xilem_web: Add `interval` `View`, which somewhat reflects `setInterval` (#486)

Simple but convenient `View`, e.g. for some kind of ticker. (Maybe needs
an additional example)

xilem_web/src/concurrent/interval.rs

@@ -0,0 +1,148 @@
+// Copyright 2024 the Xilem Authors and the Druid Authors
+// SPDX-License-Identifier: Apache-2.0
+
+use std::marker::PhantomData;
+
+use wasm_bindgen::{closure::Closure, JsCast, UnwrapThrowExt};
+use xilem_core::{MessageResult, Mut, NoElement, View, ViewId, ViewMarker};
+
+use crate::{DynMessage, OptionalAction, ViewCtx};
+
+/// Start an interval which invokes `callback` every `ms` milliseconds
+pub struct Interval<Callback, State, Action> {
+    ms: u32,
+    callback: Callback,
+    phantom: PhantomData<fn() -> (State, Action)>,
+}
+
+/// Start an interval which invokes `callback` every `ms` milliseconds
+///
+/// Currently, when `ms` changes, the previous interval is cleared, and starts with the new interval.
+/// This default behavior may change in the future, and may even be configurable.
+///
+/// # Examples
+///
+/// ```
+/// use xilem_web::{core::fork, concurrent::interval, elements::html::div, interfaces::Element};
+///
+/// fn timer(seconds: &mut u32) -> impl Element<u32> {
+///     fork(
+///         div(format!("{seconds} seconds have passed, since creating this view")),
+///         interval(
+///             1000, // in ms, when this changes, the interval is reset
+///             |seconds: &mut u32| *seconds += 1,
+///         )
+///     )
+/// }
+/// ```
+///
+/// # Panics
+///
+/// While `ms` is a `u32`, `setInterval` actually requires this to be a `i32`, so values above `2147483647` lead to a panic.
+/// See <https://developer.mozilla.org/en-US/docs/Web/API/setInterval#sect2> for more details.
+pub fn interval<State, Action, OA, Callback>(
+    ms: u32,
+    callback: Callback,
+) -> Interval<Callback, State, Action>
+where
+    State: 'static,
+    Action: 'static,
+    OA: OptionalAction<Action> + 'static,
+    Callback: Fn(&mut State) -> OA + 'static,
+{
+    Interval {
+        ms,
+        callback,
+        phantom: PhantomData,
+    }
+}
+
+pub struct IntervalState {
+    // Closures are retained so they can be called by environment
+    interval_fn: Closure<dyn FnMut()>,
+    interval_handle: i32,
+}
+
+fn start_interval(callback: &Closure<dyn FnMut()>, ms: u32) -> i32 {
+    web_sys::window()
+        .unwrap_throw()
+        .set_interval_with_callback_and_timeout_and_arguments_0(
+            callback.as_ref().unchecked_ref(),
+            ms.try_into().expect_throw(
+                "`setInterval` requires this to be an `i32`,\
+                 which is why values above `2147483647` are not possible,\
+                 see https://developer.mozilla.org/en-US/docs/Web/API/setInterval#sect2 \
+                 for more details",
+            ),
+        )
+        .unwrap_throw()
+}
+
+fn clear_interval(handle: i32) {
+    web_sys::window()
+        .unwrap_throw()
+        .clear_interval_with_handle(handle);
+}
+
+impl<Callback, State, Action> ViewMarker for Interval<Callback, State, Action> {}
+
+impl<State, Action, Callback, OA> View<State, Action, ViewCtx, DynMessage>
+    for Interval<Callback, State, Action>
+where
+    State: 'static,
+    Action: 'static,
+    OA: OptionalAction<Action> + 'static,
+    Callback: Fn(&mut State) -> OA + 'static,
+{
+    type Element = NoElement;
+
+    type ViewState = IntervalState;
+
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+        let thunk = ctx.message_thunk();
+        let interval_fn = Closure::new(move || thunk.push_message(()));
+        let state = IntervalState {
+            interval_handle: start_interval(&interval_fn, self.ms),
+            interval_fn,
+        };
+
+        (NoElement, state)
+    }
+
+    fn rebuild<'el>(
+        &self,
+        prev: &Self,
+        view_state: &mut Self::ViewState,
+        _: &mut ViewCtx,
+        (): Mut<'el, Self::Element>,
+    ) -> Mut<'el, Self::Element> {
+        if prev.ms != self.ms {
+            clear_interval(view_state.interval_handle);
+            view_state.interval_handle = start_interval(&view_state.interval_fn, self.ms);
+        }
+    }
+
+    fn teardown(
+        &self,
+        view_state: &mut Self::ViewState,
+        _: &mut ViewCtx,
+        _: Mut<'_, Self::Element>,
+    ) {
+        clear_interval(view_state.interval_handle);
+    }
+
+    fn message(
+        &self,
+        _: &mut Self::ViewState,
+        id_path: &[ViewId],
+        message: DynMessage,
+        app_state: &mut State,
+    ) -> MessageResult<Action, DynMessage> {
+        debug_assert!(id_path.is_empty());
+        message.downcast::<()>().unwrap_throw();
+        match (self.callback)(app_state).action() {
+            Some(action) => MessageResult::Action(action),
+            None => MessageResult::Nop,
+        }
+    }
+}

xilem_web/src/concurrent/mod.rs

@@ -3,6 +3,8 @@
 
 //! Async views, allowing concurrent operations, like fetching data from a server
 
-mod memoized_await;
+mod interval;
+pub use interval::{interval, Interval};
 
+mod memoized_await;
 pub use memoized_await::{memoized_await, MemoizedAwait};