Implement pure version of `todos` example :tada:

The `Widget` trait in `iced_pure` needed to change a bit to make the
implementation of `Element::map` possible.

Specifically, the `children` method has been split into `diff` and
`children_state`.

12 files changed, 261 insertions, 94 deletions

diff --git a/pure/src/lib.rs b/pure/src/lib.rs
index a179a84b..07f068cc 100644
--- a/pure/src/lib.rs
+++ b/pure/src/lib.rs
@@ -53,7 +53,8 @@ impl State {
 impl<'a, Message, Renderer> iced_native::Widget<Message, Renderer>
     for Pure<'a, Message, Renderer>
 where
-    Renderer: iced_native::Renderer,
+    Message: 'a,
+    Renderer: iced_native::Renderer + 'a,
 {
     fn width(&self) -> Length {
         self.element.as_widget().width()
diff --git a/pure/src/widget.rs b/pure/src/widget.rs
index 8df64426..302a057a 100644
--- a/pure/src/widget.rs
+++ b/pure/src/widget.rs
@@ -33,7 +33,9 @@ pub trait Widget<Message, Renderer> {
 
     fn state(&self) -> Box<dyn Any>;
 
-    fn children(&self) -> &[Element<Message, Renderer>];
+    fn diff(&self, tree: &mut Tree);
+
+    fn children_state(&self) -> Vec<Tree>;
 
     fn width(&self) -> Length;
 
@@ -99,11 +101,13 @@ pub fn row<'a, Message, Renderer>() -> Row<'a, Message, Renderer> {
     Row::new()
 }
 
-pub fn scrollable<'a, Message, Renderer>() -> Scrollable<'a, Message, Renderer>
+pub fn scrollable<'a, Message, Renderer>(
+    content: impl Into<Element<'a, Message, Renderer>>,
+) -> Scrollable<'a, Message, Renderer>
 where
     Renderer: iced_native::Renderer,
 {
-    Scrollable::new()
+    Scrollable::new(content)
 }
 
 pub fn button<'a, Message, Renderer>(
diff --git a/pure/src/widget/button.rs b/pure/src/widget/button.rs
index d0f9e53e..6dc1016c 100644
--- a/pure/src/widget/button.rs
+++ b/pure/src/widget/button.rs
@@ -85,8 +85,12 @@ where
         Box::new(State::new())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        std::slice::from_ref(&self.content)
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(std::slice::from_ref(&self.content))
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.content)]
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/checkbox.rs b/pure/src/widget/checkbox.rs
index 8ee2b7bb..1cfe89a8 100644
--- a/pure/src/widget/checkbox.rs
+++ b/pure/src/widget/checkbox.rs
@@ -22,8 +22,10 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &[]
+    fn diff(&self, _tree: &mut Tree) {}
+
+    fn children_state(&self) -> Vec<Tree> {
+        Vec::new()
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/column.rs b/pure/src/widget/column.rs
index ed097d33..68d3c4b4 100644
--- a/pure/src/widget/column.rs
+++ b/pure/src/widget/column.rs
@@ -85,8 +85,12 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &self.children
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(&self.children);
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        self.children.iter().map(Tree::new).collect()
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/container.rs b/pure/src/widget/container.rs
index 94a6b07b..85ea8039 100644
--- a/pure/src/widget/container.rs
+++ b/pure/src/widget/container.rs
@@ -132,8 +132,12 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        std::slice::from_ref(&self.content)
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(std::slice::from_ref(&self.content))
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.content)]
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/element.rs b/pure/src/widget/element.rs
index aedf5973..2a137d40 100644
--- a/pure/src/widget/element.rs
+++ b/pure/src/widget/element.rs
@@ -1,4 +1,12 @@
-use crate::Widget;
+use crate::widget::{Tree, Widget};
+
+use iced_native::event::{self, Event};
+use iced_native::layout::{self, Layout};
+use iced_native::mouse;
+use iced_native::renderer;
+use iced_native::{Clipboard, Hasher, Length, Point, Rectangle, Shell};
+
+use std::any::{self, Any};
 
 pub struct Element<'a, Message, Renderer> {
     widget: Box<dyn Widget<Message, Renderer> + 'a>,
@@ -18,4 +26,143 @@ impl<'a, Message, Renderer> Element<'a, Message, Renderer> {
     pub fn as_widget_mut(&mut self) -> &mut dyn Widget<Message, Renderer> {
         self.widget.as_mut()
     }
+
+    pub fn map<B>(
+        self,
+        f: impl Fn(Message) -> B + 'a,
+    ) -> Element<'a, B, Renderer>
+    where
+        Message: 'a,
+        Renderer: iced_native::Renderer + 'a,
+        B: 'a,
+    {
+        Element::new(Map::new(self.widget, f))
+    }
+}
+
+struct Map<'a, A, B, Renderer> {
+    widget: Box<dyn Widget<A, Renderer> + 'a>,
+    mapper: Box<dyn Fn(A) -> B + 'a>,
+}
+
+impl<'a, A, B, Renderer> Map<'a, A, B, Renderer> {
+    pub fn new<F>(
+        widget: Box<dyn Widget<A, Renderer> + 'a>,
+        mapper: F,
+    ) -> Map<'a, A, B, Renderer>
+    where
+        F: 'a + Fn(A) -> B,
+    {
+        Map {
+            widget,
+            mapper: Box::new(mapper),
+        }
+    }
+}
+
+impl<'a, A, B, Renderer> Widget<B, Renderer> for Map<'a, A, B, Renderer>
+where
+    Renderer: iced_native::Renderer + 'a,
+    A: 'a,
+    B: 'a,
+{
+    fn tag(&self) -> any::TypeId {
+        self.widget.tag()
+    }
+
+    fn state(&self) -> Box<dyn Any> {
+        self.widget.state()
+    }
+
+    fn diff(&self, tree: &mut Tree) {
+        self.widget.diff(tree)
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        self.widget.children_state()
+    }
+
+    fn width(&self) -> Length {
+        self.widget.width()
+    }
+
+    fn height(&self) -> Length {
+        self.widget.height()
+    }
+
+    fn layout(
+        &self,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.widget.layout(renderer, limits)
+    }
+
+    fn on_event(
+        &mut self,
+        tree: &mut Tree,
+        event: Event,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        renderer: &Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, B>,
+    ) -> event::Status {
+        let mut local_messages = Vec::new();
+        let mut local_shell = Shell::new(&mut local_messages);
+
+        let status = self.widget.on_event(
+            tree,
+            event,
+            layout,
+            cursor_position,
+            renderer,
+            clipboard,
+            &mut local_shell,
+        );
+
+        shell.merge(local_shell, &self.mapper);
+
+        status
+    }
+
+    fn draw(
+        &self,
+        tree: &Tree,
+        renderer: &mut Renderer,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+    ) {
+        self.widget.draw(
+            tree,
+            renderer,
+            style,
+            layout,
+            cursor_position,
+            viewport,
+        )
+    }
+
+    fn mouse_interaction(
+        &self,
+        tree: &Tree,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        viewport: &Rectangle,
+        renderer: &Renderer,
+    ) -> mouse::Interaction {
+        self.widget.mouse_interaction(
+            tree,
+            layout,
+            cursor_position,
+            viewport,
+            renderer,
+        )
+    }
+
+    fn hash_layout(&self, state: &mut Hasher) {
+        self.widget.hash_layout(state);
+    }
 }
diff --git a/pure/src/widget/row.rs b/pure/src/widget/row.rs
index 147a0850..ec7e144c 100644
--- a/pure/src/widget/row.rs
+++ b/pure/src/widget/row.rs
@@ -85,8 +85,12 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &self.children
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(&self.children)
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        self.children.iter().map(Tree::new).collect()
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/scrollable.rs b/pure/src/widget/scrollable.rs
index 69acabb7..badc9fc2 100644
--- a/pure/src/widget/scrollable.rs
+++ b/pure/src/widget/scrollable.rs
@@ -1,4 +1,4 @@
-use crate::widget::{Column, Tree};
+use crate::widget::Tree;
 use crate::{Element, Widget};
 
 use iced_native::event::{self, Event};
@@ -6,9 +6,7 @@ use iced_native::layout::{self, Layout};
 use iced_native::mouse;
 use iced_native::renderer;
 use iced_native::widget::scrollable;
-use iced_native::{
-    Alignment, Clipboard, Hasher, Length, Padding, Point, Rectangle, Shell,
-};
+use iced_native::{Clipboard, Hasher, Length, Point, Rectangle, Shell};
 
 pub use iced_style::scrollable::StyleSheet;
 
@@ -22,61 +20,33 @@ pub struct Scrollable<'a, Message, Renderer> {
     scrollbar_width: u16,
     scrollbar_margin: u16,
     scroller_width: u16,
-    content: Column<'a, Message, Renderer>,
     on_scroll: Option<Box<dyn Fn(f32) -> Message>>,
     style_sheet: Box<dyn StyleSheet + 'a>,
+    content: Element<'a, Message, Renderer>,
 }
 
 impl<'a, Message, Renderer: iced_native::Renderer>
     Scrollable<'a, Message, Renderer>
 {
-    /// Creates a new [`Scrollable`] with the given [`State`].
-    pub fn new() -> Self {
+    /// Creates a new [`Scrollable`].
+    pub fn new(content: impl Into<Element<'a, Message, Renderer>>) -> Self {
         Scrollable {
             height: Length::Shrink,
             scrollbar_width: 10,
             scrollbar_margin: 0,
             scroller_width: 10,
-            content: Column::new(),
             on_scroll: None,
             style_sheet: Default::default(),
+            content: content.into(),
         }
     }
 
-    /// Sets the vertical spacing _between_ elements.
-    ///
-    /// Custom margins per element do not exist in Iced. You should use this
-    /// method instead! While less flexible, it helps you keep spacing between
-    /// elements consistent.
-    pub fn spacing(mut self, units: u16) -> Self {
-        self.content = self.content.spacing(units);
-        self
-    }
-
-    /// Sets the [`Padding`] of the [`Scrollable`].
-    pub fn padding<P: Into<Padding>>(mut self, padding: P) -> Self {
-        self.content = self.content.padding(padding);
-        self
-    }
-
-    /// Sets the width of the [`Scrollable`].
-    pub fn width(mut self, width: Length) -> Self {
-        self.content = self.content.width(width);
-        self
-    }
-
     /// Sets the height of the [`Scrollable`].
     pub fn height(mut self, height: Length) -> Self {
         self.height = height;
         self
     }
 
-    /// Sets the horizontal alignment of the contents of the [`Scrollable`] .
-    pub fn align_items(mut self, align_items: Alignment) -> Self {
-        self.content = self.content.align_items(align_items);
-        self
-    }
-
     /// Sets the scrollbar width of the [`Scrollable`] .
     /// Silently enforces a minimum value of 1.
     pub fn scrollbar_width(mut self, scrollbar_width: u16) -> Self {
@@ -115,15 +85,6 @@ impl<'a, Message, Renderer: iced_native::Renderer>
         self.style_sheet = style_sheet.into();
         self
     }
-
-    /// Adds an element to the [`Scrollable`].
-    pub fn push<E>(mut self, child: E) -> Self
-    where
-        E: Into<Element<'a, Message, Renderer>>,
-    {
-        self.content = self.content.push(child);
-        self
-    }
 }
 
 impl<'a, Message, Renderer> Widget<Message, Renderer>
@@ -139,12 +100,16 @@ where
         Box::new(scrollable::State::new())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        self.content.children()
+    fn diff(&self, tree: &mut Tree) {
+        tree.diff_children(std::slice::from_ref(&self.content))
+    }
+
+    fn children_state(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.content)]
     }
 
     fn width(&self) -> Length {
-        Widget::<Message, Renderer>::width(&self.content)
+        self.content.as_widget().width()
     }
 
     fn height(&self) -> Length {
@@ -156,7 +121,7 @@ where
 
         self.tag().hash(state);
         self.height.hash(state);
-        self.content.hash_layout(state)
+        self.content.as_widget().hash_layout(state)
     }
 
     fn layout(
@@ -169,7 +134,9 @@ where
             limits,
             Widget::<Message, Renderer>::width(self),
             self.height,
-            |renderer, limits| self.content.layout(renderer, limits),
+            |renderer, limits| {
+                self.content.as_widget().layout(renderer, limits)
+            },
         )
     }
 
@@ -195,7 +162,7 @@ where
             self.scroller_width,
             &self.on_scroll,
             |event, layout, cursor_position, clipboard, shell| {
-                self.content.on_event(
+                self.content.as_widget_mut().on_event(
                     &mut tree.children[0],
                     event,
                     layout,
@@ -227,7 +194,7 @@ where
             self.scroller_width,
             self.style_sheet.as_ref(),
             |renderer, layout, cursor_position, viewport| {
-                self.content.draw(
+                self.content.as_widget().draw(
                     &tree.children[0],
                     renderer,
                     style,
@@ -255,7 +222,7 @@ where
             self.scrollbar_margin,
             self.scroller_width,
             |layout, cursor_position, viewport| {
-                self.content.mouse_interaction(
+                self.content.as_widget().mouse_interaction(
                     &tree.children[0],
                     layout,
                     cursor_position,
@@ -266,3 +233,16 @@ where
         )
     }
 }
+
+impl<'a, Message, Renderer> From<Scrollable<'a, Message, Renderer>>
+    for Element<'a, Message, Renderer>
+where
+    Message: 'a + Clone,
+    Renderer: 'a + iced_native::Renderer,
+{
+    fn from(
+        text_input: Scrollable<'a, Message, Renderer>,
+    ) -> Element<'a, Message, Renderer> {
+        Element::new(text_input)
+    }
+}
diff --git a/pure/src/widget/text.rs b/pure/src/widget/text.rs
index f437b48b..8f157ea0 100644
--- a/pure/src/widget/text.rs
+++ b/pure/src/widget/text.rs
@@ -21,8 +21,10 @@ where
         Box::new(())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &[]
+    fn diff(&self, _tree: &mut Tree) {}
+
+    fn children_state(&self) -> Vec<Tree> {
+        Vec::new()
     }
 
     fn width(&self) -> Length {
diff --git a/pure/src/widget/text_input.rs b/pure/src/widget/text_input.rs
index 06ab2910..e18a2bf0 100644
--- a/pure/src/widget/text_input.rs
+++ b/pure/src/widget/text_input.rs
@@ -146,8 +146,10 @@ where
         Box::new(text_input::State::new())
     }
 
-    fn children(&self) -> &[Element<Message, Renderer>] {
-        &[]
+    fn diff(&self, _tree: &mut Tree) {}
+
+    fn children_state(&self) -> Vec<Tree> {
+        Vec::new()
     }
 
     fn width(&self) -> Length {
@@ -237,3 +239,16 @@ where
         text_input::mouse_interaction(layout, cursor_position)
     }
 }
+
+impl<'a, Message, Renderer> From<TextInput<'a, Message, Renderer>>
+    for Element<'a, Message, Renderer>
+where
+    Message: 'a + Clone,
+    Renderer: 'a + text::Renderer,
+{
+    fn from(
+        text_input: TextInput<'a, Message, Renderer>,
+    ) -> Element<'a, Message, Renderer> {
+        Element::new(text_input)
+    }
+}
diff --git a/pure/src/widget/tree.rs b/pure/src/widget/tree.rs
index 98e976ad..3a5f4433 100644
--- a/pure/src/widget/tree.rs
+++ b/pure/src/widget/tree.rs
@@ -23,12 +23,7 @@ impl Tree {
         Self {
             tag: element.as_widget().tag(),
             state: State(element.as_widget().state()),
-            children: element
-                .as_widget()
-                .children()
-                .iter()
-                .map(Self::new)
-                .collect(),
+            children: element.as_widget().children_state(),
         }
     }
 
@@ -37,25 +32,30 @@ impl Tree {
         new: &Element<'_, Message, Renderer>,
     ) {
         if self.tag == new.as_widget().tag() {
-            let new_children = new.as_widget().children();
+            new.as_widget().diff(self)
+        } else {
+            *self = Self::new(new);
+        }
+    }
 
-            if self.children.len() > new_children.len() {
-                self.children.truncate(new_children.len());
-            }
+    pub fn diff_children<Message, Renderer>(
+        &mut self,
+        new_children: &[Element<'_, Message, Renderer>],
+    ) {
+        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())
-            {
-                child_state.diff(new);
-            }
+        for (child_state, new) in
+            self.children.iter_mut().zip(new_children.iter())
+        {
+            child_state.diff(new);
+        }
 
-            if self.children.len() < new_children.len() {
-                self.children.extend(
-                    new_children[self.children.len()..].iter().map(Self::new),
-                );
-            }
-        } else {
-            *self = Self::new(new);
+        if self.children.len() < new_children.len() {
+            self.children.extend(
+                new_children[self.children.len()..].iter().map(Self::new),
+            );
         }
     }
 }