Merge remote-tracking branch 'origin/master' into feat/multi-window-support

# Conflicts:
#	Cargo.toml
#	core/src/window/icon.rs
#	core/src/window/id.rs
#	core/src/window/position.rs
#	core/src/window/settings.rs
#	examples/integration/src/main.rs
#	examples/integration_opengl/src/main.rs
#	glutin/src/application.rs
#	native/src/subscription.rs
#	native/src/window.rs
#	runtime/src/window/action.rs
#	src/lib.rs
#	src/window.rs
#	winit/Cargo.toml
#	winit/src/application.rs
#	winit/src/icon.rs
#	winit/src/settings.rs
#	winit/src/window.rs

7 files changed, 1187 insertions, 0 deletions

diff --git a/core/src/widget/id.rs b/core/src/widget/id.rs
new file mode 100644
index 00000000..ae739bb7
--- /dev/null
+++ b/core/src/widget/id.rs
@@ -0,0 +1,43 @@
+use std::borrow;
+use std::sync::atomic::{self, AtomicUsize};
+
+static NEXT_ID: AtomicUsize = AtomicUsize::new(0);
+
+/// The identifier of a generic widget.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Id(Internal);
+
+impl Id {
+    /// Creates a custom [`Id`].
+    pub fn new(id: impl Into<borrow::Cow<'static, str>>) -> Self {
+        Self(Internal::Custom(id.into()))
+    }
+
+    /// Creates a unique [`Id`].
+    ///
+    /// This function produces a different [`Id`] every time it is called.
+    pub fn unique() -> Self {
+        let id = NEXT_ID.fetch_add(1, atomic::Ordering::Relaxed);
+
+        Self(Internal::Unique(id))
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+enum Internal {
+    Unique(usize),
+    Custom(borrow::Cow<'static, str>),
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Id;
+
+    #[test]
+    fn unique_generates_different_ids() {
+        let a = Id::unique();
+        let b = Id::unique();
+
+        assert_ne!(a, b);
+    }
+}
diff --git a/core/src/widget/operation.rs b/core/src/widget/operation.rs
new file mode 100644
index 00000000..ad188c36
--- /dev/null
+++ b/core/src/widget/operation.rs
@@ -0,0 +1,226 @@
+//! Query or update internal widget state.
+pub mod focusable;
+pub mod scrollable;
+pub mod text_input;
+
+pub use focusable::Focusable;
+pub use scrollable::Scrollable;
+pub use text_input::TextInput;
+
+use crate::widget::Id;
+
+use std::any::Any;
+use std::fmt;
+use std::rc::Rc;
+
+/// A piece of logic that can traverse the widget tree of an application in
+/// order to query or update some widget state.
+pub trait Operation<T> {
+    /// Operates on a widget that contains other widgets.
+    ///
+    /// The `operate_on_children` function can be called to return control to
+    /// the widget tree and keep traversing it.
+    fn container(
+        &mut self,
+        id: Option<&Id>,
+        operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+    );
+
+    /// Operates on a widget that can be focused.
+    fn focusable(&mut self, _state: &mut dyn Focusable, _id: Option<&Id>) {}
+
+    /// Operates on a widget that can be scrolled.
+    fn scrollable(&mut self, _state: &mut dyn Scrollable, _id: Option<&Id>) {}
+
+    /// Operates on a widget that has text input.
+    fn text_input(&mut self, _state: &mut dyn TextInput, _id: Option<&Id>) {}
+
+    /// Operates on a custom widget with some state.
+    fn custom(&mut self, _state: &mut dyn Any, _id: Option<&Id>) {}
+
+    /// Finishes the [`Operation`] and returns its [`Outcome`].
+    fn finish(&self) -> Outcome<T> {
+        Outcome::None
+    }
+}
+
+/// The result of an [`Operation`].
+pub enum Outcome<T> {
+    /// The [`Operation`] produced no result.
+    None,
+
+    /// The [`Operation`] produced some result.
+    Some(T),
+
+    /// The [`Operation`] needs to be followed by another [`Operation`].
+    Chain(Box<dyn Operation<T>>),
+}
+
+impl<T> fmt::Debug for Outcome<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::None => write!(f, "Outcome::None"),
+            Self::Some(output) => write!(f, "Outcome::Some({output:?})"),
+            Self::Chain(_) => write!(f, "Outcome::Chain(...)"),
+        }
+    }
+}
+
+/// Maps the output of an [`Operation`] using the given function.
+pub fn map<A, B>(
+    operation: Box<dyn Operation<A>>,
+    f: impl Fn(A) -> B + 'static,
+) -> impl Operation<B>
+where
+    A: 'static,
+    B: 'static,
+{
+    #[allow(missing_debug_implementations)]
+    struct Map<A, B> {
+        operation: Box<dyn Operation<A>>,
+        f: Rc<dyn Fn(A) -> B>,
+    }
+
+    impl<A, B> Operation<B> for Map<A, B>
+    where
+        A: 'static,
+        B: 'static,
+    {
+        fn container(
+            &mut self,
+            id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<B>),
+        ) {
+            struct MapRef<'a, A> {
+                operation: &'a mut dyn Operation<A>,
+            }
+
+            impl<'a, A, B> Operation<B> for MapRef<'a, A> {
+                fn container(
+                    &mut self,
+                    id: Option<&Id>,
+                    operate_on_children: &mut dyn FnMut(&mut dyn Operation<B>),
+                ) {
+                    let Self { operation, .. } = self;
+
+                    operation.container(id, &mut |operation| {
+                        operate_on_children(&mut MapRef { operation });
+                    });
+                }
+
+                fn scrollable(
+                    &mut self,
+                    state: &mut dyn Scrollable,
+                    id: Option<&Id>,
+                ) {
+                    self.operation.scrollable(state, id);
+                }
+
+                fn focusable(
+                    &mut self,
+                    state: &mut dyn Focusable,
+                    id: Option<&Id>,
+                ) {
+                    self.operation.focusable(state, id);
+                }
+
+                fn text_input(
+                    &mut self,
+                    state: &mut dyn TextInput,
+                    id: Option<&Id>,
+                ) {
+                    self.operation.text_input(state, id);
+                }
+
+                fn custom(&mut self, state: &mut dyn Any, id: Option<&Id>) {
+                    self.operation.custom(state, id);
+                }
+            }
+
+            let Self { operation, .. } = self;
+
+            MapRef {
+                operation: operation.as_mut(),
+            }
+            .container(id, operate_on_children);
+        }
+
+        fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
+            self.operation.focusable(state, id);
+        }
+
+        fn scrollable(&mut self, state: &mut dyn Scrollable, id: Option<&Id>) {
+            self.operation.scrollable(state, id);
+        }
+
+        fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+            self.operation.text_input(state, id);
+        }
+
+        fn custom(&mut self, state: &mut dyn Any, id: Option<&Id>) {
+            self.operation.custom(state, id);
+        }
+
+        fn finish(&self) -> Outcome<B> {
+            match self.operation.finish() {
+                Outcome::None => Outcome::None,
+                Outcome::Some(output) => Outcome::Some((self.f)(output)),
+                Outcome::Chain(next) => Outcome::Chain(Box::new(Map {
+                    operation: next,
+                    f: self.f.clone(),
+                })),
+            }
+        }
+    }
+
+    Map {
+        operation,
+        f: Rc::new(f),
+    }
+}
+
+/// Produces an [`Operation`] that applies the given [`Operation`] to the
+/// children of a container with the given [`Id`].
+pub fn scope<T: 'static>(
+    target: Id,
+    operation: impl Operation<T> + 'static,
+) -> impl Operation<T> {
+    struct ScopedOperation<Message> {
+        target: Id,
+        operation: Box<dyn Operation<Message>>,
+    }
+
+    impl<Message: 'static> Operation<Message> for ScopedOperation<Message> {
+        fn container(
+            &mut self,
+            id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<Message>),
+        ) {
+            if id == Some(&self.target) {
+                operate_on_children(self.operation.as_mut());
+            } else {
+                operate_on_children(self);
+            }
+        }
+
+        fn finish(&self) -> Outcome<Message> {
+            match self.operation.finish() {
+                Outcome::Chain(next) => {
+                    Outcome::Chain(Box::new(ScopedOperation {
+                        target: self.target.clone(),
+                        operation: next,
+                    }))
+                }
+                outcome => outcome,
+            }
+        }
+    }
+
+    ScopedOperation {
+        target,
+        operation: Box::new(operation),
+    }
+}
diff --git a/core/src/widget/operation/focusable.rs b/core/src/widget/operation/focusable.rs
new file mode 100644
index 00000000..312e4894
--- /dev/null
+++ b/core/src/widget/operation/focusable.rs
@@ -0,0 +1,203 @@
+//! Operate on widgets that can be focused.
+use crate::widget::operation::{Operation, Outcome};
+use crate::widget::Id;
+
+/// The internal state of a widget that can be focused.
+pub trait Focusable {
+    /// Returns whether the widget is focused or not.
+    fn is_focused(&self) -> bool;
+
+    /// Focuses the widget.
+    fn focus(&mut self);
+
+    /// Unfocuses the widget.
+    fn unfocus(&mut self);
+}
+
+/// A summary of the focusable widgets present on a widget tree.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Count {
+    /// The index of the current focused widget, if any.
+    pub focused: Option<usize>,
+
+    /// The total amount of focusable widgets.
+    pub total: usize,
+}
+
+/// Produces an [`Operation`] that focuses the widget with the given [`Id`].
+pub fn focus<T>(target: Id) -> impl Operation<T> {
+    struct Focus {
+        target: Id,
+    }
+
+    impl<T> Operation<T> for Focus {
+        fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.focus();
+                }
+                _ => {
+                    state.unfocus();
+                }
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    Focus { target }
+}
+
+/// Produces an [`Operation`] that generates a [`Count`] and chains it with the
+/// provided function to build a new [`Operation`].
+pub fn count<T, O>(f: fn(Count) -> O) -> impl Operation<T>
+where
+    O: Operation<T> + 'static,
+{
+    struct CountFocusable<O> {
+        count: Count,
+        next: fn(Count) -> O,
+    }
+
+    impl<T, O> Operation<T> for CountFocusable<O>
+    where
+        O: Operation<T> + 'static,
+    {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            if state.is_focused() {
+                self.count.focused = Some(self.count.total);
+            }
+
+            self.count.total += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+
+        fn finish(&self) -> Outcome<T> {
+            Outcome::Chain(Box::new((self.next)(self.count)))
+        }
+    }
+
+    CountFocusable {
+        count: Count::default(),
+        next: f,
+    }
+}
+
+/// Produces an [`Operation`] that searches for the current focused widget, and
+/// - if found, focuses the previous focusable widget.
+/// - if not found, focuses the last focusable widget.
+pub fn focus_previous<T>() -> impl Operation<T> {
+    struct FocusPrevious {
+        count: Count,
+        current: usize,
+    }
+
+    impl<T> Operation<T> for FocusPrevious {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            if self.count.total == 0 {
+                return;
+            }
+
+            match self.count.focused {
+                None if self.current == self.count.total - 1 => state.focus(),
+                Some(0) if self.current == 0 => state.unfocus(),
+                Some(0) => {}
+                Some(focused) if focused == self.current => state.unfocus(),
+                Some(focused) if focused - 1 == self.current => state.focus(),
+                _ => {}
+            }
+
+            self.current += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    count(|count| FocusPrevious { count, current: 0 })
+}
+
+/// Produces an [`Operation`] that searches for the current focused widget, and
+/// - if found, focuses the next focusable widget.
+/// - if not found, focuses the first focusable widget.
+pub fn focus_next<T>() -> impl Operation<T> {
+    struct FocusNext {
+        count: Count,
+        current: usize,
+    }
+
+    impl<T> Operation<T> for FocusNext {
+        fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
+            match self.count.focused {
+                None if self.current == 0 => state.focus(),
+                Some(focused) if focused == self.current => state.unfocus(),
+                Some(focused) if focused + 1 == self.current => state.focus(),
+                _ => {}
+            }
+
+            self.current += 1;
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    count(|count| FocusNext { count, current: 0 })
+}
+
+/// Produces an [`Operation`] that searches for the current focused widget
+/// and stores its ID. This ignores widgets that do not have an ID.
+pub fn find_focused() -> impl Operation<Id> {
+    struct FindFocused {
+        focused: Option<Id>,
+    }
+
+    impl Operation<Id> for FindFocused {
+        fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
+            if state.is_focused() && id.is_some() {
+                self.focused = id.cloned();
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<Id>),
+        ) {
+            operate_on_children(self)
+        }
+
+        fn finish(&self) -> Outcome<Id> {
+            if let Some(id) = &self.focused {
+                Outcome::Some(id.clone())
+            } else {
+                Outcome::None
+            }
+        }
+    }
+
+    FindFocused { focused: None }
+}
diff --git a/core/src/widget/operation/scrollable.rs b/core/src/widget/operation/scrollable.rs
new file mode 100644
index 00000000..f947344d
--- /dev/null
+++ b/core/src/widget/operation/scrollable.rs
@@ -0,0 +1,93 @@
+//! Operate on widgets that can be scrolled.
+use crate::widget::{Id, Operation};
+
+/// The internal state of a widget that can be scrolled.
+pub trait Scrollable {
+    /// Snaps the scroll of the widget to the given `percentage` along the horizontal & vertical axis.
+    fn snap_to(&mut self, offset: RelativeOffset);
+
+    /// Scroll the widget to the given [`AbsoluteOffset`] along the horizontal & vertical axis.
+    fn scroll_to(&mut self, offset: AbsoluteOffset);
+}
+
+/// Produces an [`Operation`] that snaps the widget with the given [`Id`] to
+/// the provided `percentage`.
+pub fn snap_to<T>(target: Id, offset: RelativeOffset) -> impl Operation<T> {
+    struct SnapTo {
+        target: Id,
+        offset: RelativeOffset,
+    }
+
+    impl<T> Operation<T> for SnapTo {
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+
+        fn scrollable(&mut self, state: &mut dyn Scrollable, id: Option<&Id>) {
+            if Some(&self.target) == id {
+                state.snap_to(self.offset);
+            }
+        }
+    }
+
+    SnapTo { target, offset }
+}
+
+/// Produces an [`Operation`] that scrolls the widget with the given [`Id`] to
+/// the provided [`AbsoluteOffset`].
+pub fn scroll_to<T>(target: Id, offset: AbsoluteOffset) -> impl Operation<T> {
+    struct ScrollTo {
+        target: Id,
+        offset: AbsoluteOffset,
+    }
+
+    impl<T> Operation<T> for ScrollTo {
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+
+        fn scrollable(&mut self, state: &mut dyn Scrollable, id: Option<&Id>) {
+            if Some(&self.target) == id {
+                state.scroll_to(self.offset);
+            }
+        }
+    }
+
+    ScrollTo { target, offset }
+}
+
+/// The amount of absolute offset in each direction of a [`Scrollable`].
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct AbsoluteOffset {
+    /// The amount of horizontal offset
+    pub x: f32,
+    /// The amount of vertical offset
+    pub y: f32,
+}
+
+/// The amount of relative offset in each direction of a [`Scrollable`].
+///
+/// A value of `0.0` means start, while `1.0` means end.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct RelativeOffset {
+    /// The amount of horizontal offset
+    pub x: f32,
+    /// The amount of vertical offset
+    pub y: f32,
+}
+
+impl RelativeOffset {
+    /// A relative offset that points to the top-left of a [`Scrollable`].
+    pub const START: Self = Self { x: 0.0, y: 0.0 };
+
+    /// A relative offset that points to the bottom-right of a [`Scrollable`].
+    pub const END: Self = Self { x: 1.0, y: 1.0 };
+}
diff --git a/core/src/widget/operation/text_input.rs b/core/src/widget/operation/text_input.rs
new file mode 100644
index 00000000..4c773e99
--- /dev/null
+++ b/core/src/widget/operation/text_input.rs
@@ -0,0 +1,131 @@
+//! Operate on widgets that have text input.
+use crate::widget::operation::Operation;
+use crate::widget::Id;
+
+/// The internal state of a widget that has text input.
+pub trait TextInput {
+    /// Moves the cursor of the text input to the front of the input text.
+    fn move_cursor_to_front(&mut self);
+    /// Moves the cursor of the text input to the end of the input text.
+    fn move_cursor_to_end(&mut self);
+    /// Moves the cursor of the text input to an arbitrary location.
+    fn move_cursor_to(&mut self, position: usize);
+    /// Selects all the content of the text input.
+    fn select_all(&mut self);
+}
+
+/// Produces an [`Operation`] that moves the cursor of the widget with the given [`Id`] to the
+/// front.
+pub fn move_cursor_to_front<T>(target: Id) -> impl Operation<T> {
+    struct MoveCursor {
+        target: Id,
+    }
+
+    impl<T> Operation<T> for MoveCursor {
+        fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.move_cursor_to_front();
+                }
+                _ => {}
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    MoveCursor { target }
+}
+
+/// Produces an [`Operation`] that moves the cursor of the widget with the given [`Id`] to the
+/// end.
+pub fn move_cursor_to_end<T>(target: Id) -> impl Operation<T> {
+    struct MoveCursor {
+        target: Id,
+    }
+
+    impl<T> Operation<T> for MoveCursor {
+        fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.move_cursor_to_end();
+                }
+                _ => {}
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    MoveCursor { target }
+}
+
+/// Produces an [`Operation`] that moves the cursor of the widget with the given [`Id`] to the
+/// provided position.
+pub fn move_cursor_to<T>(target: Id, position: usize) -> impl Operation<T> {
+    struct MoveCursor {
+        target: Id,
+        position: usize,
+    }
+
+    impl<T> Operation<T> for MoveCursor {
+        fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.move_cursor_to(self.position);
+                }
+                _ => {}
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    MoveCursor { target, position }
+}
+
+/// Produces an [`Operation`] that selects all the content of the widget with the given [`Id`].
+pub fn select_all<T>(target: Id) -> impl Operation<T> {
+    struct MoveCursor {
+        target: Id,
+    }
+
+    impl<T> Operation<T> for MoveCursor {
+        fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+            match id {
+                Some(id) if id == &self.target => {
+                    state.select_all();
+                }
+                _ => {}
+            }
+        }
+
+        fn container(
+            &mut self,
+            _id: Option<&Id>,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+        ) {
+            operate_on_children(self)
+        }
+    }
+
+    MoveCursor { target }
+}
diff --git a/core/src/widget/text.rs b/core/src/widget/text.rs
new file mode 100644
index 00000000..79df2b02
--- /dev/null
+++ b/core/src/widget/text.rs
@@ -0,0 +1,304 @@
+//! Write some text for your users to read.
+use crate::alignment;
+use crate::layout;
+use crate::mouse;
+use crate::renderer;
+use crate::text;
+use crate::widget::Tree;
+use crate::{Color, Element, Layout, Length, Pixels, Rectangle, Widget};
+
+use std::borrow::Cow;
+
+pub use text::{LineHeight, Shaping};
+
+/// A paragraph of text.
+#[allow(missing_debug_implementations)]
+pub struct Text<'a, Renderer>
+where
+    Renderer: text::Renderer,
+    Renderer::Theme: StyleSheet,
+{
+    content: Cow<'a, str>,
+    size: Option<f32>,
+    line_height: LineHeight,
+    width: Length,
+    height: Length,
+    horizontal_alignment: alignment::Horizontal,
+    vertical_alignment: alignment::Vertical,
+    font: Option<Renderer::Font>,
+    shaping: Shaping,
+    style: <Renderer::Theme as StyleSheet>::Style,
+}
+
+impl<'a, Renderer> Text<'a, Renderer>
+where
+    Renderer: text::Renderer,
+    Renderer::Theme: StyleSheet,
+{
+    /// Create a new fragment of [`Text`] with the given contents.
+    pub fn new(content: impl Into<Cow<'a, str>>) -> Self {
+        Text {
+            content: content.into(),
+            size: None,
+            line_height: LineHeight::default(),
+            font: None,
+            width: Length::Shrink,
+            height: Length::Shrink,
+            horizontal_alignment: alignment::Horizontal::Left,
+            vertical_alignment: alignment::Vertical::Top,
+            shaping: Shaping::Basic,
+            style: Default::default(),
+        }
+    }
+
+    /// Sets the size of the [`Text`].
+    pub fn size(mut self, size: impl Into<Pixels>) -> Self {
+        self.size = Some(size.into().0);
+        self
+    }
+
+    /// Sets the [`LineHeight`] of the [`Text`].
+    pub fn line_height(mut self, line_height: impl Into<LineHeight>) -> Self {
+        self.line_height = line_height.into();
+        self
+    }
+
+    /// Sets the [`Font`] of the [`Text`].
+    ///
+    /// [`Font`]: crate::text::Renderer::Font
+    pub fn font(mut self, font: impl Into<Renderer::Font>) -> Self {
+        self.font = Some(font.into());
+        self
+    }
+
+    /// Sets the style of the [`Text`].
+    pub fn style(
+        mut self,
+        style: impl Into<<Renderer::Theme as StyleSheet>::Style>,
+    ) -> Self {
+        self.style = style.into();
+        self
+    }
+
+    /// Sets the width of the [`Text`] boundaries.
+    pub fn width(mut self, width: impl Into<Length>) -> Self {
+        self.width = width.into();
+        self
+    }
+
+    /// Sets the height of the [`Text`] boundaries.
+    pub fn height(mut self, height: impl Into<Length>) -> Self {
+        self.height = height.into();
+        self
+    }
+
+    /// Sets the [`alignment::Horizontal`] of the [`Text`].
+    pub fn horizontal_alignment(
+        mut self,
+        alignment: alignment::Horizontal,
+    ) -> Self {
+        self.horizontal_alignment = alignment;
+        self
+    }
+
+    /// Sets the [`alignment::Vertical`] of the [`Text`].
+    pub fn vertical_alignment(
+        mut self,
+        alignment: alignment::Vertical,
+    ) -> Self {
+        self.vertical_alignment = alignment;
+        self
+    }
+
+    /// Sets the [`Shaping`] strategy of the [`Text`].
+    pub fn shaping(mut self, shaping: Shaping) -> Self {
+        self.shaping = shaping;
+        self
+    }
+}
+
+impl<'a, Message, Renderer> Widget<Message, Renderer> for Text<'a, Renderer>
+where
+    Renderer: text::Renderer,
+    Renderer::Theme: StyleSheet,
+{
+    fn width(&self) -> Length {
+        self.width
+    }
+
+    fn height(&self) -> Length {
+        self.height
+    }
+
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        let limits = limits.width(self.width).height(self.height);
+
+        let size = self.size.unwrap_or_else(|| renderer.default_size());
+
+        let bounds = renderer.measure(
+            &self.content,
+            size,
+            self.line_height,
+            self.font.unwrap_or_else(|| renderer.default_font()),
+            limits.max(),
+            self.shaping,
+        );
+
+        let size = limits.resolve(bounds);
+
+        layout::Node::new(size)
+    }
+
+    fn draw(
+        &self,
+        _state: &Tree,
+        renderer: &mut Renderer,
+        theme: &Renderer::Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        _cursor_position: mouse::Cursor,
+        _viewport: &Rectangle,
+    ) {
+        draw(
+            renderer,
+            style,
+            layout,
+            &self.content,
+            self.size,
+            self.line_height,
+            self.font,
+            theme.appearance(self.style.clone()),
+            self.horizontal_alignment,
+            self.vertical_alignment,
+            self.shaping,
+        );
+    }
+}
+
+/// Draws text using the same logic as the [`Text`] widget.
+///
+/// Specifically:
+///
+/// * If no `size` is provided, the default text size of the `Renderer` will be
+///   used.
+/// * If no `color` is provided, the [`renderer::Style::text_color`] will be
+///   used.
+/// * The alignment attributes do not affect the position of the bounds of the
+///   [`Layout`].
+pub fn draw<Renderer>(
+    renderer: &mut Renderer,
+    style: &renderer::Style,
+    layout: Layout<'_>,
+    content: &str,
+    size: Option<f32>,
+    line_height: LineHeight,
+    font: Option<Renderer::Font>,
+    appearance: Appearance,
+    horizontal_alignment: alignment::Horizontal,
+    vertical_alignment: alignment::Vertical,
+    shaping: Shaping,
+) where
+    Renderer: text::Renderer,
+{
+    let bounds = layout.bounds();
+
+    let x = match horizontal_alignment {
+        alignment::Horizontal::Left => bounds.x,
+        alignment::Horizontal::Center => bounds.center_x(),
+        alignment::Horizontal::Right => bounds.x + bounds.width,
+    };
+
+    let y = match vertical_alignment {
+        alignment::Vertical::Top => bounds.y,
+        alignment::Vertical::Center => bounds.center_y(),
+        alignment::Vertical::Bottom => bounds.y + bounds.height,
+    };
+
+    let size = size.unwrap_or_else(|| renderer.default_size());
+
+    renderer.fill_text(crate::Text {
+        content,
+        size,
+        line_height,
+        bounds: Rectangle { x, y, ..bounds },
+        color: appearance.color.unwrap_or(style.text_color),
+        font: font.unwrap_or_else(|| renderer.default_font()),
+        horizontal_alignment,
+        vertical_alignment,
+        shaping,
+    });
+}
+
+impl<'a, Message, Renderer> From<Text<'a, Renderer>>
+    for Element<'a, Message, Renderer>
+where
+    Renderer: text::Renderer + 'a,
+    Renderer::Theme: StyleSheet,
+{
+    fn from(text: Text<'a, Renderer>) -> Element<'a, Message, Renderer> {
+        Element::new(text)
+    }
+}
+
+impl<'a, Renderer> Clone for Text<'a, Renderer>
+where
+    Renderer: text::Renderer,
+    Renderer::Theme: StyleSheet,
+{
+    fn clone(&self) -> Self {
+        Self {
+            content: self.content.clone(),
+            size: self.size,
+            line_height: self.line_height,
+            width: self.width,
+            height: self.height,
+            horizontal_alignment: self.horizontal_alignment,
+            vertical_alignment: self.vertical_alignment,
+            font: self.font,
+            style: self.style.clone(),
+            shaping: self.shaping,
+        }
+    }
+}
+
+impl<'a, Renderer> From<&'a str> for Text<'a, Renderer>
+where
+    Renderer: text::Renderer,
+    Renderer::Theme: StyleSheet,
+{
+    fn from(content: &'a str) -> Self {
+        Self::new(content)
+    }
+}
+
+impl<'a, Message, Renderer> From<&'a str> for Element<'a, Message, Renderer>
+where
+    Renderer: text::Renderer + 'a,
+    Renderer::Theme: StyleSheet,
+{
+    fn from(content: &'a str) -> Self {
+        Text::from(content).into()
+    }
+}
+
+/// The style sheet of some text.
+pub trait StyleSheet {
+    /// The supported style of the [`StyleSheet`].
+    type Style: Default + Clone;
+
+    /// Produces the [`Appearance`] of some text.
+    fn appearance(&self, style: Self::Style) -> Appearance;
+}
+
+/// The apperance of some text.
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Appearance {
+    /// The [`Color`] of the text.
+    ///
+    /// The default, `None`, means using the inherited color.
+    pub color: Option<Color>,
+}
diff --git a/core/src/widget/tree.rs b/core/src/widget/tree.rs
new file mode 100644
index 00000000..da269632
--- /dev/null
+++ b/core/src/widget/tree.rs
@@ -0,0 +1,187 @@
+//! Store internal widget state in a state tree to ensure continuity.
+use crate::Widget;
+
+use std::any::{self, Any};
+use std::borrow::Borrow;
+use std::fmt;
+
+/// A persistent state widget tree.
+///
+/// A [`Tree`] is normally associated with a specific widget in the widget tree.
+#[derive(Debug)]
+pub struct Tree {
+    /// The tag of the [`Tree`].
+    pub tag: Tag,
+
+    /// The [`State`] of the [`Tree`].
+    pub state: State,
+
+    /// The children of the root widget of the [`Tree`].
+    pub children: Vec<Tree>,
+}
+
+impl Tree {
+    /// Creates an empty, stateless [`Tree`] with no children.
+    pub fn empty() -> Self {
+        Self {
+            tag: Tag::stateless(),
+            state: State::None,
+            children: Vec::new(),
+        }
+    }
+
+    /// Creates a new [`Tree`] for the provided [`Widget`].
+    pub fn new<'a, Message, Renderer>(
+        widget: impl Borrow<dyn Widget<Message, Renderer> + 'a>,
+    ) -> Self
+    where
+        Renderer: crate::Renderer,
+    {
+        let widget = widget.borrow();
+
+        Self {
+            tag: widget.tag(),
+            state: widget.state(),
+            children: widget.children(),
+        }
+    }
+
+    /// Reconciliates the current tree with the provided [`Widget`].
+    ///
+    /// If the tag of the [`Widget`] matches the tag of the [`Tree`], then the
+    /// [`Widget`] proceeds with the reconciliation (i.e. [`Widget::diff`] is called).
+    ///
+    /// Otherwise, the whole [`Tree`] is recreated.
+    ///
+    /// [`Widget::diff`]: crate::Widget::diff
+    pub fn diff<'a, Message, Renderer>(
+        &mut self,
+        new: impl Borrow<dyn Widget<Message, Renderer> + 'a>,
+    ) where
+        Renderer: crate::Renderer,
+    {
+        if self.tag == new.borrow().tag() {
+            new.borrow().diff(self)
+        } else {
+            *self = Self::new(new);
+        }
+    }
+
+    /// Reconciles the children of the tree with the provided list of widgets.
+    pub fn diff_children<'a, Message, Renderer>(
+        &mut self,
+        new_children: &[impl Borrow<dyn Widget<Message, Renderer> + 'a>],
+    ) where
+        Renderer: crate::Renderer,
+    {
+        self.diff_children_custom(
+            new_children,
+            |tree, widget| tree.diff(widget.borrow()),
+            |widget| Self::new(widget.borrow()),
+        )
+    }
+
+    /// Reconciliates the children of the tree with the provided list of widgets using custom
+    /// logic both for diffing and creating new widget state.
+    pub fn diff_children_custom<T>(
+        &mut self,
+        new_children: &[T],
+        diff: impl Fn(&mut Tree, &T),
+        new_state: impl Fn(&T) -> Self,
+    ) {
+        if self.children.len() > new_children.len() {
+            self.children.truncate(new_children.len());
+        }
+
+        for (child_state, new) in
+            self.children.iter_mut().zip(new_children.iter())
+        {
+            diff(child_state, new);
+        }
+
+        if self.children.len() < new_children.len() {
+            self.children.extend(
+                new_children[self.children.len()..].iter().map(new_state),
+            );
+        }
+    }
+}
+
+/// The identifier of some widget state.
+#[derive(Debug, Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Hash)]
+pub struct Tag(any::TypeId);
+
+impl Tag {
+    /// Creates a [`Tag`] for a state of type `T`.
+    pub fn of<T>() -> Self
+    where
+        T: 'static,
+    {
+        Self(any::TypeId::of::<T>())
+    }
+
+    /// Creates a [`Tag`] for a stateless widget.
+    pub fn stateless() -> Self {
+        Self::of::<()>()
+    }
+}
+
+/// The internal [`State`] of a widget.
+pub enum State {
+    /// No meaningful internal state.
+    None,
+
+    /// Some meaningful internal state.
+    Some(Box<dyn Any>),
+}
+
+impl State {
+    /// Creates a new [`State`].
+    pub fn new<T>(state: T) -> Self
+    where
+        T: 'static,
+    {
+        State::Some(Box::new(state))
+    }
+
+    /// Downcasts the [`State`] to `T` and returns a reference to it.
+    ///
+    /// # Panics
+    /// This method will panic if the downcast fails or the [`State`] is [`State::None`].
+    pub fn downcast_ref<T>(&self) -> &T
+    where
+        T: 'static,
+    {
+        match self {
+            State::None => panic!("Downcast on stateless state"),
+            State::Some(state) => {
+                state.downcast_ref().expect("Downcast widget state")
+            }
+        }
+    }
+
+    /// Downcasts the [`State`] to `T` and returns a mutable reference to it.
+    ///
+    /// # Panics
+    /// This method will panic if the downcast fails or the [`State`] is [`State::None`].
+    pub fn downcast_mut<T>(&mut self) -> &mut T
+    where
+        T: 'static,
+    {
+        match self {
+            State::None => panic!("Downcast on stateless state"),
+            State::Some(state) => {
+                state.downcast_mut().expect("Downcast widget state")
+            }
+        }
+    }
+}
+
+impl fmt::Debug for State {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::None => write!(f, "State::None"),
+            Self::Some(_) => write!(f, "State::Some"),
+        }
+    }
+}