Add Ref lens and use move semantics for lens! macro index.

CHANGELOG.md

@@ -19,6 +19,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - `request_update` in `EventCtx`. ([#1128] by [@raphlinus])
 - `ExtEventSink`s can now be obtained from widget methods. ([#1152] by [@jneem])
 - 'Scope' widget to allow encapsulation of reactive state. ([#1151] by [@rjwittams])
+- `Ref` lens that applies `AsRef` and thus allow indexing arrays. ([#1171] by [@finnerale])
 
 ### Changed
 
@@ -27,8 +28,8 @@ You can find its changes [documented below](#060---2020-06-01).
 - `Container::rounded` takes `KeyOrValue<f64>` instead of `f64`. ([#1054] by [@binomial0])
 - `request_anim_frame` no longer invalidates the entire window. ([#1057] by [@jneem])
 - Use new Piet text api ([#1143] by [@cmyr])
-- `Env::try_get` (and related methods) return a `Result` instead of an `Option`.
-  ([#1172] by [@cmyr])
+- `Env::try_get` (and related methods) return a `Result` instead of an `Option`. ([#1172] by [@cmyr])
+- `lens!` macro to use move semantics for the index. ([#1171] by [@finnerale])
 
 ### Deprecated
 
@@ -405,7 +406,8 @@ Last release without a changelog :(
 [#1151]: https://github.com/linebender/druid/pull/1151
 [#1152]: https://github.com/linebender/druid/pull/1152
 [#1157]: https://github.com/linebender/druid/pull/1157
-[#1172]: https://github.com/linebender/druid/pull/1157
+[#1171]: https://github.com/linebender/druid/pull/1171
+[#1172]: https://github.com/linebender/druid/pull/1172
 
 [Unreleased]: https://github.com/linebender/druid/compare/v0.6.0...master
 [0.6.0]: https://github.com/linebender/druid/compare/v0.5.0...v0.6.0

druid/src/lens/lens.rs

@@ -125,6 +125,53 @@ pub trait LensExt<A: ?Sized, B: ?Sized>: Lens<A, B> {
         self.then(Deref)
     }
 
+    /// Invoke a type's `AsRef` and `AsMut` impl.
+    ///
+    /// It also allows indexing arrays with the [`index`] lens as shown in the example.
+    /// This is necessary, because the `Index` trait in Rust is only implemented
+    /// for slices (`[T]`), but not for arrays (`[T; N]`).
+    ///
+    /// # Examples
+    ///
+    /// Using `ref` this works:
+    ///
+    /// ```
+    /// use druid::{widget::TextBox, Data, Lens, LensExt, Widget, WidgetExt};
+    ///
+    /// #[derive(Clone, Default, Data, Lens)]
+    /// struct State {
+    ///     data: [String; 2],
+    /// }
+    ///
+    /// fn with_ref() -> impl Widget<State> {
+    ///     TextBox::new().lens(State::data.as_ref().index(1))
+    /// }
+    /// ```
+    ///
+    /// While this fails:
+    ///
+    /// ```compile_fail
+    /// # use druid::*;
+    /// # #[derive(Clone, Default, Data, Lens)]
+    /// # struct State {
+    /// #     data: [String; 2],
+    /// # }
+    /// fn without_ref() -> impl Widget<State> {
+    ///     // results in: `[std::string::String; 2]` cannot be mutably indexed by `usize`
+    ///     TextBox::new().lens(State::data.index(1))
+    /// }
+    /// ```
+    ///
+    /// [`Lens`]: ./trait.Lens.html
+    /// [`index`]: #method.index
+    fn as_ref<T: ?Sized>(self) -> Then<Self, Ref, B>
+    where
+        B: AsRef<T> + AsMut<T>,
+        Self: Sized,
+    {
+        self.then(Ref)
+    }
+
     /// Access an index in a container
     ///
     /// ```
@@ -307,10 +354,10 @@ where
 #[macro_export]
 macro_rules! lens {
     ($ty:ty, [$index:expr]) => {
-        $crate::lens::Field::new::<$ty, _>(|x| &x[$index], |x| &mut x[$index])
+        $crate::lens::Field::new::<$ty, _>(move |x| &x[$index], move |x| &mut x[$index])
     };
     ($ty:ty, $field:tt) => {
-        $crate::lens::Field::new::<$ty, _>(|x| &x.$field, |x| &mut x.$field)
+        $crate::lens::Field::new::<$ty, _>(move |x| &x.$field, move |x| &mut x.$field)
     };
 }
 
@@ -421,6 +468,28 @@ where
     }
 }
 
+/// [`Lens`] for invoking `AsRef` and `AsMut` on a type.
+///
+/// [`LensExt::ref`] offers an easy way to apply this,
+/// as well as more information and examples.
+///
+/// [`Lens`]: ../trait.Lens.html
+/// [`LensExt::ref`]: ../trait.LensExt.html#method.as_ref
+#[derive(Debug, Copy, Clone)]
+pub struct Ref;
+
+impl<T: ?Sized, U: ?Sized> Lens<T, U> for Ref
+where
+    T: AsRef<U> + AsMut<U>,
+{
+    fn with<V, F: FnOnce(&U) -> V>(&self, data: &T, f: F) -> V {
+        f(data.as_ref())
+    }
+    fn with_mut<V, F: FnOnce(&mut U) -> V>(&self, data: &mut T, f: F) -> V {
+        f(data.as_mut())
+    }
+}
+
 /// `Lens` for indexing containers
 #[derive(Debug, Copy, Clone)]
 pub struct Index<I> {

druid/src/lens/mod.rs

@@ -48,6 +48,6 @@
 
 #[allow(clippy::module_inception)]
 mod lens;
-pub use lens::{Deref, Field, Id, InArc, Index, Map, Then};
+pub use lens::{Deref, Field, Id, InArc, Index, Map, Ref, Then};
 #[doc(hidden)]
 pub use lens::{Lens, LensExt, LensWrap};