add checkbox to widget example (#568)

This PR adds another widget to the widgets example, and also draws boxes
around each widget to separate them visually. I wanted to use flex wrap
to display them, but I don't think that's implemented yet in Xilem, and
the layout I did use isn't bad.

It also provides a simple example of the use of `Adapt`

---------

Co-authored-by: Daniel McNab <36049421+DJMcNab@users.noreply.github.com>

xilem/examples/widgets.rs

@@ -7,50 +7,95 @@ use masonry::dpi::LogicalSize;
 use masonry::event_loop_runner::{EventLoop, EventLoopBuilder};
 use winit::error::EventLoopError;
 use winit::window::Window;
-use xilem::view::{button, checkbox, flex, label, progress_bar, FlexSpacer};
-use xilem::{WidgetView, Xilem};
+use xilem::core::adapt;
+use xilem::view::{button, checkbox, flex, flex_item, progress_bar, sized_box, Axis, FlexSpacer};
+use xilem::{Color, WidgetView, Xilem};
+
+const SPACER_WIDTH: f64 = 10.;
 
 /// The state of the entire application.
 ///
 /// This is owned by Xilem, used to construct the view tree, and updated by event handlers.
 struct WidgetGallery {
     progress: Option<f64>,
+    checked: bool,
 }
 
-fn app_logic(data: &mut WidgetGallery) -> impl WidgetView<WidgetGallery> {
+fn progress_bar_view(data: Option<f64>) -> impl WidgetView<Option<f64>> {
     flex((
-        label("this 'widgets' example currently only has 1 widget"),
-        FlexSpacer::Flex(1.),
-        progress_bar(data.progress),
+        progress_bar(data),
         checkbox(
             "set indeterminate progress",
-            data.progress.is_none(),
-            |state: &mut WidgetGallery, checked| {
+            data.is_none(),
+            |state: &mut Option<f64>, checked| {
                 if checked {
-                    state.progress = None;
+                    *state = None;
                 } else {
-                    state.progress = Some(0.5);
+                    *state = Some(0.5);
                 }
             },
         ),
-        button("change progress", |state: &mut WidgetGallery| {
-            match state.progress {
+        button("change progress", |state: &mut Option<f64>| match state {
             Some(ref mut v) => *v = (*v + 0.1).rem_euclid(1.),
-                None => state.progress = Some(0.5),
-            }
+            None => *state = Some(0.5),
         }),
-        FlexSpacer::Flex(1.),
     ))
 }
 
+fn checkbox_view(data: bool) -> impl WidgetView<bool> {
+    checkbox("a simple checkbox", data, |data, new_state| {
+        *data = new_state;
+    })
+}
+
+/// Wrap `inner` in a box with a border
+fn border_box<State: 'static, Action: 'static>(
+    inner: impl WidgetView<State, Action>,
+) -> impl WidgetView<State, Action> {
+    sized_box(
+        flex((
+            FlexSpacer::Flex(1.),
+            flex((FlexSpacer::Flex(1.), inner, FlexSpacer::Flex(1.))),
+            FlexSpacer::Flex(1.),
+        ))
+        .direction(Axis::Horizontal),
+    )
+    .border(Color::WHITE, 2.)
+    .width(450.)
+    .height(200.)
+}
+
+/// Top-level view
+fn app_logic(data: &mut WidgetGallery) -> impl WidgetView<WidgetGallery> {
+    // Use a `sized_box` to pad the window contents
+    sized_box(
+        flex((
+            adapt(
+                flex_item(border_box(progress_bar_view(data.progress)), 1.),
+                |data: &mut WidgetGallery, thunk| thunk.call(&mut data.progress),
+            ),
+            adapt(
+                flex_item(border_box(checkbox_view(data.checked)), 1.),
+                |data: &mut WidgetGallery, thunk| thunk.call(&mut data.checked),
+            ),
+        ))
+        .gap(SPACER_WIDTH)
+        .direction(Axis::Horizontal),
+    )
+    .border(Color::TRANSPARENT, SPACER_WIDTH)
+}
+
 fn run(event_loop: EventLoopBuilder) -> Result<(), EventLoopError> {
+    // Set up the initial state of the app
     let data = WidgetGallery {
         progress: Some(0.5),
+        checked: false,
     };
 
+    // Instantiate and run the UI using the passed event loop.
     let app = Xilem::new(data, app_logic);
     let min_window_size = LogicalSize::new(300., 200.);
-    let window_size = LogicalSize::new(450., 300.);
+    let window_size = LogicalSize::new(650., 500.);
     let window_attributes = Window::default_attributes()
         .with_title("Xilem Widgets")
         .with_resizable(true)

xilem/src/view/mod.rs

@@ -1,6 +1,8 @@
 // Copyright 2024 the Xilem Authors
 // SPDX-License-Identifier: Apache-2.0
 
+//! Views for the widgets which are built-in to Masonry. These are the primitives your Xilem app's view tree will generally be constructed from.
+
 mod task;
 pub use task::*;