode<'src>],
}

/// An element like a "div" with children, listeners, and attributes.
pub struct VElement<'a> {
    pub tag_name: &'static str,

    pub namespace: Option<&'static str>,

    pub key: Option<&'a str>,

    pub dom_id: Cell<Option<ElementId>>,

    pub parent_id: Cell<Option<ElementId>>,

    pub listeners: &'a [Listener<'a>],

    pub attributes: &'a [Attribute<'a>],

    pub children: &'a [VNode<'a>],
}

impl Debug for VElement<'_> {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("VElement")
            .field("tag_name", &self.tag_name)
            .field("namespace", &self.namespace)
            .field("key", &self.key)
            .field("dom_id", &self.dom_id)
            .field("parent_id", &self.parent_id)
            .field("listeners", &self.listeners.len())
            .field("attributes", &self.attributes)
            .field("children", &self.children)
            .finish()
    }
}

/// A trait for any generic Dioxus Element.
///
/// This trait provides the ability to use custom elements in the `rsx!` macro.
///
/// ```rust
/// struct my_element;
///
/// impl DioxusElement for my_element {
///     const TAG_NAME: "my_element";
///     const NAME_SPACE: None;
/// }
///
/// let _ = rsx!{
///     my_element {}
/// };
/// ```
pub trait DioxusElement {
    const TAG_NAME: &'static str;
    const NAME_SPACE: Option<&'static str>;
    #[inline]
    fn tag_name(&self) -> &'static str {
        Self::TAG_NAME
    }
    #[inline]
    fn namespace(&self) -> Option<&'static str> {
        Self::NAME_SPACE
    }
}

/// An attribute on a DOM node, such as `id="my-thing"` or
/// `href="https://example.com"`.
#[derive(Clone, Debug)]
pub struct Attribute<'a> {
    pub name: &'static str,

    pub value: &'a str,

    pub is_static: bool,

    pub is_volatile: bool,

    // Doesn't exist in the html spec.
    // Used in Dioxus to denote "style" tags.
    pub namespace: Option<&'static str>,
}

/// An event listener.
/// IE onclick, onkeydown, etc
pub struct Listener<'bump> {
    /// The ID of the node that this listener is mounted to
    /// Used to generate the event listener's ID on the DOM
    pub mounted_node: Cell<Option<ElementId>>,

    /// The type of event to listen for.
    ///
    /// IE "click" - whatever the renderer needs to attach the listener by name.
    pub event: &'static str,

    /// The actual callback that the user specified
    pub(crate) callback: RefCell<Option<BumpBox<'bump, dyn FnMut(Box<dyn Any + Send>) + 'bump>>>,
}

/// Virtual Components for custom user-defined components
/// Only supports the functional syntax
pub struct VComponent<'src> {
    pub key: Option<&'src str>,

    pub associated_scope: Cell<Option<ScopeId>>,
    // pub associated_scope: Cell<Option<*mut ScopeInner>>,

    // Function pointer to the FC that was used to generate this component
    pub user_fc: *const (),
    pub(crate) can_memoize: bool,

    // Raw pointer into the bump arena for the props of the component
    pub(crate) raw_props: *const (),

    // during the "teardown" process we'll take the caller out so it can be dropped properly
    pub(crate) caller: Option<VCompCaller<'src>>,
    pub(crate) comparator: Option<BumpBox<'src, dyn Fn(&VComponent) -> bool + 'src>>,
}

pub enum VCompCaller<'src> {
    Borrowed(BumpBox<'src, dyn for<'b> Fn(&'b ScopeState) -> Element + 'src>),
    Owned(Box<dyn for<'b> Fn(&'b ScopeState) -> Element>),
}

pub struct VSuspended<'a> {
    pub task_id: u64,
    pub dom_id: Cell<Option<ElementId>>,

    #[allow(clippy::type_complexity)]
    pub callback: RefCell<Option<BumpBox<'a, dyn FnMut() -> Element + 'a>>>,
}

/// This struct provides an ergonomic API to quickly build VNodes.
///
/// NodeFactory is used to build VNodes in the component's memory space.
/// This struct adds metadata to the final VNode about listeners, attributes, and children
#[derive(Copy, Clone)]
pub struct NodeFactory<'a> {
    pub(crate) bump: &'a Bump,
}

impl<'a> NodeFactory<'a> {
    pub fn new(bump: &'a Bump) -> NodeFactory<'a> {
        NodeFactory { bump }
    }

    #[inline]
    pub fn bump(&self) -> &'a bumpalo::Bump {
        self.bump
    }

    /// Directly pass in text blocks without the need to use the format_args macro.
    pub fn static_text(&self, text: &'static str) -> VNode<'a> {
        VNode::Text(self.bump.alloc(VText {
            dom_id: empty_cell(),
            text,
            is_static: true,
        }))
    }

    /// Parses a lazy text Arguments and returns a string and a flag indicating if the text is 'static
    ///
    /// Text that's static may be pointer compared, making it cheaper to diff
    pub fn raw_text(&self, args: Arguments) -> (&'a str, bool) {
        match args.as_str() {
            Some(static_str) => (static_str, true),
            None => {
                use bumpalo::core_alloc::fmt::Write;
                let mut str_buf = bumpalo::collections::String::new_in(self.bump());
                str_buf.write_fmt(args).unwrap();
                (str_buf.into_bump_str(), false)
            }
        }
    }

    /// Create some text that's allocated along with the other vnodes
    ///
    pub fn text(&self, args: Arguments) -> VNode<'a> {
        let (text, is_static) = self.raw_text(args);

        VNode::Text(self.bump.alloc(VText {
            text,
            is_static,
            dom_id: empty_cell(),
        }))
    }

    pub fn element<L, A, V>(
        &self,
        el: impl DioxusElement,
        listeners: L,
        attributes: A,
        children: V,
        key: Option<Arguments>,
    ) -> VNode<'a>
    where
        L: 'a + AsRef<[Listener<'a>]>,
        A: 'a + AsRef<[Attribute<'a>]>,
        V: 'a + AsRef<[VNode<'a>]>,
    {
        self.raw_element(
            el.tag_name(),
            el.namespace(),
            listeners,
            attributes,
            children,
            key,
        )
    }

    pub fn raw_element<L, A, V>(
        &self,
        tag_name: &'static str,
        namespace: Option<&'static str>,
        listeners: L,
        attributes: A,
        children: V,
        key: Option<Arguments>,
    ) -> VNode<'a>
    where
        L: 'a + AsRef<[Listener<'a>]>,
        A: 'a + AsRef<[Attribute<'a>]>,
        V: 'a + AsRef<[VNode<'a>]>,
    {
        let listeners: &'a L = self.bump().alloc(listeners);
        let listeners = listeners.as_ref();

        let attributes: &'a A = self.bump().alloc(attributes);
        let attributes = attributes.as_ref();

        let children: &'a V = self.bump().alloc(children);
        let children = children.as_ref();

        let key = key.map(|f| self.raw_text(f).0);

        VNode::Element(self.bump().alloc(VElement {
            tag_name,
            key,
            namespace,
            listeners,
            attributes,
            children,
            dom_id: empty_cell(),
            parent_id: empty_cell(),
        }))
    }

    pub fn attr(
        &self,
        name: &'static str,
        val: Arguments,
        namespace: Option<&'static str>,
        is_volatile: bool,
    ) -> Attribute<'a> {
        let (value, is_static) = self.raw_text(val);
        Attribute {
            name,
            value,
            is_static,
            namespace,
            is_volatile,
        }
    }

    pub fn component<P>(
        &self,
        component: fn(Scope<'a, P>) -> Element,
        props: P,
        key: Option<Arguments>,
    ) -> VNode<'a>
    where
        P: Properties + 'a,
    {
        /*
        our strategy:
        - unsafe hackery
        - lol

        - we don't want to hit the global allocator
        - allocate into our bump arena
        - if the props aren't static, then we convert them into a box which we pass off between renders
        */

        let bump = self.bump();

        // let p = BumpBox::new_in(x, a)

        // the best place to allocate the props are the other component's arena
        // the second best place is the global allocator

        // // if the props are static
        // let boxed = if P::IS_STATIC {
        //     todo!()
        // } else {
        //     todo!()
        // }

        // let caller = Box::new(|f: &ScopeInner| -> Element {
        //     //
        //     component((f, &props))
        // });

        let user_fc = component as *const ();

        // let comparator: &mut dyn Fn(&VComponent) -> bool = bump.alloc_with(|| {
        //     move |other: &VComponent| {
        //         if user_fc == other.user_fc {
        //             // Safety
        //             // - We guarantee that FC<P> is the same by function pointer
        //             // - Because FC<P> is the same, then P must be the same (even with generics)
        //             // - Non-static P are autoderived to memoize as false
        //             // - This comparator is only called on a corresponding set of bumpframes
        //             let props_memoized = unsafe {
        //                 let real_other: &P = &*(other.raw_props as *const _ as *const P);
        //                 props.memoize(real_other)
        //             };

        //             // It's only okay to memoize if there are no children and the props can be memoized
        //             // Implementing memoize is unsafe and done automatically with the props trait
        //             props_memoized
        //         } else {
        //             false
        //         }
        //     }
        // });
        // let comparator = Some(unsafe { BumpBox::from_raw(comparator) });

        let key = key.map(|f| self.raw_text(f).0);

        let caller = match P::IS_STATIC {
            true => {
                // it just makes sense to box the props
                let boxed_props: Box<P> = Box::new(props);
                let props_we_know_are_static = todo!();
                VCompCaller::Owned(Box::new(|f| {
                    //

                    let p = todo!();

                    todo!()
                }))
            }
            false => VCompCaller::Borrowed({
                //

                todo!()
                // let caller = bump.alloc()
            }),
        };

        todo!()
        // let caller: &'a mut dyn for<'b> Fn(&'b ScopeInner) -> Element<'b> =
        //     bump.alloc(move |scope: &ScopeInner| -> Element {
        //         log::debug!("calling component renderr {:?}", scope.our_arena_idx);
        //         let props: &'_ P = unsafe { &*(raw_props as *const P) };

        //         let scp: &'a ScopeInner = unsafe { std::mem::transmute(scope) };
        //         let s: Scope<'a, P> = (scp, props);

        //         let res: Element = component(s);
        //         unsafe { std::mem::transmute(res) }
        //     });

        // let caller = unsafe { BumpBox::from_raw(caller) };

        // VNode::Component(bump.alloc(VComponent {
        //     user_fc,
        //     comparator,
        //     raw_props,
        //     caller,
        //     is_static: P::IS_STATIC,
        //     key,
        //     can_memoize: P::IS_STATIC,
        //     drop_props,
        //     associated_scope: Cell::new(None),
        // }))
    }

    pub fn listener(
        self,
        event: &'static str,
        callback: BumpBox<'a, dyn FnMut(Box<dyn Any + Send>) + 'a>,
    ) -> Listener<'a> {
        Listener {
            mounted_node: Cell::new(None),
            event,
            callback: RefCell::new(Some(callback)),
        }
    }

    pub fn fragment_from_iter(
        self,
        node_iter: impl IntoIterator<Item = impl IntoVNode<'a>>,
    ) -> VNode<'a> {
        let bump = self.bump();
        let mut nodes = bumpalo::collections::Vec::new_in(bump);

        for node in node_iter {
            nodes.push(node.into_vnode(self));
        }

        if nodes.is_empty() {
            nodes.push(VNode::Anchor(bump.alloc(VAnchor {
                dom_id: empty_cell(),
            })));
        }

        let children = nodes.into_bump_slice();

        // TODO
        // We need a dedicated path in the rsx! macro that will trigger the "you need keys" warning
        //
        // if cfg!(debug_assertions) {
        //     if children.len() > 1 {
        //         if children.last().unwrap().key().is_none() {
        //             log::error!(
        //                 r#"
        // Warning: Each child in an array or iterator should have a unique "key" prop.
        // Not providing a key will lead to poor performance with lists.
        // See docs.rs/dioxus for more information.
        // ---
        // To help you identify where this error is coming from, we've generated a backtrace.
        //                         "#,
        //             );
        //         }
        //     }
        // }

        VNode::Fragment(VFragment {
            children,
            key: None,
        })
    }

    pub fn annotate_lazy<'z, 'b>(
        f: impl FnOnce(NodeFactory<'z>) -> VNode<'z> + 'b,
    ) -> Option<LazyNodes<'z, 'b>> {
        Some(LazyNodes::new(f))
    }
}

impl Debug for NodeFactory<'_> {
    fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        Ok(())
    }
}

/// Trait implementations for use in the rsx! and html! macros.
///
/// ## Details
///
/// This section provides convenience methods and trait implementations for converting common structs into a format accepted
/// by the macros.
///
/// All dynamic content in the macros must flow in through `fragment_from_iter`. Everything else must be statically layed out.
/// We pipe basically everything through `fragment_from_iter`, so we expect a very specific type:
/// ```
/// impl IntoIterator<Item = impl IntoVNode<'a>>
/// ```
///
/// As such, all node creation must go through the factory, which is only available in the component context.
/// These strict requirements make it possible to manage lifetimes and state.
pub trait IntoVNode<'a> {
    fn into_vnode(self, cx: NodeFactory<'a>) -> VNode<'a>;
}

// For the case where a rendered VNode is passed into the rsx! macro through curly braces
impl<'a> IntoIterator for VNode<'a> {
    type Item = VNode<'a>;
    type IntoIter = std::iter::Once<Self::Item>;
    fn into_iter(self) -> Self::IntoIter {
        std::iter::once(self)
    }
}

// TODO: do we even need this? It almost seems better not to
// // For the case where a rendered VNode is passed into the rsx! macro through curly braces
impl<'a> IntoVNode<'a> for VNode<'a> {
    fn into_vnode(self, _: NodeFactory<'a>) -> VNode<'a> {
        self
    }
}

// Conveniently, we also support "null" (nothing) passed in
impl IntoVNode<'_> for () {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        cx.fragment_from_iter(None as Option<VNode>)
    }
}

// Conveniently, we also support "None"
impl IntoVNode<'_> for Option<()> {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        cx.fragment_from_iter(None as Option<VNode>)
    }
}

// Conveniently, we also support "None"
impl IntoVNode<'_> for Option<NodeLink> {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        todo!()
        // cx.fragment_from_iter(None as Option<VNode>)
    }
}
// Conveniently, we also support "None"
impl IntoVNode<'_> for NodeLink {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        todo!()
        // cx.fragment_from_iter(None as Option<VNode>)
    }
}

impl<'a> IntoVNode<'a> for Option<VNode<'a>> {
    fn into_vnode(self, cx: NodeFactory<'a>) -> VNode<'a> {
        self.unwrap_or_else(|| cx.fragment_from_iter(None as Option<VNode>))
    }
}

impl<'a> IntoVNode<'a> for Option<LazyNodes<'a, '_>> {
    fn into_vnode(self, cx: NodeFactory<'a>) -> VNode<'a> {
        match self {
            Some(lazy) => lazy.call(cx),
            None => VNode::Fragment(VFragment {
                children: &[],
                key: None,
            }),
        }
    }
}

impl<'a> IntoVNode<'a> for LazyNodes<'a, '_> {
    fn into_vnode(self, cx: NodeFactory<'a>) -> VNode<'a> {
        self.call(cx)
    }
}

impl IntoVNode<'_> for &'static str {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        cx.static_text(self)
    }
}

impl IntoVNode<'_> for Arguments<'_> {
    fn into_vnode(self, cx: NodeFactory) -> VNode {
        cx.text(self)
    }
}

/// Access the children elements passed into the component
///
/// This enables patterns where a component is passed children from its parent.
///
/// ## Details
///
/// Unlike React, Dioxus allows *only* lists of children to be passed from parent to child - not arbitrary functions
/// or classes. If you want to generate nodes instead of accepting them as a list, consider declaring a closure
/// on the props that takes Context.
///
/// If a parent passes children into a component, the child will always re-render when the parent re-renders. In other
/// words, a component cannot be automatically memoized if it borrows nodes from its parent, even if the component's
/// props are valid for the static lifetime.
///
/// ## Example
///
/// ```rust
/// const App: FC<()> = |(cx, props)|{
///     cx.render(rsx!{
///         CustomCard {
///             h1 {}
///             p {}
///         }
///     })
/// }
///
/// const CustomCard: FC<()> = |(cx, props)|{
///     cx.render(rsx!{
///         div {
///             h1 {"Title card"}
///             {props.children}
///         }
///     })
/// }
/// ```
///
/// ## Notes:
///
/// This method returns a "ScopeChildren" object. This object is copy-able and preserve the correct lifetime.
pub struct ScopeChildren<'a> {
    root: Option<VNode<'a>>,
}

impl Default for ScopeChildren<'_> {
    fn default() -> Self {
        Self { root: None }
    }
}

impl<'a> ScopeChildren<'a> {
    pub fn new(root: VNode<'a>) -> Self {
        Self { root: Some(root) }
    }
    pub fn new_option(root: Option<VNode<'a>>) -> Self {
        Self { root }
    }
}

impl IntoIterator for &ScopeChildren<'_> {
    type Item = Self;

    type IntoIter = std::iter::Once<Self>;

    fn into_iter(self) -> Self::IntoIter {
        std::iter::once(self)
    }
}

impl<'a> IntoVNode<'a> for &ScopeChildren<'a> {
    fn into_vnode(self, cx: NodeFactory<'a>) -> VNode<'a> {
        match &self.root {
            Some(n) => n.decouple(),
            None => cx.fragment_from_iter(None as Option<VNode>),
        }
    }
}