Implement pop widget 🎉

core/src/rectangle.rs

@@ -143,6 +143,20 @@ impl Rectangle<f32> {
             && point.y < self.y + self.height
     }
 
+    /// Returns the minimum distance from the given [`Point`] to any of the edges
+    /// of the [`Rectangle`].
+    pub fn distance(&self, point: Point) -> f32 {
+        let center = self.center();
+
+        let distance_x =
+            ((point.x - center.x).abs() - self.width / 2.0).max(0.0);
+
+        let distance_y =
+            ((point.y - center.y).abs() - self.height / 2.0).max(0.0);
+
+        distance_x.hypot(distance_y)
+    }
+
     /// Returns true if the current [`Rectangle`] is completely within the given
     /// `container`.
     pub fn is_within(&self, container: &Rectangle) -> bool {

widget/src/helpers.rs

@@ -24,7 +24,7 @@ use crate::text_input::{self, TextInput};
 use crate::toggler::{self, Toggler};
 use crate::tooltip::{self, Tooltip};
 use crate::vertical_slider::{self, VerticalSlider};
-use crate::{Column, MouseArea, Pin, Row, Space, Stack, Themer};
+use crate::{Column, MouseArea, Pin, Pop, Row, Space, Stack, Themer};
 
 use std::borrow::Borrow;
 use std::ops::RangeInclusive;
@@ -970,6 +970,20 @@ where
     })
 }
 
+/// Creates a new [`Pop`] widget.
+///
+/// A [`Pop`] widget can generate messages when it pops in and out of view.
+/// It can even notify you with anticipation at a given distance!
+pub fn pop<'a, Message, Theme, Renderer>(
+    content: impl Into<Element<'a, Message, Theme, Renderer>>,
+) -> Pop<'a, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer,
+    Message: Clone,
+{
+    Pop::new(content)
+}
+
 /// Creates a new [`Scrollable`] with the provided content.
 ///
 /// Scrollables let users navigate an endless amount of content with a scrollbar.

widget/src/lib.rs

@@ -25,6 +25,7 @@ pub mod keyed;
 pub mod overlay;
 pub mod pane_grid;
 pub mod pick_list;
+pub mod pop;
 pub mod progress_bar;
 pub mod radio;
 pub mod rule;
@@ -66,6 +67,8 @@ pub use pick_list::PickList;
 #[doc(no_inline)]
 pub use pin::Pin;
 #[doc(no_inline)]
+pub use pop::Pop;
+#[doc(no_inline)]
 pub use progress_bar::ProgressBar;
 #[doc(no_inline)]
 pub use radio::Radio;

widget/src/pop.rs

@@ -0,0 +1,232 @@
+//! Generate messages when content pops in and out of view.
+use crate::core::layout;
+use crate::core::mouse;
+use crate::core::overlay;
+use crate::core::renderer;
+use crate::core::widget;
+use crate::core::widget::tree::{self, Tree};
+use crate::core::window;
+use crate::core::{
+    self, Clipboard, Element, Event, Layout, Length, Pixels, Rectangle, Shell,
+    Size, Vector, Widget,
+};
+
+/// A widget that can generate messages when its content pops in and out of view.
+///
+/// It can even notify you with anticipation at a given distance!
+#[allow(missing_debug_implementations)]
+pub struct Pop<'a, Message, Theme = crate::Theme, Renderer = crate::Renderer> {
+    content: Element<'a, Message, Theme, Renderer>,
+    on_show: Option<Message>,
+    on_hide: Option<Message>,
+    anticipate: Pixels,
+}
+
+impl<'a, Message, Theme, Renderer> Pop<'a, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer,
+    Message: Clone,
+{
+    /// Creates a new [`Pop`] widget with the given content.
+    pub fn new(
+        content: impl Into<Element<'a, Message, Theme, Renderer>>,
+    ) -> Self {
+        Self {
+            content: content.into(),
+            on_show: None,
+            on_hide: None,
+            anticipate: Pixels::ZERO,
+        }
+    }
+
+    /// Sets the message to be produced when the content pops into view.
+    pub fn on_show(mut self, on_show: Message) -> Self {
+        self.on_show = Some(on_show);
+        self
+    }
+
+    /// Sets the message to be produced when the content pops out of view.
+    pub fn on_hide(mut self, on_hide: Message) -> Self {
+        self.on_hide = Some(on_hide);
+        self
+    }
+
+    /// Sets the distance in [`Pixels`] to use in anticipation of the
+    /// content popping into view.
+    ///
+    /// This can be quite useful to lazily load items in a long scrollable
+    /// behind the scenes before the user can notice it!
+    pub fn anticipate(mut self, distance: impl Into<Pixels>) -> Self {
+        self.anticipate = distance.into();
+        self
+    }
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+struct State {
+    has_popped_in: bool,
+}
+
+impl<Message, Theme, Renderer> Widget<Message, Theme, Renderer>
+    for Pop<'_, Message, Theme, Renderer>
+where
+    Message: Clone,
+    Renderer: core::Renderer,
+{
+    fn tag(&self) -> tree::Tag {
+        tree::Tag::of::<State>()
+    }
+
+    fn state(&self) -> tree::State {
+        tree::State::new(State::default())
+    }
+
+    fn children(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.content)]
+    }
+
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(&[&self.content]);
+    }
+
+    fn update(
+        &mut self,
+        tree: &mut Tree,
+        event: Event,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        renderer: &Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, Message>,
+        viewport: &Rectangle,
+    ) {
+        if let Event::Window(window::Event::RedrawRequested(_)) = &event {
+            let state = tree.state.downcast_mut::<State>();
+            let bounds = layout.bounds();
+
+            let top_left_distance = viewport.distance(bounds.position());
+
+            let bottom_right_distance = viewport
+                .distance(bounds.position() + Vector::from(bounds.size()));
+
+            let distance = top_left_distance.min(bottom_right_distance);
+
+            if state.has_popped_in {
+                if let Some(on_hide) = &self.on_hide {
+                    if distance > self.anticipate.0 {
+                        state.has_popped_in = false;
+                        shell.publish(on_hide.clone());
+                    }
+                }
+            } else if let Some(on_show) = &self.on_show {
+                if distance <= self.anticipate.0 {
+                    state.has_popped_in = true;
+                    shell.publish(on_show.clone());
+                }
+            }
+        }
+
+        self.content.as_widget_mut().update(
+            tree, event, layout, cursor, renderer, clipboard, shell, viewport,
+        );
+    }
+
+    fn size(&self) -> Size<Length> {
+        self.content.as_widget().size()
+    }
+
+    fn size_hint(&self) -> Size<Length> {
+        self.content.as_widget().size_hint()
+    }
+
+    fn layout(
+        &self,
+        tree: &mut Tree,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.content
+            .as_widget()
+            .layout(&mut tree.children[0], renderer, limits)
+    }
+
+    fn draw(
+        &self,
+        tree: &Tree,
+        renderer: &mut Renderer,
+        theme: &Theme,
+        style: &renderer::Style,
+        layout: layout::Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+    ) {
+        self.content.as_widget().draw(
+            &tree.children[0],
+            renderer,
+            theme,
+            style,
+            layout,
+            cursor,
+            viewport,
+        );
+    }
+
+    fn operate(
+        &self,
+        tree: &mut Tree,
+        layout: core::Layout<'_>,
+        renderer: &Renderer,
+        operation: &mut dyn widget::Operation,
+    ) {
+        self.content.as_widget().operate(
+            &mut tree.children[0],
+            layout,
+            renderer,
+            operation,
+        );
+    }
+
+    fn mouse_interaction(
+        &self,
+        tree: &Tree,
+        layout: core::Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+        renderer: &Renderer,
+    ) -> mouse::Interaction {
+        self.content.as_widget().mouse_interaction(
+            &tree.children[0],
+            layout,
+            cursor,
+            viewport,
+            renderer,
+        )
+    }
+
+    fn overlay<'b>(
+        &'b mut self,
+        tree: &'b mut Tree,
+        layout: core::Layout<'_>,
+        renderer: &Renderer,
+        translation: core::Vector,
+    ) -> Option<overlay::Element<'b, Message, Theme, Renderer>> {
+        self.content.as_widget_mut().overlay(
+            &mut tree.children[0],
+            layout,
+            renderer,
+            translation,
+        )
+    }
+}
+
+impl<'a, Message, Theme, Renderer> From<Pop<'a, Message, Theme, Renderer>>
+    for Element<'a, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer + 'a,
+    Theme: 'a,
+    Message: Clone + 'a,
+{
+    fn from(pop: Pop<'a, Message, Theme, Renderer>) -> Self {
+        Element::new(pop)
+    }
+}