Implement AnyView in xilem_masonry

crates/xilem_masonry/examples/mason.rs

@@ -2,7 +2,7 @@
 #![windows_subsystem = "windows"]
 
 use xilem_masonry::view::{button, flex};
-use xilem_masonry::{MasonryView, Xilem};
+use xilem_masonry::{BoxedMasonryView, MasonryView, Xilem};
 
 fn app_logic(data: &mut AppData) -> impl MasonryView<AppData> {
     // here's some logic, deriving state for the view from our state
@@ -20,18 +20,39 @@ fn app_logic(data: &mut AppData) -> impl MasonryView<AppData> {
         .collect::<Vec<_>>();
     flex((
         button(label, |data: &mut AppData| data.count += 1),
+        toggleable(data),
         button("Decrement", |data: &mut AppData| data.count -= 1),
         button("Reset", |data: &mut AppData| data.count = 0),
         sequence,
     ))
 }
 
+fn toggleable(data: &mut AppData) -> impl MasonryView<AppData> {
+    let inner_view: BoxedMasonryView<_, _> = if data.active {
+        Box::new(flex((
+            button("Deactivate", |data: &mut AppData| {
+                data.active = false;
+            }),
+            button("Unlimited Power", |data: &mut AppData| {
+                data.count = -1_000_000;
+            }),
+        )))
+    } else {
+        Box::new(button("Activate", |data: &mut AppData| data.active = true))
+    };
+    inner_view
+}
+
 struct AppData {
     count: i32,
+    active: bool,
 }
 
 fn main() {
-    let data = AppData { count: 0 };
+    let data = AppData {
+        count: 0,
+        active: false,
+    };
 
     let app = Xilem::new(data, app_logic);
     app.run_windowed("First Example".into()).unwrap();

crates/xilem_masonry/src/any_view.rs

@@ -0,0 +1,177 @@
+use std::ops::Deref;
+
+use masonry::{
+    declare_widget,
+    widget::{StoreInWidgetMut, WidgetMut, WidgetRef},
+    BoxConstraints, EventCtx, LayoutCtx, LifeCycle, LifeCycleCtx, PaintCtx, Point, PointerEvent,
+    Size, StatusChange, TextEvent, Widget, WidgetPod,
+};
+use smallvec::SmallVec;
+use vello::Scene;
+
+use crate::{ChangeFlags, MasonryView, MessageResult, ViewCx, ViewId};
+
+/// A view which can have any underlying view type.
+///
+/// This can be used to return type erased views (such as from a trait),
+/// or used to implement conditional display and switching of views.
+///
+/// Note that `Option` can also be used for conditionally displaying
+/// views when placed in a ViewSequence.
+// TODO: Mention `Either` when we have implemented that?
+pub type BoxedMasonryView<T, A = ()> = Box<dyn AnyMasonryView<T, A>>;
+
+impl<T: 'static, A: 'static> MasonryView<T, A> for BoxedMasonryView<T, A> {
+    type Element = DynWidget;
+
+    fn build(&self, cx: &mut ViewCx) -> masonry::WidgetPod<Self::Element> {
+        self.deref().dyn_build(cx)
+    }
+
+    fn message(
+        &self,
+        id_path: &[ViewId],
+        message: Box<dyn std::any::Any>,
+        app_state: &mut T,
+    ) -> crate::MessageResult<A> {
+        self.deref().dyn_message(id_path, message, app_state)
+    }
+
+    fn rebuild(
+        &self,
+        cx: &mut ViewCx,
+        prev: &Self,
+        // _id: &mut Id,
+        element: masonry::widget::WidgetMut<Self::Element>,
+    ) -> ChangeFlags {
+        self.deref().dyn_rebuild(cx, prev, element)
+    }
+}
+
+/// A trait enabling type erasure of views.
+pub trait AnyMasonryView<T, A = ()>: Send {
+    fn as_any(&self) -> &dyn std::any::Any;
+
+    fn dyn_build(&self, cx: &mut ViewCx) -> WidgetPod<DynWidget>;
+
+    fn dyn_rebuild(
+        &self,
+        cx: &mut ViewCx,
+        prev: &dyn AnyMasonryView<T, A>,
+        element: WidgetMut<DynWidget>,
+    ) -> ChangeFlags;
+
+    fn dyn_message(
+        &self,
+        id_path: &[ViewId],
+        message: Box<dyn std::any::Any>,
+        app_state: &mut T,
+    ) -> MessageResult<A>;
+}
+
+impl<T, A, V: MasonryView<T, A> + 'static> AnyMasonryView<T, A> for V {
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+
+    fn dyn_build(&self, cx: &mut ViewCx) -> WidgetPod<DynWidget> {
+        let element = self.build(cx);
+        WidgetPod::new(DynWidget {
+            inner: element.boxed(),
+        })
+    }
+
+    fn dyn_rebuild(
+        &self,
+        cx: &mut ViewCx,
+        prev: &dyn AnyMasonryView<T, A>,
+        mut element: WidgetMut<DynWidget>,
+    ) -> ChangeFlags {
+        // TODO: Does this need to have a custom view id to enable events sent
+        // to an outdated view path to be caught and returned?
+        // Should we store this generation in `element`? Seems plausible
+        if let Some(prev) = prev.as_any().downcast_ref() {
+            // If we were previously of this type, then do a normal rebuild
+            element.downcast(|element| {
+                if let Some(element) = element {
+                    self.rebuild(cx, prev, element)
+                } else {
+                    eprintln!("downcast of element failed in dyn_rebuild");
+                    ChangeFlags::UNCHANGED
+                }
+            })
+        } else {
+            // Otherwise, replace the element
+            let new_element = self.build(cx);
+            element.replace_inner(new_element.boxed());
+            ChangeFlags::CHANGED
+        }
+    }
+
+    fn dyn_message(
+        &self,
+        id_path: &[ViewId],
+        message: Box<dyn std::any::Any>,
+        app_state: &mut T,
+    ) -> MessageResult<A> {
+        self.message(id_path, message, app_state)
+    }
+}
+
+/// A widget whose only child can be dynamically replaced
+///
+/// `WidgetPod<Box<dyn Widget>>` doesn't expose this possibility
+pub struct DynWidget {
+    inner: WidgetPod<Box<dyn Widget>>,
+}
+
+/// Forwards all events to the child widget
+impl Widget for DynWidget {
+    fn on_pointer_event(&mut self, ctx: &mut EventCtx, event: &PointerEvent) {
+        self.inner.on_pointer_event(ctx, event);
+    }
+    fn on_text_event(&mut self, ctx: &mut EventCtx, event: &TextEvent) {
+        self.inner.on_text_event(ctx, event);
+    }
+
+    fn on_status_change(&mut self, _: &mut LifeCycleCtx, _: &StatusChange) {
+        // Intentionally do nothing
+    }
+
+    fn lifecycle(&mut self, ctx: &mut LifeCycleCtx, event: &LifeCycle) {
+        self.inner.lifecycle(ctx, event);
+    }
+
+    fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints) -> Size {
+        let size = self.inner.layout(ctx, bc);
+        ctx.place_child(&mut self.inner, Point::ORIGIN);
+        size
+    }
+
+    fn paint(&mut self, ctx: &mut PaintCtx, scene: &mut Scene) {
+        self.inner.paint(ctx, scene);
+    }
+
+    fn children(&self) -> SmallVec<[WidgetRef<'_, dyn Widget>; 16]> {
+        let mut vec = SmallVec::new();
+        vec.push(self.inner.as_dyn());
+        vec
+    }
+}
+
+declare_widget!(DynWidgetMut, DynWidget);
+
+impl DynWidgetMut<'_> {
+    pub(crate) fn replace_inner(&mut self, widget: WidgetPod<Box<dyn Widget>>) {
+        self.widget.inner = widget;
+        self.ctx.children_changed();
+    }
+
+    pub(crate) fn downcast<W: Widget + StoreInWidgetMut, R>(
+        &mut self,
+        f: impl FnOnce(Option<WidgetMut<'_, W>>) -> R,
+    ) -> R {
+        let mut get_mut = self.ctx.get_mut(&mut self.widget.inner);
+        f(get_mut.downcast())
+    }
+}

crates/xilem_masonry/src/lib.rs

@@ -17,9 +17,11 @@ use winit::{
     window::{Window, WindowBuilder},
 };
 
+mod any_view;
 mod id;
 mod sequence;
 mod vec_splice;
+pub use any_view::{AnyMasonryView, BoxedMasonryView};
 pub mod view;
 pub use id::ViewId;
 pub use sequence::{ElementSplice, ViewSequence};
@@ -192,19 +194,21 @@ where
 pub trait MasonryView<State, Action = ()>: Send + 'static {
     type Element: Widget + StoreInWidgetMut;
     fn build(&self, cx: &mut ViewCx) -> WidgetPod<Self::Element>;
-    fn message(
-        &self,
-        id_path: &[ViewId],
-        message: Box<dyn Any>,
-        app_state: &mut State,
-    ) -> MessageResult<Action>;
+
     fn rebuild(
         &self,
         _cx: &mut ViewCx,
         prev: &Self,
         // _id: &mut Id,
         element: WidgetMut<Self::Element>,
     ) -> ChangeFlags;
+
+    fn message(
+        &self,
+        id_path: &[ViewId],
+        message: Box<dyn Any>,
+        app_state: &mut State,
+    ) -> MessageResult<Action>;
 }
 
 #[must_use]