Reserve ability to create Pod to ViewCtx (#597)

This is another intermediary PR for the "restrict widget creation to
Mutate pass" feature.

The basic idea is that, in the near future, it will be impossible to
create a WidgetPod without a handle to the arena. In my current WIP
code, that handle is passed through ViewCtx. Therefore, this PR changes
all the sites in xilem that create a Pod and has them use a ViewCtx
method instead.

I've tested most xilem examples and they all worked (except for
variable_clock, which currently panics in the last main commit).

xilem/src/any_view.rs

@@ -25,12 +25,12 @@ use crate::{Pod, ViewCtx};
 pub type AnyWidgetView<State, Action = ()> =
     dyn AnyView<State, Action, ViewCtx, Pod<DynWidget>> + Send + Sync;
 
-impl<W: Widget> SuperElement<Pod<W>> for Pod<DynWidget> {
+impl<W: Widget> SuperElement<Pod<W>, ViewCtx> for Pod<DynWidget> {
-    fn upcast(child: Pod<W>) -> Self {
+    fn upcast(ctx: &mut ViewCtx, child: Pod<W>) -> Self {
-        WidgetPod::new(DynWidget {
+        let boxed_pod = ctx.boxed_pod(child);
-            inner: child.inner.boxed(),
+        ctx.new_pod(DynWidget {
+            inner: boxed_pod.inner.boxed(),
         })
-        .into()
     }
 
     fn with_downcast_val<R>(
@@ -47,7 +47,7 @@ impl<W: Widget> SuperElement<Pod<W>> for Pod<DynWidget> {
     }
 }
 
-impl<W: Widget> AnyElement<Pod<W>> for Pod<DynWidget> {
+impl<W: Widget> AnyElement<Pod<W>, ViewCtx> for Pod<DynWidget> {
     fn replace_inner(mut this: Self::Mut<'_>, child: Pod<W>) -> Self::Mut<'_> {
         DynWidget::replace_inner(&mut this, child.inner.boxed());
         this

xilem/src/lib.rs

@@ -156,26 +156,13 @@ pub struct Pod<W: Widget> {
     pub inner: WidgetPod<W>,
 }
 
-impl<W: Widget> Pod<W> {
-    /// Create a new `Pod` for `inner`.
-    pub fn new(inner: W) -> Self {
-        Self::from(WidgetPod::new(inner))
-    }
-}
-
-impl<W: Widget> From<WidgetPod<W>> for Pod<W> {
-    fn from(inner: WidgetPod<W>) -> Self {
-        Pod { inner }
-    }
-}
-
 impl<W: Widget> ViewElement for Pod<W> {
     type Mut<'a> = WidgetMut<'a, W>;
 }
 
-impl<W: Widget> SuperElement<Pod<W>> for Pod<Box<dyn Widget>> {
+impl<W: Widget> SuperElement<Pod<W>, ViewCtx> for Pod<Box<dyn Widget>> {
-    fn upcast(child: Pod<W>) -> Self {
+    fn upcast(ctx: &mut ViewCtx, child: Pod<W>) -> Self {
-        child.inner.boxed().into()
+        ctx.boxed_pod(child)
     }
 
     fn with_downcast_val<R>(
@@ -274,6 +261,18 @@ impl ViewPathTracker for ViewCtx {
 }
 
 impl ViewCtx {
+    pub fn new_pod<W: Widget>(&mut self, widget: W) -> Pod<W> {
+        Pod {
+            inner: WidgetPod::new(widget),
+        }
+    }
+
+    pub fn boxed_pod<W: Widget>(&mut self, pod: Pod<W>) -> Pod<Box<dyn Widget>> {
+        Pod {
+            inner: pod.inner.boxed(),
+        }
+    }
+
     pub fn mark_changed(&mut self) {
         if cfg!(debug_assertions) {
             self.view_tree_changed = true;

xilem/src/one_of.rs

@@ -120,6 +120,7 @@ impl<
         }
     }
     fn upcast_one_of_element(
+        &mut self,
         elem: xilem_core::one_of::OneOf<
             Pod<A>,
             Pod<B>,
@@ -133,15 +134,15 @@ impl<
         >,
     ) -> Self::OneOfElement {
         match elem {
-            xilem_core::one_of::OneOf::A(w) => Pod::new(OneOfWidget::A(w.inner)),
+            xilem_core::one_of::OneOf::A(w) => self.new_pod(OneOfWidget::A(w.inner)),
-            xilem_core::one_of::OneOf::B(w) => Pod::new(OneOfWidget::B(w.inner)),
+            xilem_core::one_of::OneOf::B(w) => self.new_pod(OneOfWidget::B(w.inner)),
-            xilem_core::one_of::OneOf::C(w) => Pod::new(OneOfWidget::C(w.inner)),
+            xilem_core::one_of::OneOf::C(w) => self.new_pod(OneOfWidget::C(w.inner)),
-            xilem_core::one_of::OneOf::D(w) => Pod::new(OneOfWidget::D(w.inner)),
+            xilem_core::one_of::OneOf::D(w) => self.new_pod(OneOfWidget::D(w.inner)),
-            xilem_core::one_of::OneOf::E(w) => Pod::new(OneOfWidget::E(w.inner)),
+            xilem_core::one_of::OneOf::E(w) => self.new_pod(OneOfWidget::E(w.inner)),
-            xilem_core::one_of::OneOf::F(w) => Pod::new(OneOfWidget::F(w.inner)),
+            xilem_core::one_of::OneOf::F(w) => self.new_pod(OneOfWidget::F(w.inner)),
-            xilem_core::one_of::OneOf::G(w) => Pod::new(OneOfWidget::G(w.inner)),
+            xilem_core::one_of::OneOf::G(w) => self.new_pod(OneOfWidget::G(w.inner)),
-            xilem_core::one_of::OneOf::H(w) => Pod::new(OneOfWidget::H(w.inner)),
+            xilem_core::one_of::OneOf::H(w) => self.new_pod(OneOfWidget::H(w.inner)),
-            xilem_core::one_of::OneOf::I(w) => Pod::new(OneOfWidget::I(w.inner)),
+            xilem_core::one_of::OneOf::I(w) => self.new_pod(OneOfWidget::I(w.inner)),
         }
     }
 

xilem/src/view/button.rs

@@ -50,7 +50,7 @@ where
     type ViewState = ();
 
     fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        ctx.with_leaf_action_widget(|_| Pod::new(widget::Button::new(self.label.clone())))
+        ctx.with_leaf_action_widget(|ctx| ctx.new_pod(widget::Button::new(self.label.clone())))
     }
 
     fn rebuild<'el>(

xilem/src/view/checkbox.rs

@@ -36,8 +36,8 @@ where
     type ViewState = ();
 
     fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        ctx.with_leaf_action_widget(|_| {
+        ctx.with_leaf_action_widget(|ctx| {
-            Pod::new(masonry::widget::Checkbox::new(
+            ctx.new_pod(masonry::widget::Checkbox::new(
                 self.checked,
                 self.label.clone(),
             ))

xilem/src/view/flex.rs

@@ -115,7 +115,7 @@ where
                 FlexElement::FlexSpacer(flex) => widget.with_flex_spacer(flex),
             }
         }
-        (Pod::new(widget), seq_state)
+        (ctx.new_pod(widget), seq_state)
     }
 
     fn rebuild<'el>(
@@ -209,8 +209,8 @@ impl ViewElement for FlexElement {
     type Mut<'w> = FlexElementMut<'w>;
 }
 
-impl SuperElement<FlexElement> for FlexElement {
+impl SuperElement<FlexElement, ViewCtx> for FlexElement {
-    fn upcast(child: FlexElement) -> Self {
+    fn upcast(_ctx: &mut ViewCtx, child: FlexElement) -> Self {
         child
     }
 
@@ -230,9 +230,9 @@ impl SuperElement<FlexElement> for FlexElement {
     }
 }
 
-impl<W: Widget> SuperElement<Pod<W>> for FlexElement {
+impl<W: Widget> SuperElement<Pod<W>, ViewCtx> for FlexElement {
-    fn upcast(child: Pod<W>) -> Self {
+    fn upcast(ctx: &mut ViewCtx, child: Pod<W>) -> Self {
-        FlexElement::Child(child.inner.boxed().into(), FlexParams::default())
+        FlexElement::Child(ctx.boxed_pod(child), FlexParams::default())
     }
 
     fn with_downcast_val<R>(
@@ -450,10 +450,7 @@ where
 
     fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
         let (pod, state) = self.view.build(ctx);
-        (
+        (FlexElement::Child(ctx.boxed_pod(pod), self.params), state)
-            FlexElement::Child(pod.inner.boxed().into(), self.params),
-            state,
-        )
     }
 
     fn rebuild<'el>(

xilem/src/view/grid.rs

@@ -74,7 +74,7 @@ where
                 GridElement::Child(child, params) => widget.with_child_pod(child.inner, params),
             }
         }
-        (Pod::new(widget), seq_state)
+        (ctx.new_pod(widget), seq_state)
     }
 
     fn rebuild<'el>(
@@ -133,8 +133,8 @@ impl ViewElement for GridElement {
 }
 
 // Used to allow the item to be used as a generic item in ViewSequence.
-impl SuperElement<GridElement> for GridElement {
+impl SuperElement<GridElement, ViewCtx> for GridElement {
-    fn upcast(child: GridElement) -> Self {
+    fn upcast(_ctx: &mut ViewCtx, child: GridElement) -> Self {
         child
     }
 
@@ -154,14 +154,14 @@ impl SuperElement<GridElement> for GridElement {
     }
 }
 
-impl<W: Widget> SuperElement<Pod<W>> for GridElement {
+impl<W: Widget> SuperElement<Pod<W>, ViewCtx> for GridElement {
-    fn upcast(child: Pod<W>) -> Self {
+    fn upcast(ctx: &mut ViewCtx, child: Pod<W>) -> Self {
         // Getting here means that the widget didn't use .grid_item or .grid_pos.
         // This currently places the widget in the top left cell.
         // There is not much else, beyond purposefully failing, that can be done here,
         // because there isn't enough information to determine an appropriate spot
         // for the widget.
-        GridElement::Child(child.inner.boxed().into(), GridParams::new(1, 1, 1, 1))
+        GridElement::Child(ctx.boxed_pod(child), GridParams::new(1, 1, 1, 1))
     }
 
     fn with_downcast_val<R>(
@@ -362,12 +362,9 @@ where
 
     type ViewState = V::ViewState;
 
-    fn build(&self, cx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        let (pod, state) = self.view.build(cx);
+        let (pod, state) = self.view.build(ctx);
-        (
+        (GridElement::Child(ctx.boxed_pod(pod), self.params), state)
-            GridElement::Child(pod.inner.boxed().into(), self.params),
-            state,
-        )
     }
 
     fn rebuild<'el>(

xilem/src/view/image.rs

@@ -49,8 +49,8 @@ impl<State, Action> View<State, Action, ViewCtx> for Image {
     type Element = Pod<widget::Image>;
     type ViewState = ();
 
-    fn build(&self, _: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        (Pod::new(widget::Image::new(self.image.clone())), ())
+        (ctx.new_pod(widget::Image::new(self.image.clone())), ())
     }
 
     fn rebuild<'el>(

xilem/src/view/label.rs

@@ -48,8 +48,8 @@ impl<State, Action> View<State, Action, ViewCtx> for Label {
     type Element = Pod<widget::Label>;
     type ViewState = ();
 
-    fn build(&self, _ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        let widget_pod = Pod::new(
+        let widget_pod = ctx.new_pod(
             widget::Label::new(self.label.clone())
                 .with_text_brush(self.text_brush.clone())
                 .with_text_alignment(self.alignment)

xilem/src/view/portal.rs

@@ -40,7 +40,7 @@ where
         // The Portal `View` doesn't get any messages directly (yet - scroll events?), so doesn't need to
         // use ctx.with_id.
         let (child, child_state) = self.child.build(ctx);
-        let widget_pod = Pod::new(widget::Portal::new_pod(child.inner));
+        let widget_pod = ctx.new_pod(widget::Portal::new_pod(child.inner));
         (widget_pod, child_state)
     }
 

xilem/src/view/progress_bar.rs

@@ -20,7 +20,9 @@ impl<State, Action> View<State, Action, ViewCtx> for ProgressBar {
     type ViewState = ();
 
     fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        ctx.with_leaf_action_widget(|_| Pod::new(masonry::widget::ProgressBar::new(self.progress)))
+        ctx.with_leaf_action_widget(|ctx| {
+            ctx.new_pod(masonry::widget::ProgressBar::new(self.progress))
+        })
     }
 
     fn rebuild<'el>(

xilem/src/view/prose.rs

@@ -49,8 +49,8 @@ impl<State, Action> View<State, Action, ViewCtx> for Prose {
     type Element = Pod<widget::Prose>;
     type ViewState = ();
 
-    fn build(&self, _ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        let widget_pod = Pod::new(
+        let widget_pod = ctx.new_pod(
             widget::Prose::new(self.content.clone())
                 .with_text_brush(self.text_brush.clone())
                 .with_text_alignment(self.alignment)

xilem/src/view/sized_box.rs

@@ -135,7 +135,7 @@ where
         if let Some(border) = &self.border {
             widget = widget.border(border.color, border.width);
         }
-        (Pod::new(widget), child_state)
+        (ctx.new_pod(widget), child_state)
     }
 
     fn rebuild<'el>(

xilem/src/view/spinner.rs

@@ -55,8 +55,8 @@ impl<State, Action> View<State, Action, ViewCtx> for Spinner {
     type Element = Pod<widget::Spinner>;
     type ViewState = ();
 
-    fn build(&self, _: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        (Pod::new(widget::Spinner::new()), ())
+        (ctx.new_pod(widget::Spinner::new()), ())
     }
 
     fn rebuild<'el>(

xilem/src/view/textbox.rs

@@ -69,8 +69,8 @@ impl<State: 'static, Action: 'static> View<State, Action, ViewCtx> for Textbox<S
     type ViewState = ();
 
     fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        ctx.with_leaf_action_widget(|_| {
+        ctx.with_leaf_action_widget(|ctx| {
-            Pod::new(
+            ctx.new_pod(
                 masonry::widget::Textbox::new(self.contents.clone())
                     .with_text_brush(self.text_brush.clone())
                     .with_text_alignment(self.alignment),

xilem/src/view/variable_label.rs

@@ -100,8 +100,8 @@ impl<State, Action> View<State, Action, ViewCtx> for VariableLabel {
     type Element = Pod<widget::VariableLabel>;
     type ViewState = ();
 
-    fn build(&self, _ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
+    fn build(&self, ctx: &mut ViewCtx) -> (Self::Element, Self::ViewState) {
-        let widget_pod = Pod::new(
+        let widget_pod = ctx.new_pod(
             widget::VariableLabel::new(self.label.clone())
                 .with_text_brush(self.text_brush.clone())
                 .with_line_break_mode(widget::LineBreaking::WordWrap)

xilem_core/examples/filesystem.rs

@@ -98,8 +98,8 @@ impl<V, State, Action> FileView<State, Action> for V where
 
 type DynFileView<State, Action = ()> = Box<dyn AnyView<State, Action, ViewCtx, FsPath>>;
 
-impl SuperElement<FsPath> for FsPath {
+impl SuperElement<FsPath, ViewCtx> for FsPath {
-    fn upcast(child: FsPath) -> Self {
+    fn upcast(_ctx: &mut ViewCtx, child: FsPath) -> Self {
         child
     }
 
@@ -112,7 +112,7 @@ impl SuperElement<FsPath> for FsPath {
     }
 }
 
-impl AnyElement<FsPath> for FsPath {
+impl AnyElement<FsPath, ViewCtx> for FsPath {
     fn replace_inner(this: Self::Mut<'_>, child: FsPath) -> Self::Mut<'_> {
         *this = child.0;
         this

xilem_core/examples/user_interface.rs

@@ -99,8 +99,8 @@ impl Widget for ButtonWidget {
     }
 }
 
-impl<W: Widget> SuperElement<WidgetPod<W>> for WidgetPod<Box<dyn Widget>> {
+impl<W: Widget> SuperElement<WidgetPod<W>, ViewCtx> for WidgetPod<Box<dyn Widget>> {
-    fn upcast(child: WidgetPod<W>) -> Self {
+    fn upcast(_ctx: &mut ViewCtx, child: WidgetPod<W>) -> Self {
         WidgetPod {
             widget: Box::new(child.widget),
         }

xilem_core/src/any_view.rs

@@ -66,7 +66,7 @@ pub trait AnyView<State, Action, Context, Element: ViewElement, Message = DynMes
 impl<State, Action, Context, DynamicElement, Message, V>
     AnyView<State, Action, Context, DynamicElement, Message> for V
 where
-    DynamicElement: AnyElement<V::Element>,
+    DynamicElement: AnyElement<V::Element, Context>,
     Context: ViewPathTracker,
     V: View<State, Action, Context, Message> + 'static,
     V::ViewState: 'static,
@@ -80,7 +80,7 @@ where
         let generation = 0;
         let (element, view_state) = ctx.with_id(ViewId::new(generation), |ctx| self.build(ctx));
         (
-            DynamicElement::upcast(element),
+            DynamicElement::upcast(ctx, element),
             AnyViewState {
                 inner_state: Box::new(view_state),
                 generation,

xilem_core/src/element.rs

@@ -42,12 +42,12 @@ pub type Mut<'el, E> = <E as ViewElement>::Mut<'el>;
 ///
 /// [`AnyView`]: crate::AnyView
 /// [`ViewSequence`]: crate::ViewSequence
-pub trait SuperElement<Child>: ViewElement
+pub trait SuperElement<Child, Context>: ViewElement
 where
     Child: ViewElement,
 {
     /// Convert from the child to this element type.
-    fn upcast(child: Child) -> Self;
+    fn upcast(ctx: &mut Context, child: Child) -> Self;
 
     /// Perform a reborrowing downcast to the child reference type.
     ///
@@ -75,7 +75,7 @@ where
 }
 
 /// An element which can be used for an [`AnyView`](crate::AnyView) containing `Child`.
-pub trait AnyElement<Child>: SuperElement<Child>
+pub trait AnyElement<Child, Context>: SuperElement<Child, Context>
 where
     Child: ViewElement,
 {
@@ -95,8 +95,8 @@ impl ViewElement for NoElement {
     type Mut<'a> = ();
 }
 
-impl SuperElement<NoElement> for NoElement {
+impl<Context> SuperElement<NoElement, Context> for NoElement {
-    fn upcast(child: NoElement) -> Self {
+    fn upcast(_ctx: &mut Context, child: NoElement) -> Self {
         child
     }
 

xilem_core/src/sequence.rs

@@ -150,14 +150,14 @@ impl<State, Action, Context, V, Element, Message>
 where
     Context: ViewPathTracker,
     V: View<State, Action, Context, Message> + ViewMarker,
-    Element: SuperElement<V::Element>,
+    Element: SuperElement<V::Element, Context>,
     V::Element: ViewElement,
 {
     type SeqState = V::ViewState;
 
     fn seq_build(&self, ctx: &mut Context, elements: &mut AppendVec<Element>) -> Self::SeqState {
         let (element, view_state) = self.build(ctx);
-        elements.push(Element::upcast(element));
+        elements.push(Element::upcast(ctx, element));
         view_state
     }
     fn seq_rebuild(

xilem_core/src/views/one_of.rs

@@ -137,7 +137,10 @@ pub trait OneOfCtx<
     fn with_downcast_i(elem: &mut Mut<'_, Self::OneOfElement>, f: impl FnOnce(Mut<'_, I>));
 
     /// Creates the wrapping element, this is used in `View::build` to wrap the inner view element variant
-    fn upcast_one_of_element(elem: OneOf<A, B, C, D, E, F, G, H, I>) -> Self::OneOfElement;
+    fn upcast_one_of_element(
+        &mut self,
+        elem: OneOf<A, B, C, D, E, F, G, H, I>,
+    ) -> Self::OneOfElement;
 
     /// When the variant of the inner view element has changed, the wrapping element needs to be updated, this is used in `View::rebuild`
     fn update_one_of_element_mut(
@@ -233,7 +236,7 @@ where
             }
         });
         (
-            Context::upcast_one_of_element(element),
+            ctx.upcast_one_of_element(element),
             OneOfState {
                 generation,
                 inner_state: state,

xilem_core/tests/common/mod.rs

@@ -210,8 +210,8 @@ impl<const N: u32> View<(), Action, TestCtx> for OperationView<N> {
     }
 }
 
-impl SuperElement<TestElement> for TestElement {
+impl SuperElement<TestElement, TestCtx> for TestElement {
-    fn upcast(child: TestElement) -> Self {
+    fn upcast(_ctx: &mut TestCtx, child: TestElement) -> Self {
         child
     }
 
@@ -224,7 +224,7 @@ impl SuperElement<TestElement> for TestElement {
     }
 }
 
-impl AnyElement<TestElement> for TestElement {
+impl AnyElement<TestElement, TestCtx> for TestElement {
     fn replace_inner(this: Self::Mut<'_>, child: TestElement) -> Self::Mut<'_> {
         assert_eq!(child.operations.len(), 1);
         let Operation::Build(child_id) = child.operations.first().unwrap() else {

xilem_core/tests/one_of.rs

@@ -41,6 +41,7 @@ impl
     type OneOfElement = TestElement;
 
     fn upcast_one_of_element(
+        &mut self,
         elem: OneOf<
             TestElement,
             TestElement,

xilem_web/src/lib.rs

@@ -269,8 +269,8 @@ impl<E: DomNode<P>, P: 'static> ViewElement for Pod<E, P> {
     type Mut<'a> = PodMut<'a, E, P>;
 }
 
-impl<E: DomNode<P>, P: 'static> SuperElement<Pod<E, P>> for AnyPod {
+impl<E: DomNode<P>, P: 'static> SuperElement<Pod<E, P>, ViewCtx> for AnyPod {
-    fn upcast(child: Pod<E, P>) -> Self {
+    fn upcast(_ctx: &mut ViewCtx, child: Pod<E, P>) -> Self {
         Pod::into_dyn_node(child.node, child.props)
     }
 
@@ -284,7 +284,7 @@ impl<E: DomNode<P>, P: 'static> SuperElement<Pod<E, P>> for AnyPod {
     }
 }
 
-impl<E: DomNode<P>, P: 'static> AnyElement<Pod<E, P>> for AnyPod {
+impl<E: DomNode<P>, P: 'static> AnyElement<Pod<E, P>, ViewCtx> for AnyPod {
     fn replace_inner(mut this: Self::Mut<'_>, child: Pod<E, P>) -> Self::Mut<'_> {
         Pod::replace_inner(&mut this, child);
         this

xilem_web/src/one_of.rs

@@ -50,6 +50,7 @@ where
         Pod<OneOf<N1, N2, N3, N4, N5, N6, N7, N8, N9>, OneOf<P1, P2, P3, P4, P5, P6, P7, P8, P9>>;
 
     fn upcast_one_of_element(
+        &mut self,
         elem: OneOf<
             Pod<N1, P1>,
             Pod<N2, P2>,