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// Copyright 2019 The xi-editor Authors. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // http://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. //! Support for lenses, a way of focusing on subfields of data. use std::marker::PhantomData; use std::ops; use std::sync::Arc; pub use druid_derive::Lens; use crate::kurbo::Size; use crate::{BoxConstraints, Data, Env, Event, EventCtx, LayoutCtx, PaintCtx, UpdateCtx, Widget}; /// A lens is a datatype that gives access to a part of a larger /// data structure. /// /// A simple example of a lens is a field of a struct; in this case, /// the lens itself is zero-sized. Another case is accessing an array /// element, in which case the lens contains the array index. /// /// Many `Lens` implementations will be derived by macro, but custom /// implementations are practical as well. /// /// The name "lens" is inspired by the [Haskell lens] package, which /// has generally similar goals. It's likely we'll develop more /// sophistication, for example combinators to combine lenses. /// /// [Haskell lens]: http://hackage.haskell.org/package/lens pub trait Lens<T: ?Sized, U: ?Sized> { /// Get non-mut access to the field. /// /// Runs the supplied closure with a reference to the data. It's /// structured this way, as opposed to simply returning a reference, /// so that the data might be synthesized on-the-fly by the lens. fn with<V, F: FnOnce(&U) -> V>(&self, data: &T, f: F) -> V; /// Get mutable access to the field. /// /// This method is defined in terms of a closure, rather than simply /// yielding a mutable reference, because it is intended to be used /// with value-type data (also known as immutable data structures). /// For example, a lens for an immutable list might be implemented by /// cloning the list, giving the closure mutable access to the clone, /// then updating the reference after the closure returns. fn with_mut<V, F: FnOnce(&mut U) -> V>(&self, data: &mut T, f: F) -> V; } /// Helpers for manipulating `Lens`es pub trait LensExt<A: ?Sized, B: ?Sized>: Lens<A, B> { /// Copy the targeted value out of `data` fn get(&self, data: &A) -> B where B: Clone, { self.with(data, |x| x.clone()) } /// Set the targeted value in `data` to `value` fn put(&self, data: &mut A, value: B) where B: Sized, { self.with_mut(data, |x| *x = value); } /// Compose a `Lens<A, B>` with a `Lens<B, C>` to produce a `Lens<A, C>` /// /// ``` /// # use druid::*; /// struct Foo { x: (u32, bool) } /// let lens = lens!(Foo, x).then(lens!((u32, bool), 1)); /// assert_eq!(lens.get(&Foo { x: (0, true) }), true); /// ``` fn then<Other, C>(self, other: Other) -> Then<Self, Other, B> where Other: Lens<B, C> + Sized, C: ?Sized, Self: Sized, { Then::new(self, other) } /// Combine a `Lens<A, B>` with a function that can transform a `B` and its inverse. /// /// Useful for cases where the desired value doesn't physically exist in `A`, but can be /// computed. For example, a lens like the following might be used to adapt a value with the /// range 0-2 for use with a `Widget<f64>` like `Slider` that has a range of 0-1: /// /// ``` /// # use druid::*; /// let lens = lens!((bool, f64), 1); /// assert_eq!(lens.map(|x| x / 2.0, |x, y| *x = y * 2.0).get(&(true, 2.0)), 1.0); /// ``` /// /// The computed `C` may represent a whole or only part of the original `B`. fn map<Get, Put, C>(self, get: Get, put: Put) -> Then<Self, Map<Get, Put>, B> where Get: Fn(&B) -> C, Put: Fn(&mut B, C), Self: Sized, { self.then(Map::new(get, put)) } /// Invoke a type's `Deref` impl /// /// ``` /// # use druid::*; /// assert_eq!(lens::Id.deref().get(&Box::new(42)), 42); /// ``` fn deref(self) -> Then<Self, Deref, B> where B: ops::Deref + ops::DerefMut, Self: Sized, { self.then(Deref) } /// Access an index in a container /// /// ``` /// # use druid::*; /// assert_eq!(lens::Id.index(2).get(&vec![0u32, 1, 2, 3]), 2); /// ``` fn index<I>(self, index: I) -> Then<Self, Index<I>, B> where I: Clone, B: ops::Index<I> + ops::IndexMut<I>, Self: Sized, { self.then(Index::new(index)) } /// Adapt to operate on the contents of an `Arc` with efficient copy-on-write semantics /// /// ``` /// # use druid::*; use std::sync::Arc; /// let lens = lens::Id.index(2).in_arc(); /// let mut x = Arc::new(vec![0, 1, 2, 3]); /// let original = x.clone(); /// assert_eq!(lens.get(&x), 2); /// lens.put(&mut x, 2); /// assert!(Arc::ptr_eq(&original, &x), "no-op writes don't cause a deep copy"); /// lens.put(&mut x, 42); /// assert_eq!(&*x, &[0, 1, 42, 3]); /// ``` fn in_arc(self) -> InArc<Self> where A: Clone, B: Data, Self: Sized, { InArc::new(self) } } impl<A: ?Sized, B: ?Sized, T: Lens<A, B>> LensExt<A, B> for T {} // A case can be made this should be in the `widget` module. /// A wrapper for its widget subtree to have access to a part /// of its parent's data. /// /// Every widget in druid is instantiated with access to data of some /// type; the root widget has access to the entire application data. /// Often, a part of the widget hierarchy is only concerned with a part /// of that data. The `LensWrap` widget is a way to "focus" the data /// reference down, for the subtree. One advantage is performance; /// data changes that don't intersect the scope of the lens aren't /// propagated. /// /// Another advantage is generality and reuse. If a widget (or tree of /// widgets) is designed to work with some chunk of data, then with a /// lens that same code can easily be reused across all occurrences of /// that chunk within the application state. /// /// This wrapper takes a [`Lens`] as an argument, which is a specification /// of a struct field, or some other way of narrowing the scope. /// /// [`Lens`]: trait.Lens.html pub struct LensWrap<U, L, W> { inner: W, lens: L, // The following is a workaround for otherwise getting E0207. phantom: PhantomData<U>, } impl<U, L, W> LensWrap<U, L, W> { /// Wrap a widget with a lens. /// /// When the lens has type `Lens<T, U>`, the inner widget has data /// of type `U`, and the wrapped widget has data of type `T`. pub fn new(inner: W, lens: L) -> LensWrap<U, L, W> { LensWrap { inner, lens, phantom: Default::default(), } } } impl<T, U, L, W> Widget<T> for LensWrap<U, L, W> where T: Data, U: Data, L: Lens<T, U>, W: Widget<U>, { fn event(&mut self, ctx: &mut EventCtx, event: &Event, data: &mut T, env: &Env) { let inner = &mut self.inner; self.lens .with_mut(data, |data| inner.event(ctx, event, data, env)) } fn update(&mut self, ctx: &mut UpdateCtx, old_data: Option<&T>, data: &T, env: &Env) { let inner = &mut self.inner; let lens = &self.lens; if let Some(old_data) = old_data { lens.with(old_data, |old_data| { lens.with(data, |data| { if !old_data.same(data) { inner.update(ctx, Some(old_data), data, env); } }) }) } else { lens.with(data, |data| inner.update(ctx, None, data, env)); } } fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints, data: &T, env: &Env) -> Size { let inner = &mut self.inner; self.lens .with(data, |data| inner.layout(ctx, bc, data, env)) } fn paint(&mut self, paint_ctx: &mut PaintCtx, data: &T, env: &Env) { let inner = &mut self.inner; self.lens .with(data, |data| inner.paint(paint_ctx, data, env)); } } /// Lens accessing a member of some type using accessor functions /// /// See also the `lens` macro. /// /// ``` /// let lens = druid::lens::Field::new(|x: &Vec<u32>| &x[42], |x| &mut x[42]); /// ``` pub struct Field<Get, GetMut> { get: Get, get_mut: GetMut, } impl<Get, GetMut> Field<Get, GetMut> { /// Construct a lens from a pair of getter functions pub fn new<T: ?Sized, U: ?Sized>(get: Get, get_mut: GetMut) -> Self where Get: Fn(&T) -> &U, GetMut: Fn(&mut T) -> &mut U, { Self { get, get_mut } } } impl<T, U, Get, GetMut> Lens<T, U> for Field<Get, GetMut> where T: ?Sized, U: ?Sized, Get: Fn(&T) -> &U, GetMut: Fn(&mut T) -> &mut U, { fn with<V, F: FnOnce(&U) -> V>(&self, data: &T, f: F) -> V { f((self.get)(data)) } fn with_mut<V, F: FnOnce(&mut U) -> V>(&self, data: &mut T, f: F) -> V { f((self.get_mut)(data)) } } /// Construct a lens accessing a type's field /// /// This is a convenience macro for constructing `Field` lenses for fields or indexable elements. /// /// ``` /// struct Foo { x: u32 } /// let lens = druid::lens!(Foo, x); /// let lens = druid::lens!((u32, bool), 1); /// let lens = druid::lens!([u8], [4]); /// ``` #[macro_export] macro_rules! lens { ($ty:ty, [$index:expr]) => { $crate::lens::Field::new::<$ty, _>(|x| &x[$index], |x| &mut x[$index]) }; ($ty:ty, $field:tt) => { $crate::lens::Field::new::<$ty, _>(|x| &x.$field, |x| &mut x.$field) }; } /// `Lens` composed of two lenses joined together #[derive(Debug, Copy)] pub struct Then<T, U, B: ?Sized> { left: T, right: U, _marker: PhantomData<B>, } impl<T, U, B: ?Sized> Then<T, U, B> { /// Compose two lenses /// /// See also `LensExt::then`. pub fn new<A: ?Sized, C: ?Sized>(left: T, right: U) -> Self where T: Lens<A, B>, U: Lens<B, C>, { Self { left, right, _marker: PhantomData, } } } impl<T, U, A, B, C> Lens<A, C> for Then<T, U, B> where A: ?Sized, B: ?Sized, C: ?Sized, T: Lens<A, B>, U: Lens<B, C>, { fn with<V, F: FnOnce(&C) -> V>(&self, data: &A, f: F) -> V { self.left.with(data, |b| self.right.with(b, f)) } fn with_mut<V, F: FnOnce(&mut C) -> V>(&self, data: &mut A, f: F) -> V { self.left.with_mut(data, |b| self.right.with_mut(b, f)) } } impl<T: Clone, U: Clone, B> Clone for Then<T, U, B> { fn clone(&self) -> Self { Self { left: self.left.clone(), right: self.right.clone(), _marker: PhantomData, } } } /// `Lens` built from a getter and a setter #[derive(Debug, Copy, Clone)] pub struct Map<Get, Put> { get: Get, put: Put, } impl<Get, Put> Map<Get, Put> { /// Construct a mapping /// /// See also `LensExt::map` pub fn new<A: ?Sized, B>(get: Get, put: Put) -> Self where Get: Fn(&A) -> B, Put: Fn(&mut A, B), { Self { get, put } } } impl<A: ?Sized, B, Get, Put> Lens<A, B> for Map<Get, Put> where Get: Fn(&A) -> B, Put: Fn(&mut A, B), { fn with<V, F: FnOnce(&B) -> V>(&self, data: &A, f: F) -> V { f(&(self.get)(data)) } fn with_mut<V, F: FnOnce(&mut B) -> V>(&self, data: &mut A, f: F) -> V { let mut temp = (self.get)(data); let x = f(&mut temp); (self.put)(data, temp); x } } /// `Lens` for invoking `Deref` and `DerefMut` on a type /// /// See also `LensExt::deref`. #[derive(Debug, Copy, Clone)] pub struct Deref; impl<T: ?Sized> Lens<T, T::Target> for Deref where T: ops::Deref + ops::DerefMut, { fn with<V, F: FnOnce(&T::Target) -> V>(&self, data: &T, f: F) -> V { f(data.deref()) } fn with_mut<V, F: FnOnce(&mut T::Target) -> V>(&self, data: &mut T, f: F) -> V { f(data.deref_mut()) } } /// `Lens` for indexing containers #[derive(Debug, Copy, Clone)] pub struct Index<I> { index: I, } impl<I> Index<I> { /// Construct a lens that accesses a particular index /// /// See also `LensExt::index`. pub fn new(index: I) -> Self { Self { index } } } impl<T, I> Lens<T, T::Output> for Index<I> where T: ?Sized + ops::Index<I> + ops::IndexMut<I>, I: Clone, { fn with<V, F: FnOnce(&T::Output) -> V>(&self, data: &T, f: F) -> V { f(&data[self.index.clone()]) } fn with_mut<V, F: FnOnce(&mut T::Output) -> V>(&self, data: &mut T, f: F) -> V { f(&mut data[self.index.clone()]) } } /// The identity lens: the lens which does nothing, i.e. exposes exactly the original value. /// /// Useful for starting a lens combinator chain, or passing to lens-based interfaces. #[derive(Debug, Copy, Clone)] pub struct Id; impl<A: ?Sized> Lens<A, A> for Id { fn with<V, F: FnOnce(&A) -> V>(&self, data: &A, f: F) -> V { f(data) } fn with_mut<V, F: FnOnce(&mut A) -> V>(&self, data: &mut A, f: F) -> V { f(data) } } /// A `Lens` that exposes data within an `Arc` with copy-on-write semantics /// /// A copy is only made in the event that a different value is written. #[derive(Debug, Copy, Clone)] pub struct InArc<L> { inner: L, } impl<L> InArc<L> { /// Adapt a lens to operate on an `Arc` /// /// See also `LensExt::in_arc` pub fn new<A, B>(inner: L) -> Self where A: Clone, B: Data, L: Lens<A, B>, { Self { inner } } } impl<A, B, L> Lens<Arc<A>, B> for InArc<L> where A: Clone, B: Data, L: Lens<A, B>, { fn with<V, F: FnOnce(&B) -> V>(&self, data: &Arc<A>, f: F) -> V { self.inner.with(data, f) } fn with_mut<V, F: FnOnce(&mut B) -> V>(&self, data: &mut Arc<A>, f: F) -> V { let mut temp = self.inner.with(data, |x| x.clone()); let v = f(&mut temp); if self.inner.with(data, |x| !x.same(&temp)) { self.inner.with_mut(Arc::make_mut(data), |x| *x = temp); } v } }