Scope widget cherry-picked from binding-scroll branch.

AUTHORS

@@ -11,3 +11,4 @@ Kaiyin Zhong
 Kaur Kuut
 Leopold Luley
 Andrey Kabylin
+Robert Wittams

CHANGELOG.md

@@ -17,6 +17,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - Close requests from the shell can now be intercepted ([#1118] by [@jneem])
 - The Lens derive now supports an `ignore` attribute. ([#1133] by [@jneem])
 - `request_update` in `EventCtx`. ([#1128] by [@raphlinus])
+- 'Scope' widget to allow encapsulation of reactive state. ([#1151] by [@rjwittams])
 
 ### Changed
 
@@ -397,6 +398,7 @@ Last release without a changelog :(
 [#1133]: https://github.com/linebender/druid/pull/1133
 [#1143]: https://github.com/linebender/druid/pull/1143
 [#1145]: https://github.com/linebender/druid/pull/1145
+[#1151]: https://github.com/linebender/druid/pull/1151
 
 [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/contexts.rs

@@ -501,6 +501,12 @@ impl EventCtx<'_, '_> {
     }
 }
 
+impl UpdateCtx<'_, '_> {
+    pub fn has_requested_update(&mut self) -> bool {
+        self.widget_state.request_update
+    }
+}
+
 impl LifeCycleCtx<'_, '_> {
     /// Registers a child widget.
     ///

druid/src/lens/lens.rs

@@ -232,7 +232,7 @@ where
         let lens = &self.lens;
         lens.with(old_data, |old_data| {
             lens.with(data, |data| {
-                if !old_data.same(data) {
+                if ctx.has_requested_update() || !old_data.same(data) {
                     inner.update(ctx, old_data, data, env);
                 }
             })

druid/src/widget/mod.rs

@@ -34,6 +34,7 @@ mod painter;
 mod parse;
 mod progress_bar;
 mod radio;
+mod scope;
 mod scroll;
 mod sized_box;
 mod slider;
@@ -69,6 +70,7 @@ pub use painter::{BackgroundBrush, Painter};
 pub use parse::Parse;
 pub use progress_bar::ProgressBar;
 pub use radio::{Radio, RadioGroup};
+pub use scope::{DefaultScopePolicy, LensScopeTransfer, Scope, ScopePolicy, ScopeTransfer};
 pub use scroll::Scroll;
 pub use sized_box::SizedBox;
 pub use slider::Slider;

druid/src/widget/scope.rs

@@ -0,0 +1,278 @@
+use crate::kurbo::{Point, Rect};
+use crate::{
+    BoxConstraints, Data, Env, Event, EventCtx, LayoutCtx, Lens, LifeCycle, LifeCycleCtx, PaintCtx,
+    Size, UpdateCtx, Widget, WidgetPod,
+};
+use std::marker::PhantomData;
+
+/// A policy that controls how a Scope will interact with its surrounding application data.
+/// Specifically, how to create an initial State from the Input, and how to synchronise the two.
+pub trait ScopePolicy {
+    /// The type of data that comes in from the surrounding application or scope.
+    type In: Data;
+    /// The type of data that the Scope will maintain internally.
+    /// This will usually be larger than the input data, and will embed the input data.
+    type State: Data;
+    /// The type of transfer that will be used to synchronise internal and application state
+    type Transfer: ScopeTransfer<In = Self::In, State = Self::State>;
+    /// Make a new state and transfer from the input.
+    /// This consumes the policy, so non cloneable items can make their way into the state this way.
+    fn create(self, inner: &Self::In) -> (Self::State, Self::Transfer);
+}
+
+pub trait ScopeTransfer {
+    type In: Data;
+    type State: Data;
+
+    /// Replace the input we have within our State with a new one from outside
+    fn read_input(&self, state: &mut Self::State, inner: &Self::In);
+    /// Take the modifications we have made and write them back
+    /// to our input.
+    fn write_back_input(&self, state: &Self::State, inner: &mut Self::In);
+}
+
+/// A default implementation of scope policy that takes a function and a transfer
+pub struct DefaultScopePolicy<F: FnOnce(Transfer::In) -> Transfer::State, Transfer: ScopeTransfer> {
+    make_state: F,
+    transfer: Transfer,
+}
+
+impl<F: FnOnce(Transfer::In) -> Transfer::State, Transfer: ScopeTransfer>
+    DefaultScopePolicy<F, Transfer>
+{
+    pub fn new(make_state: F, transfer: Transfer) -> Self {
+        DefaultScopePolicy {
+            make_state,
+            transfer,
+        }
+    }
+}
+
+impl<F: FnOnce(In) -> State, L: Lens<State, In>, In: Data, State: Data>
+    DefaultScopePolicy<F, LensScopeTransfer<L, In, State>>
+{
+    pub fn from_lens(make_state: F, lens: L) -> Self {
+        Self::new(make_state, LensScopeTransfer::new(lens))
+    }
+}
+
+impl<F: Fn(Transfer::In) -> Transfer::State, Transfer: ScopeTransfer> ScopePolicy
+    for DefaultScopePolicy<F, Transfer>
+{
+    type In = Transfer::In;
+    type State = Transfer::State;
+    type Transfer = Transfer;
+
+    fn create(self, inner: &Self::In) -> (Self::State, Self::Transfer) {
+        let state = (self.make_state)(inner.clone());
+        (state, self.transfer)
+    }
+}
+
+/// A scope transfer that uses a Lens to synchronise between a large internal state and a small input.
+pub struct LensScopeTransfer<L: Lens<State, In>, In, State> {
+    lens: L,
+    phantom_in: PhantomData<In>,
+    phantom_state: PhantomData<State>,
+}
+
+impl<L: Lens<State, In>, In, State> LensScopeTransfer<L, In, State> {
+    pub fn new(lens: L) -> Self {
+        LensScopeTransfer {
+            lens,
+            phantom_in: PhantomData::default(),
+            phantom_state: PhantomData::default(),
+        }
+    }
+}
+
+impl<L: Lens<State, In>, In: Data, State: Data> ScopeTransfer for LensScopeTransfer<L, In, State> {
+    type In = In;
+    type State = State;
+
+    fn read_input(&self, state: &mut State, data: &In) {
+        self.lens.with_mut(state, |inner| {
+            if !inner.same(&data) {
+                *inner = data.clone()
+            }
+        });
+    }
+
+    fn write_back_input(&self, state: &State, data: &mut In) {
+        self.lens.with(state, |inner| {
+            if !inner.same(&data) {
+                *data = inner.clone();
+            }
+        });
+    }
+}
+
+enum ScopeContent<SP: ScopePolicy> {
+    Policy {
+        policy: Option<SP>,
+    },
+    Transfer {
+        state: SP::State,
+        transfer: SP::Transfer,
+    },
+}
+
+/// A widget that allows encapsulation of application state.
+///
+/// This is useful in circumstances where
+/// * A (potentially reusable) widget is composed of a tree of multiple cooperating child widgets
+/// * Those widgets communicate amongst themselves using Druids reactive data mechanisms
+/// * It is undesirable to complicate the surrounding application state with the internal details
+///   of the widget.
+///
+///
+/// Examples include:
+/// * In a tabs widget composed of a tab bar, and a widget switching body, those widgets need to
+///   cooperate on which tab is selected. However not every user of a tabs widget wishes to
+///   encumber their application state with this internal detail - especially as many tabs widgets may
+///   reasonably exist in an involved application.
+/// * In a table/grid widget composed of various internal widgets, many things need to be synchronised.
+///   Scroll position, heading moves, drag operations, sort/filter operations. For many applications
+///   access to this internal data outside of the table widget isn't needed.
+///   For this reason it may be useful to use a Scope to establish private state.
+///
+/// A scope embeds some input state (from its surrounding application or parent scope)
+/// into a larger piece of internal state. This is controlled by a user provided policy.
+///
+/// The ScopePolicy needs to do two things
+/// a) Create a new scope from the initial value of its input,
+/// b) Provide two way synchronisation between the input and the state via a ScopeTransfer
+///
+/// Convenience methods are provided to make a policy from a function and a lens.
+/// It may sometimes be advisable to implement ScopePolicy directly if you need to
+/// mention the type of a Scope.
+///
+/// # Examples
+/// ```
+/// #[derive(Data, Lens)]
+/// struct AppState{
+///     name: String
+/// }
+///
+/// #[derive(Data, Lens)]
+/// struct PrivateState{
+///     text: String,
+///     other: u32
+/// }
+///
+/// impl PrivateState{
+///     pub fn new(text: String) -> Self {
+///         PrivateState{ text, other: 0 }
+///     }
+/// }
+///
+/// fn main(){
+///     let scope = Scope::from_lens(PrivateState::new,
+///                                  PrivateState::text,
+///                                  MyWidget::new() );
+/// }
+/// ```
+pub struct Scope<SP: ScopePolicy, W: Widget<SP::State>> {
+    content: ScopeContent<SP>,
+    inner: WidgetPod<SP::State, W>,
+}
+
+impl<SP: ScopePolicy, W: Widget<SP::State>> Scope<SP, W> {
+    /// Create a new scope from a policy and an inner widget
+    pub fn new(policy: SP, inner: W) -> Self {
+        Scope {
+            content: ScopeContent::Policy {
+                policy: Some(policy),
+            },
+            inner: WidgetPod::new(inner),
+        }
+    }
+
+    fn with_state<V>(
+        &mut self,
+        data: &SP::In,
+        mut f: impl FnMut(&mut SP::State, &mut WidgetPod<SP::State, W>) -> V,
+    ) -> V {
+        match &mut self.content {
+            ScopeContent::Policy { policy } => {
+                // We know that the policy is a Some - it is an option to allow
+                // us to take ownership before replacing the content.
+                let (mut state, policy) = policy.take().unwrap().create(data);
+                let v = f(&mut state, &mut self.inner);
+                self.content = ScopeContent::Transfer {
+                    state,
+                    transfer: policy,
+                };
+                v
+            }
+            ScopeContent::Transfer {
+                ref mut state,
+                transfer,
+            } => {
+                transfer.read_input(state, data);
+                f(state, &mut self.inner)
+            }
+        }
+    }
+
+    fn write_back_input(&mut self, data: &mut SP::In) {
+        if let ScopeContent::Transfer { state, transfer } = &mut self.content {
+            transfer.write_back_input(state, data)
+        }
+    }
+}
+
+impl<
+        F: Fn(Transfer::In) -> Transfer::State,
+        Transfer: ScopeTransfer,
+        W: Widget<Transfer::State>,
+    > Scope<DefaultScopePolicy<F, Transfer>, W>
+{
+    /// Create a new policy from a function creating the state, and a ScopeTransfer synchronising it
+    pub fn from_function(make_state: F, transfer: Transfer, inner: W) -> Self {
+        Self::new(DefaultScopePolicy::new(make_state, transfer), inner)
+    }
+}
+
+impl<In: Data, State: Data, F: Fn(In) -> State, L: Lens<State, In>, W: Widget<State>>
+    Scope<DefaultScopePolicy<F, LensScopeTransfer<L, In, State>>, W>
+{
+    /// Create a new policy from a function creating the state, and a Lens synchronising it
+    pub fn from_lens(make_state: F, lens: L, inner: W) -> Self {
+        Self::new(DefaultScopePolicy::from_lens(make_state, lens), inner)
+    }
+}
+
+impl<SP: ScopePolicy, W: Widget<SP::State>> Widget<SP::In> for Scope<SP, W> {
+    fn event(&mut self, ctx: &mut EventCtx, event: &Event, data: &mut SP::In, env: &Env) {
+        self.with_state(data, |state, inner| inner.event(ctx, event, state, env));
+        self.write_back_input(data);
+        ctx.request_update()
+    }
+
+    fn lifecycle(&mut self, ctx: &mut LifeCycleCtx, event: &LifeCycle, data: &SP::In, env: &Env) {
+        self.with_state(data, |state, inner| inner.lifecycle(ctx, event, state, env));
+    }
+
+    fn update(&mut self, ctx: &mut UpdateCtx, _old_data: &SP::In, data: &SP::In, env: &Env) {
+        self.with_state(data, |state, inner| inner.update(ctx, state, env));
+    }
+
+    fn layout(
+        &mut self,
+        ctx: &mut LayoutCtx,
+        bc: &BoxConstraints,
+        data: &SP::In,
+        env: &Env,
+    ) -> Size {
+        self.with_state(data, |state, inner| {
+            let size = inner.layout(ctx, bc, state, env);
+            inner.set_layout_rect(ctx, state, env, Rect::from_origin_size(Point::ORIGIN, size));
+            size
+        })
+    }
+
+    fn paint(&mut self, ctx: &mut PaintCtx, data: &SP::In, env: &Env) {
+        self.with_state(data, |state, inner| inner.paint_raw(ctx, state, env));
+    }
+}