Create iced_widget subcrate and re-organize the whole codebase

core/src/widget/operation/scrollable.rs

@@ -0,0 +1,54 @@
+//! 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);
+}
+
+/// 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 }
+}
+
+/// The amount of 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 };
+}