Use ViewMarker trait instead of the generic Marker to allow `impl…

… ViewSequence` as return type (#472)

This allows returning `impl ViewSequence` as this was previously not
possibly as the `Marker` generic parameter couldn't be named (easily) in
`ViewSequence<..., Marker>`.

This also provides specialized `ViewSequence`s for xilem
(`WidgetViewSequence` and `FlexSequence`) as well as for xilem_web
(`DomFragment`). Additional doc-tests/documentation and a small example
(in `counter`) for xilem_web is provided as well.

This has the drawback to not being able to reeimplement `ViewSequence`
for types that already implement `View`, which was previously possible
(as seen by the now removed `NoElementView` `ViewSequence`
implementation).
And additionally by introducing more boilerplate by having to implement
`ViewMarker` for every type that implements `View`.

---------

Co-authored-by: Daniel McNab <[email protected]>

xilem/examples/calc.rs

@@ -5,7 +5,7 @@ use masonry::widget::{CrossAxisAlignment, MainAxisAlignment};
 use winit::dpi::LogicalSize;
 use winit::error::EventLoopError;
 use winit::window::Window;
-use xilem::view::Flex;
+use xilem::view::{Flex, FlexSequence};
 use xilem::EventLoopBuilder;
 use xilem::{
     view::{button, flex, label, sized_box, Axis, FlexExt as _, FlexSpacer},
@@ -254,7 +254,7 @@ fn app_logic(data: &mut Calculator) -> impl WidgetView<Calculator> {
 }
 
 /// Creates a horizontal centered flex row designed for the display portion of the calculator.
-pub fn centered_flex_row<Seq, Marker>(sequence: Seq) -> Flex<Seq, Marker> {
+pub fn centered_flex_row<State, Seq: FlexSequence<State>>(sequence: Seq) -> Flex<Seq, State> {
     flex(sequence)
         .direction(Axis::Horizontal)
         .cross_axis_alignment(CrossAxisAlignment::Center)
@@ -263,7 +263,7 @@ pub fn centered_flex_row<Seq, Marker>(sequence: Seq) -> Flex<Seq, Marker> {
 }
 
 /// Creates a horizontal filled flex row designed to be used in a grid.
-pub fn flex_row<Seq, Marker>(sequence: Seq) -> Flex<Seq, Marker> {
+pub fn flex_row<State, Seq: FlexSequence<State>>(sequence: Seq) -> Flex<Seq, State> {
     flex(sequence)
         .direction(Axis::Horizontal)
         .cross_axis_alignment(CrossAxisAlignment::Fill)

xilem/examples/elm.rs

@@ -26,7 +26,7 @@ enum CountMessage {
 
 // `map_action()` is basically how elm works, i.e. provide a message that the parent view has to handle to update the state.
 // In this case the parent adjusts the count that is given to this view according to the message
-fn elm_counter<T>(count: i32) -> impl WidgetView<T, CountMessage> {
+fn elm_counter<T: 'static>(count: i32) -> impl WidgetView<T, CountMessage> {
     flex((
         label(format!("elm count: {count}")),
         button("+", |_| CountMessage::Increment),

xilem/examples/mason.rs

@@ -109,16 +109,18 @@ fn app_logic(data: &mut AppData) -> impl WidgetView<AppData> {
 
 fn toggleable(data: &mut AppData) -> impl WidgetView<AppData> {
     if data.active {
-        flex((
-            button("Deactivate", |data: &mut AppData| {
-                data.active = false;
-            }),
-            button("Unlimited Power", |data: &mut AppData| {
-                data.count = -1_000_000;
-            }),
+        fork(
+            flex((
+                button("Deactivate", |data: &mut AppData| {
+                    data.active = false;
+                }),
+                button("Unlimited Power", |data: &mut AppData| {
+                    data.count = -1_000_000;
+                }),
+            ))
+            .direction(Axis::Horizontal),
             run_once(|| tracing::warn!("The pathway to unlimited power has been revealed")),
-        ))
-        .direction(Axis::Horizontal)
+        )
         .boxed()
     } else {
         button("Activate", |data: &mut AppData| data.active = true).boxed()

xilem/src/lib.rs

@@ -19,6 +19,7 @@ use winit::{
 };
 use xilem_core::{
     AsyncCtx, MessageResult, RawProxy, SuperElement, View, ViewElement, ViewId, ViewPathTracker,
+    ViewSequence,
 };
 
 pub use masonry::{
@@ -209,6 +210,30 @@ where
     type Widget = W;
 }
 
+/// An ordered sequence of widget views, it's used for `0..N` views.
+/// See [`ViewSequence`] for more technical details.
+///
+/// # Examples
+///
+/// ```
+/// use xilem::{view::prose, WidgetViewSequence};
+///
+/// fn prose_sequence<State: 'static>(
+///     texts: impl Iterator<Item = &'static str>,
+/// ) -> impl WidgetViewSequence<State> {
+///     texts.map(prose).collect::<Vec<_>>()
+/// }
+/// ```
+pub trait WidgetViewSequence<State, Action = ()>:
+    ViewSequence<State, Action, ViewCtx, Pod<any_view::DynWidget>>
+{
+}
+
+impl<Seq, State, Action> WidgetViewSequence<State, Action> for Seq where
+    Seq: ViewSequence<State, Action, ViewCtx, Pod<any_view::DynWidget>>
+{
+}
+
 type WidgetMap = HashMap<WidgetId, Vec<ViewId>>;
 
 pub struct ViewCtx {

xilem/src/view/async_repeat.rs

@@ -4,7 +4,9 @@
 use std::{future::Future, marker::PhantomData, sync::Arc};
 
 use tokio::task::JoinHandle;
-use xilem_core::{DynMessage, Message, MessageProxy, NoElement, View, ViewId, ViewPathTracker};
+use xilem_core::{
+    DynMessage, Message, MessageProxy, NoElement, View, ViewId, ViewMarker, ViewPathTracker,
+};
 
 use crate::ViewCtx;
 
@@ -70,6 +72,7 @@ pub struct AsyncRepeat<F, H, M> {
     message: PhantomData<fn() -> M>,
 }
 
+impl<F, H, M> ViewMarker for AsyncRepeat<F, H, M> {}
 impl<State, Action, F, H, M, Fut> View<State, Action, ViewCtx> for AsyncRepeat<F, H, M>
 where
     F: Fn(MessageProxy<M>) -> Fut + 'static,

xilem/src/view/button.rs

@@ -3,7 +3,7 @@
 
 use crate::{core::View, Pod};
 use masonry::{widget, ArcStr};
-use xilem_core::Mut;
+use xilem_core::{Mut, ViewMarker};
 
 pub use masonry::PointerButton;
 
@@ -41,6 +41,7 @@ pub struct Button<F> {
     callback: F,
 }
 
+impl<F> ViewMarker for Button<F> {}
 impl<F, State, Action> View<State, Action, ViewCtx> for Button<F>
 where
     F: Fn(&mut State, PointerButton) -> MessageResult<Action> + Send + Sync + 'static,

xilem/src/view/checkbox.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use masonry::{widget, ArcStr};
-use xilem_core::Mut;
+use xilem_core::{Mut, ViewMarker};
 
 use crate::{MessageResult, Pod, View, ViewCtx, ViewId};
 
@@ -27,6 +27,7 @@ pub struct Checkbox<F> {
     callback: F,
 }
 
+impl<F> ViewMarker for Checkbox<F> {}
 impl<F, State, Action> View<State, Action, ViewCtx> for Checkbox<F>
 where
     F: Fn(&mut State, bool) -> Action + Send + Sync + 'static,

xilem/src/view/flex.rs

@@ -9,36 +9,38 @@ use masonry::{
 };
 use xilem_core::{
     AppendVec, DynMessage, ElementSplice, MessageResult, Mut, SuperElement, View, ViewElement,
-    ViewId, ViewPathTracker, ViewSequence,
+    ViewId, ViewMarker, ViewPathTracker, ViewSequence,
 };
 
 use crate::{AnyWidgetView, Pod, ViewCtx, WidgetView};
 
 pub use masonry::widget::{Axis, CrossAxisAlignment, FlexParams, MainAxisAlignment};
 
-pub fn flex<Seq, Marker>(sequence: Seq) -> Flex<Seq, Marker> {
+pub fn flex<State, Action, Seq: FlexSequence<State, Action>>(
+    sequence: Seq,
+) -> Flex<Seq, State, Action> {
     Flex {
-        phantom: PhantomData,
         sequence,
         axis: Axis::Vertical,
         cross_axis_alignment: CrossAxisAlignment::Center,
         main_axis_alignment: MainAxisAlignment::Start,
         fill_major_axis: false,
         gap: None,
+        phantom: PhantomData,
     }
 }
 
-pub struct Flex<Seq, Marker> {
+pub struct Flex<Seq, State, Action = ()> {
     sequence: Seq,
     axis: Axis,
     cross_axis_alignment: CrossAxisAlignment,
     main_axis_alignment: MainAxisAlignment,
     fill_major_axis: bool,
-    phantom: PhantomData<fn() -> Marker>,
     gap: Option<f64>,
+    phantom: PhantomData<fn() -> (State, Action)>,
 }
 
-impl<Seq, Marker> Flex<Seq, Marker> {
+impl<Seq, State, Action> Flex<Seq, State, Action> {
     pub fn direction(mut self, axis: Axis) -> Self {
         self.axis = axis;
         self
@@ -85,9 +87,12 @@ impl<Seq, Marker> Flex<Seq, Marker> {
     }
 }
 
-impl<State, Action, Seq, Marker: 'static> View<State, Action, ViewCtx> for Flex<Seq, Marker>
+impl<Seq, State, Action> ViewMarker for Flex<Seq, State, Action> {}
+impl<State, Action, Seq> View<State, Action, ViewCtx> for Flex<Seq, State, Action>
 where
-    Seq: ViewSequence<State, Action, ViewCtx, FlexElement, Marker>,
+    State: 'static,
+    Action: 'static,
+    Seq: FlexSequence<State, Action>,
 {
     type Element = Pod<widget::Flex>;
 
@@ -303,6 +308,31 @@ impl ElementSplice<FlexElement> for FlexSplice<'_> {
     }
 }
 
+/// An ordered sequence of views for a [`Flex`] view.
+/// See [`ViewSequence`] for more technical details.
+///
+/// # Examples
+///
+/// ```
+/// use xilem::view::{label, FlexSequence, FlexExt as _};
+///
+/// fn label_sequence<State: 'static>(
+///     labels: impl Iterator<Item = &'static str>,
+///     flex: f64,
+/// ) -> impl FlexSequence<State> {
+///     labels.map(|l| label(l).flex(flex)).collect::<Vec<_>>()
+/// }
+/// ```
+pub trait FlexSequence<State, Action = ()>:
+    ViewSequence<State, Action, ViewCtx, FlexElement>
+{
+}
+
+impl<Seq, State, Action> FlexSequence<State, Action> for Seq where
+    Seq: ViewSequence<State, Action, ViewCtx, FlexElement>
+{
+}
+
 /// A trait which extends a [`WidgetView`] with methods to provide parameters for a flex item, or being able to use it interchangeably with a spacer
 pub trait FlexExt<State, Action>: WidgetView<State, Action> {
     /// Applies [`impl Into<FlexParams>`](`FlexParams`) to this view, can be used as child of a [`Flex`] [`View`]
@@ -407,6 +437,7 @@ where
     }
 }
 
+impl<V, State, Action> ViewMarker for FlexItem<V, State, Action> {}
 impl<State, Action, V> View<State, Action, ViewCtx> for FlexItem<V, State, Action>
 where
     State: 'static,
@@ -485,6 +516,7 @@ impl<State, Action> From<FlexSpacer> for AnyFlexChild<State, Action> {
     }
 }
 
+impl ViewMarker for FlexSpacer {}
 // This impl doesn't require a view id, as it neither receives, nor sends any messages
 // If this should ever change, it's necessary to adjust the `AnyFlexChild` `View` impl
 impl<State, Action> View<State, Action, ViewCtx> for FlexSpacer {
@@ -593,6 +625,7 @@ pub struct AnyFlexChildState<State: 'static, Action: 'static> {
     generation: u64,
 }
 
+impl<State, Action> ViewMarker for AnyFlexChild<State, Action> {}
 impl<State, Action> View<State, Action, ViewCtx> for AnyFlexChild<State, Action>
 where
     State: 'static,

xilem/src/view/label.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use masonry::{text::TextBrush, widget, ArcStr};
-use xilem_core::Mut;
+use xilem_core::{Mut, ViewMarker};
 
 use crate::{Color, MessageResult, Pod, TextAlignment, View, ViewCtx, ViewId};
 
@@ -43,6 +43,7 @@ impl Label {
     }
 }
 
+impl ViewMarker for Label {}
 impl<State, Action> View<State, Action, ViewCtx> for Label {
     type Element = Pod<widget::Label>;
     type ViewState = ();

xilem/src/view/prose.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use masonry::{text::TextBrush, widget, ArcStr};
-use xilem_core::Mut;
+use xilem_core::{Mut, ViewMarker};
 
 use crate::{Color, MessageResult, Pod, TextAlignment, View, ViewCtx, ViewId};
 
@@ -44,6 +44,7 @@ impl Prose {
     }
 }
 
+impl ViewMarker for Prose {}
 impl<State, Action> View<State, Action, ViewCtx> for Prose {
     type Element = Pod<widget::Prose>;
     type ViewState = ();

xilem/src/view/sized_box.rs

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use masonry::widget;
+use xilem_core::ViewMarker;
 
 use crate::{
     core::{Mut, View, ViewId},
@@ -74,6 +75,7 @@ impl<V> SizedBox<V> {
     }
 }
 
+impl<V> ViewMarker for SizedBox<V> {}
 impl<V, State, Action> View<State, Action, ViewCtx> for SizedBox<V>
 where
     V: WidgetView<State, Action>,

xilem/src/view/textbox.rs

@@ -2,7 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use masonry::{text::TextBrush, widget};
-use xilem_core::{Mut, View};
+use xilem_core::{Mut, View, ViewMarker};
 
 use crate::{Color, MessageResult, Pod, TextAlignment, ViewCtx, ViewId};
 
@@ -63,6 +63,7 @@ impl<State, Action> Textbox<State, Action> {
     }
 }
 
+impl<State, Action> ViewMarker for Textbox<State, Action> {}
 impl<State: 'static, Action: 'static> View<State, Action, ViewCtx> for Textbox<State, Action> {
     type Element = Pod<widget::Textbox>;
     type ViewState = ();

xilem_core/examples/filesystem.rs

@@ -4,7 +4,7 @@
 use std::{io::stdin, path::PathBuf};
 
 use xilem_core::{
-    AnyElement, AnyView, Mut, SuperElement, View, ViewElement, ViewId, ViewPathTracker,
+    AnyElement, AnyView, Mut, SuperElement, View, ViewElement, ViewId, ViewMarker, ViewPathTracker,
 };
 
 #[derive(Debug)]
@@ -145,6 +145,7 @@ impl ViewElement for FsPath {
     type Mut<'a> = &'a mut PathBuf;
 }
 
+impl ViewMarker for File {}
 impl<State, Action> View<State, Action, ViewCtx> for File {
     type Element = FsPath;
     type ViewState = ();

xilem_core/examples/user_interface.rs

@@ -6,7 +6,8 @@
 use core::any::Any;
 
 use xilem_core::{
-    DynMessage, MessageResult, Mut, SuperElement, View, ViewElement, ViewId, ViewPathTracker,
+    DynMessage, MessageResult, Mut, SuperElement, View, ViewElement, ViewId, ViewMarker,
+    ViewPathTracker,
 };
 
 pub fn app_logic(_: &mut u32) -> impl WidgetView<u32> {
@@ -44,6 +45,7 @@ impl<W: Widget> ViewElement for WidgetPod<W> {
     type Mut<'a> = WidgetMut<'a, W>;
 }
 
+impl ViewMarker for Button {}
 impl<State, Action> View<State, Action, ViewCtx> for Button {
     type Element = WidgetPod<ButtonWidget>;
     type ViewState = ();