Introduce `black_box` and `chain` in `widget::operation`

7 files changed, 200 insertions, 35 deletions

diff --git a/core/src/element.rs b/core/src/element.rs
index 385d8295..6ebb8a15 100644
--- a/core/src/element.rs
+++ b/core/src/element.rs
@@ -304,7 +304,7 @@ where
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
-        operation: &mut dyn widget::Operation<()>,
+        operation: &mut dyn widget::Operation,
     ) {
         self.widget.operate(tree, layout, renderer, operation);
     }
@@ -440,7 +440,7 @@ where
         state: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
-        operation: &mut dyn widget::Operation<()>,
+        operation: &mut dyn widget::Operation,
     ) {
         self.element
             .widget
diff --git a/core/src/overlay.rs b/core/src/overlay.rs
index 3b79970e..f09de831 100644
--- a/core/src/overlay.rs
+++ b/core/src/overlay.rs
@@ -41,7 +41,7 @@ where
         &mut self,
         _layout: Layout<'_>,
         _renderer: &Renderer,
-        _operation: &mut dyn widget::Operation<()>,
+        _operation: &mut dyn widget::Operation,
     ) {
     }
 
diff --git a/core/src/overlay/element.rs b/core/src/overlay/element.rs
index 61e75e8a..32e987a3 100644
--- a/core/src/overlay/element.rs
+++ b/core/src/overlay/element.rs
@@ -92,7 +92,7 @@ where
         &mut self,
         layout: Layout<'_>,
         renderer: &Renderer,
-        operation: &mut dyn widget::Operation<()>,
+        operation: &mut dyn widget::Operation,
     ) {
         self.overlay.operate(layout, renderer, operation);
     }
@@ -144,7 +144,7 @@ where
         &mut self,
         layout: Layout<'_>,
         renderer: &Renderer,
-        operation: &mut dyn widget::Operation<()>,
+        operation: &mut dyn widget::Operation,
     ) {
         self.content.operate(layout, renderer, operation);
     }
diff --git a/core/src/overlay/group.rs b/core/src/overlay/group.rs
index cd12eac9..6541d311 100644
--- a/core/src/overlay/group.rs
+++ b/core/src/overlay/group.rs
@@ -132,7 +132,7 @@ where
         &mut self,
         layout: Layout<'_>,
         renderer: &Renderer,
-        operation: &mut dyn widget::Operation<()>,
+        operation: &mut dyn widget::Operation,
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children.iter_mut().zip(layout.children()).for_each(
diff --git a/core/src/widget.rs b/core/src/widget.rs
index 08cfa55b..c5beea54 100644
--- a/core/src/widget.rs
+++ b/core/src/widget.rs
@@ -105,7 +105,7 @@ where
         _state: &mut Tree,
         _layout: Layout<'_>,
         _renderer: &Renderer,
-        _operation: &mut dyn Operation<()>,
+        _operation: &mut dyn Operation,
     ) {
     }
 
diff --git a/core/src/widget/operation.rs b/core/src/widget/operation.rs
index 3e4ed618..741e1a5f 100644
--- a/core/src/widget/operation.rs
+++ b/core/src/widget/operation.rs
@@ -12,11 +12,12 @@ use crate::{Rectangle, Vector};
 
 use std::any::Any;
 use std::fmt;
+use std::marker::PhantomData;
 use std::sync::Arc;
 
 /// 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>: Send {
+pub trait Operation<T = ()>: Send {
     /// Operates on a widget that contains other widgets.
     ///
     /// The `operate_on_children` function can be called to return control to
@@ -53,6 +54,46 @@ pub trait Operation<T>: Send {
     }
 }
 
+impl<T, O> Operation<O> for Box<T>
+where
+    T: Operation<O> + ?Sized,
+{
+    fn container(
+        &mut self,
+        id: Option<&Id>,
+        bounds: Rectangle,
+        operate_on_children: &mut dyn FnMut(&mut dyn Operation<O>),
+    ) {
+        self.as_mut().container(id, bounds, operate_on_children);
+    }
+
+    fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
+        self.as_mut().focusable(state, id);
+    }
+
+    fn scrollable(
+        &mut self,
+        state: &mut dyn Scrollable,
+        id: Option<&Id>,
+        bounds: Rectangle,
+        translation: Vector,
+    ) {
+        self.as_mut().scrollable(state, id, bounds, translation);
+    }
+
+    fn text_input(&mut self, state: &mut dyn TextInput, id: Option<&Id>) {
+        self.as_mut().text_input(state, id);
+    }
+
+    fn custom(&mut self, state: &mut dyn Any, id: Option<&Id>) {
+        self.as_mut().custom(state, id);
+    }
+
+    fn finish(&self) -> Outcome<O> {
+        self.as_ref().finish()
+    }
+}
+
 /// The result of an [`Operation`].
 pub enum Outcome<T> {
     /// The [`Operation`] produced no result.
@@ -78,9 +119,62 @@ where
     }
 }
 
+/// Wraps the [`Operation`] in a black box, erasing its returning type.
+pub fn black_box<'a, T, O>(
+    operation: &'a mut dyn Operation<T>,
+) -> impl Operation<O> + 'a
+where
+    T: 'a,
+{
+    struct BlackBox<'a, T> {
+        operation: &'a mut dyn Operation<T>,
+    }
+
+    impl<'a, T, O> Operation<O> for BlackBox<'a, T> {
+        fn container(
+            &mut self,
+            id: Option<&Id>,
+            bounds: Rectangle,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<O>),
+        ) {
+            self.operation.container(id, bounds, &mut |operation| {
+                operate_on_children(&mut BlackBox { operation });
+            });
+        }
+
+        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>,
+            bounds: Rectangle,
+            translation: Vector,
+        ) {
+            self.operation.scrollable(state, id, bounds, translation);
+        }
+
+        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<O> {
+            Outcome::None
+        }
+    }
+
+    BlackBox { operation }
+}
+
 /// Maps the output of an [`Operation`] using the given function.
 pub fn map<A, B>(
-    operation: Box<dyn Operation<A>>,
+    operation: impl Operation<A>,
     f: impl Fn(A) -> B + Send + Sync + 'static,
 ) -> impl Operation<B>
 where
@@ -88,13 +182,14 @@ where
     B: 'static,
 {
     #[allow(missing_debug_implementations)]
-    struct Map<A, B> {
-        operation: Box<dyn Operation<A>>,
+    struct Map<O, A, B> {
+        operation: O,
         f: Arc<dyn Fn(A) -> B + Send + Sync>,
     }
 
-    impl<A, B> Operation<B> for Map<A, B>
+    impl<O, A, B> Operation<B> for Map<O, A, B>
     where
+        O: Operation<A>,
         A: 'static,
         B: 'static,
     {
@@ -155,10 +250,7 @@ where
 
             let Self { operation, .. } = self;
 
-            MapRef {
-                operation: operation.as_mut(),
-            }
-            .container(id, bounds, operate_on_children);
+            MapRef { operation }.container(id, bounds, operate_on_children);
         }
 
         fn focusable(&mut self, state: &mut dyn Focusable, id: Option<&Id>) {
@@ -201,6 +293,87 @@ where
     }
 }
 
+/// Chains the output of an [`Operation`] with the provided function to
+/// build a new [`Operation`].
+pub fn chain<A, B, O>(
+    operation: impl Operation<A> + 'static,
+    f: fn(A) -> O,
+) -> impl Operation<B>
+where
+    A: 'static,
+    B: Send + 'static,
+    O: Operation<B> + 'static,
+{
+    struct Chain<T, O, A, B>
+    where
+        T: Operation<A>,
+        O: Operation<B>,
+    {
+        operation: T,
+        next: fn(A) -> O,
+        _result: PhantomData<B>,
+    }
+
+    impl<T, O, A, B> Operation<B> for Chain<T, O, A, B>
+    where
+        T: Operation<A> + 'static,
+        O: Operation<B> + 'static,
+        A: 'static,
+        B: Send + 'static,
+    {
+        fn container(
+            &mut self,
+            id: Option<&Id>,
+            bounds: Rectangle,
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<B>),
+        ) {
+            self.operation.container(id, bounds, &mut |operation| {
+                operate_on_children(&mut black_box(operation));
+            });
+        }
+
+        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>,
+            bounds: Rectangle,
+            translation: crate::Vector,
+        ) {
+            self.operation.scrollable(state, id, bounds, translation);
+        }
+
+        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 std::any::Any, id: Option<&Id>) {
+            self.operation.custom(state, id);
+        }
+
+        fn finish(&self) -> Outcome<B> {
+            match self.operation.finish() {
+                Outcome::None => Outcome::None,
+                Outcome::Some(value) => {
+                    Outcome::Chain(Box::new((self.next)(value)))
+                }
+                Outcome::Chain(operation) => {
+                    Outcome::Chain(Box::new(chain(operation, self.next)))
+                }
+            }
+        }
+    }
+
+    Chain {
+        operation,
+        next: f,
+        _result: PhantomData,
+    }
+}
+
 /// Produces an [`Operation`] that applies the given [`Operation`] to the
 /// children of a container with the given [`Id`].
 pub fn scope<T: 'static>(
diff --git a/core/src/widget/operation/focusable.rs b/core/src/widget/operation/focusable.rs
index 68c22faa..0a6f2e96 100644
--- a/core/src/widget/operation/focusable.rs
+++ b/core/src/widget/operation/focusable.rs
@@ -1,5 +1,5 @@
 //! Operate on widgets that can be focused.
-use crate::widget::operation::{Operation, Outcome};
+use crate::widget::operation::{self, Operation, Outcome};
 use crate::widget::Id;
 use crate::Rectangle;
 
@@ -58,19 +58,12 @@ pub fn focus<T>(target: Id) -> impl Operation<T> {
 
 /// 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> {
+pub fn count() -> impl Operation<Count> {
+    struct CountFocusable {
         count: Count,
-        next: fn(Count) -> O,
     }
 
-    impl<T, O> Operation<T> for CountFocusable<O>
-    where
-        O: Operation<T> + 'static,
-    {
+    impl Operation<Count> for CountFocusable {
         fn focusable(&mut self, state: &mut dyn Focusable, _id: Option<&Id>) {
             if state.is_focused() {
                 self.count.focused = Some(self.count.total);
@@ -83,26 +76,25 @@ where
             &mut self,
             _id: Option<&Id>,
             _bounds: Rectangle,
-            operate_on_children: &mut dyn FnMut(&mut dyn Operation<T>),
+            operate_on_children: &mut dyn FnMut(&mut dyn Operation<Count>),
         ) {
             operate_on_children(self);
         }
 
-        fn finish(&self) -> Outcome<T> {
-            Outcome::Chain(Box::new((self.next)(self.count)))
+        fn finish(&self) -> Outcome<Count> {
+            Outcome::Some(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> {
+pub fn focus_previous() -> impl Operation {
     struct FocusPrevious {
         count: Count,
         current: usize,
@@ -136,13 +128,13 @@ pub fn focus_previous<T>() -> impl Operation<T> {
         }
     }
 
-    count(|count| FocusPrevious { count, current: 0 })
+    operation::chain(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> {
+pub fn focus_next() -> impl Operation {
     struct FocusNext {
         count: Count,
         current: usize,
@@ -170,7 +162,7 @@ pub fn focus_next<T>() -> impl Operation<T> {
         }
     }
 
-    count(|count| FocusNext { count, current: 0 })
+    operation::chain(count(), |count| FocusNext { count, current: 0 })
 }
 
 /// Produces an [`Operation`] that searches for the current focused widget