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//! Store internal widget state in a state tree to ensure continuity.
use crate::Element;
use std::any::{self, Any};
/// A persistent state widget tree.
///
/// A [`Tree`] is normally associated with a specific widget in the widget tree.
pub struct Tree {
/// The tag of the [`Tree`].
pub tag: Tag,
/// The [`State`] of the [`Tree`].
pub state: State,
/// The children of the root widget of the [`Tree`].
pub children: Vec<Tree>,
}
impl Tree {
/// Creates an empty, stateless [`Tree`] with no children.
pub fn empty() -> Self {
Self {
tag: Tag::stateless(),
state: State::None,
children: Vec::new(),
}
}
/// Creates a new [`Tree`] for the provided [`Element`].
pub fn new<Message, Renderer>(
element: &Element<'_, Message, Renderer>,
) -> Self {
Self {
tag: element.as_widget().tag(),
state: element.as_widget().state(),
children: element.as_widget().children(),
}
}
/// Reconciliates the current tree with the provided [`Element`].
///
/// If the tag of the [`Element`] matches the tag of the [`Tree`], then the
/// [`Element`] proceeds with the reconciliation (i.e. [`Widget::diff`] is called).
///
/// Otherwise, the whole [`Tree`] is recreated.
///
/// [`Widget::diff`]: crate::Widget::diff
pub fn diff<Message, Renderer>(
&mut self,
new: &Element<'_, Message, Renderer>,
) {
if self.tag == new.as_widget().tag() {
new.as_widget().diff(self)
} else {
*self = Self::new(new);
}
}
/// Reconciliates the children of the tree with the provided list of [`Element`].
pub fn diff_children<Message, Renderer>(
&mut self,
new_children: &[Element<'_, Message, Renderer>],
) {
self.diff_children_custom(new_children, Self::diff, Self::new)
}
/// Reconciliates the children of the tree with the provided list of [`Element`] using custom
/// logic both for diffing and creating new widget state.
pub fn diff_children_custom<T>(
&mut self,
new_children: &[T],
diff: impl Fn(&mut Tree, &T),
new_state: impl Fn(&T) -> Self,
) {
if self.children.len() > new_children.len() {
self.children.truncate(new_children.len());
}
for (child_state, new) in
self.children.iter_mut().zip(new_children.iter())
{
diff(child_state, new);
}
if self.children.len() < new_children.len() {
self.children.extend(
new_children[self.children.len()..].iter().map(new_state),
);
}
}
}
/// The identifier of some widget state.
#[derive(Debug, Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Hash)]
pub struct Tag(any::TypeId);
impl Tag {
/// Creates a [`Tag`] for a state of type `T`.
pub fn of<T>() -> Self
where
T: 'static,
{
Self(any::TypeId::of::<T>())
}
/// Creates a [`Tag`] for a stateless widget.
pub fn stateless() -> Self {
Self::of::<()>()
}
}
/// The internal [`State`] of a widget.
pub enum State {
/// No meaningful internal state.
None,
/// Some meaningful internal state.
Some(Box<dyn Any>),
}
impl State {
/// Creates a new [`State`].
pub fn new<T>(state: T) -> Self
where
T: 'static,
{
State::Some(Box::new(state))
}
/// Downcasts the [`State`] to `T` and returns a reference to it.
///
/// # Panics
/// This method will panic if the downcast fails or the [`State`] is [`State::None`].
pub fn downcast_ref<T>(&self) -> &T
where
T: 'static,
{
match self {
State::None => panic!("Downcast on stateless state"),
State::Some(state) => {
state.downcast_ref().expect("Downcast widget state")
}
}
}
/// Downcasts the [`State`] to `T` and returns a mutable reference to it.
///
/// # Panics
/// This method will panic if the downcast fails or the [`State`] is [`State::None`].
pub fn downcast_mut<T>(&mut self) -> &mut T
where
T: 'static,
{
match self {
State::None => panic!("Downcast on stateless state"),
State::Some(state) => {
state.downcast_mut().expect("Downcast widget state")
}
}
}
}
|
