Expose scope state (#2082)

* Expose scope state.

This is mainly for composite widgets that contain a scope: using this they can affect the widgets inside the scope.
Using this facilty, allow getting and setting the tab index of a Tabs widget.
This offers part of a solution to #1390, although it is in terms of the index and not the key.

* Add suggestions from review

Co-authored-by: Robert Wittams <robert@wittams.com>

CHANGELOG.md

@@ -61,6 +61,8 @@ You can find its changes [documented below](#070---2021-01-01).
 - `scroll_to_view` and `scroll_area_to_view` methods on `UpdateCtx`, `LifecycleCtx` and `EventCtx` ([#1976] by [@xarvic])
 - `Notification::route` ([#1978] by [@xarvic])
 - Build on OpenBSD ([#1993] by [@klemensn])
+- Scope: expose scoped state using state() and state_mut() ([#2082] by [@rjwittams]
+- Tabs: allow getting and setting the tab index of a Tabs widget ([#2082] by [@rjwittams]
 
 ### Changed
 

druid/examples/tabs.rs

@@ -236,7 +236,8 @@ fn build_tab_widget(tab_config: &TabConfig) -> impl Widget<AppState> {
         .with_tab("Page 3", Label::new("Page 3 content"))
         .with_tab("Page 4", Label::new("Page 4 content"))
         .with_tab("Page 5", Label::new("Page 5 content"))
-        .with_tab("Page 6", Label::new("Page 6 content"));
+        .with_tab("Page 6", Label::new("Page 6 content"))
+        .with_tab_index(1);
 
     Split::rows(main_tabs, dyn_tabs).draggable(true)
 }

druid/src/widget/scope.rs

@@ -41,10 +41,10 @@ pub trait ScopeTransfer {
     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);
+    fn read_input(&self, state: &mut Self::State, input: &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);
+    fn write_back_input(&self, state: &Self::State, input: &mut Self::In);
 }
 
 /// A default implementation of [`ScopePolicy`] that takes a function and a transfer.
@@ -84,8 +84,8 @@ impl<F: FnOnce(Transfer::In) -> Transfer::State, Transfer: ScopeTransfer> ScopeP
     type State = Transfer::State;
     type Transfer = Transfer;
 
-    fn create(self, inner: &Self::In) -> (Self::State, Self::Transfer) {
-        let state = (self.make_state)(inner.clone());
+    fn create(self, input: &Self::In) -> (Self::State, Self::Transfer) {
+        let state = (self.make_state)(input.clone());
         (state, self.transfer)
     }
 }
@@ -215,6 +215,34 @@ impl<SP: ScopePolicy, W: Widget<SP::State>> Scope<SP, W> {
         }
     }
 
+    /// A reference to the contents of the `Scope`'s state.
+    ///
+    /// This allows you to access the content from outside the widget.
+    pub fn state(&self) -> Option<&SP::State> {
+        if let ScopeContent::Transfer { ref state, .. } = &self.content {
+            Some(state)
+        } else {
+            None
+        }
+    }
+
+    /// A mutable reference to the contents of the [`Scope`]'s state.
+    ///
+    /// This allows you to mutably access the content of the `Scope`' s state from
+    /// outside the widget. Mainly useful for composite widgets.
+    ///
+    /// # Note:
+    ///
+    /// If you modify the state through this reference, the Scope will not
+    /// call update on its children until the next event it receives.
+    pub fn state_mut(&mut self) -> Option<&mut SP::State> {
+        if let ScopeContent::Transfer { ref mut state, .. } = &mut self.content {
+            Some(state)
+        } else {
+            None
+        }
+    }
+
     fn with_state<V>(
         &mut self,
         data: &SP::In,

druid/src/widget/tabs.rs

@@ -588,7 +588,14 @@ impl<TP: TabsPolicy> Widget<TabsState<TP>> for TabsBody<TP> {
         }
 
         if let (Some(t_state), Event::AnimFrame(interval)) = (&mut self.transition_state, event) {
-            t_state.current_time += *interval;
+            // We can get a high interval on the first frame due to other widgets or old animations.
+            let interval = if t_state.current_time == 0 {
+                1
+            } else {
+                *interval
+            };
+
+            t_state.current_time += interval;
             if t_state.live() {
                 ctx.request_anim_frame();
             } else {
@@ -794,9 +801,11 @@ impl<T: Data> InitialTab<T> {
 enum TabsContent<TP: TabsPolicy> {
     Building {
         tabs: TP::Build,
+        index: TabIndex,
     },
     Complete {
         tabs: TP,
+        index: TabIndex,
     },
     Running {
         scope: WidgetPod<TP::Input, TabsScope<TP>>,
@@ -862,7 +871,7 @@ impl<TP: TabsPolicy> Tabs<TP> {
     /// Create a Tabs widget using the provided policy.
     /// This is useful for tabs derived from data.
     pub fn for_policy(tabs: TP) -> Self {
-        Self::of_content(TabsContent::Complete { tabs })
+        Self::of_content(TabsContent::Complete { tabs, index: 0 })
     }
 
     // This could be public if there is a case for custom policies that support static tabs - ie the AddTab method.
@@ -874,6 +883,7 @@ impl<TP: TabsPolicy> Tabs<TP> {
     {
         Self::of_content(TabsContent::Building {
             tabs: tabs_from_data,
+            index: 0,
         })
     }
 
@@ -909,6 +919,12 @@ impl<TP: TabsPolicy> Tabs<TP> {
         self
     }
 
+    /// A builder-style method to specify the (zero-based) index of the selected tab.
+    pub fn with_tab_index(mut self, idx: TabIndex) -> Self {
+        self.set_tab_index(idx);
+        self
+    }
+
     /// Available when the policy implements AddTab - e.g StaticTabs.
     /// Return this Tabs widget with the named tab added.
     pub fn add_tab(
@@ -918,14 +934,44 @@ impl<TP: TabsPolicy> Tabs<TP> {
     ) where
         TP: AddTab,
     {
-        if let TabsContent::Building { tabs } = &mut self.content {
+        if let TabsContent::Building { tabs, .. } = &mut self.content {
             TP::add_tab(tabs, name, child)
         } else {
             tracing::warn!("Can't add static tabs to a running or complete tabs instance!")
         }
     }
 
-    fn make_scope(&self, tabs_from_data: TP) -> WidgetPod<TP::Input, TabsScope<TP>> {
+    /// The (zero-based) index of the currently selected tab.
+    pub fn tab_index(&self) -> TabIndex {
+        let index = match &self.content {
+            TabsContent::Running { scope, .. } => scope.widget().state().map(|s| s.selected),
+            TabsContent::Building { index, .. } | TabsContent::Complete { index, .. } => {
+                Some(*index)
+            }
+            TabsContent::Swapping => None,
+        };
+        index.unwrap_or(0)
+    }
+
+    /// Set the selected (zero-based) tab index.
+    ///
+    /// This tab will become visible if it exists. If animations are enabled
+    /// (and the widget is laid out), the tab transition will be animated.
+    pub fn set_tab_index(&mut self, idx: TabIndex) {
+        match &mut self.content {
+            TabsContent::Running { scope, .. } => {
+                if let Some(state) = scope.widget_mut().state_mut() {
+                    state.selected = idx
+                }
+            }
+            TabsContent::Building { index, .. } | TabsContent::Complete { index, .. } => {
+                *index = idx;
+            }
+            TabsContent::Swapping => (),
+        }
+    }
+
+    fn make_scope(&self, tabs_from_data: TP, idx: TabIndex) -> WidgetPod<TP::Input, TabsScope<TP>> {
         let tabs_bar = TabBar::new(self.axis, self.edge);
         let tabs_body = TabsBody::new(self.axis, self.transition)
             .padding(5.)
@@ -941,7 +987,7 @@ impl<TP: TabsPolicy> Tabs<TP> {
         };
 
         WidgetPod::new(Scope::new(
-            TabsScopePolicy::new(tabs_from_data, 0),
+            TabsScopePolicy::new(tabs_from_data, idx),
             Box::new(layout),
         ))
     }
@@ -967,16 +1013,16 @@ impl<TP: TabsPolicy> Widget<TP::Input> for Tabs<TP> {
             let content = std::mem::replace(&mut self.content, TabsContent::Swapping);
 
             self.content = match content {
-                TabsContent::Building { tabs } => {
+                TabsContent::Building { tabs, index } => {
                     ctx.children_changed();
                     TabsContent::Running {
-                        scope: self.make_scope(TP::build(tabs)),
+                        scope: self.make_scope(TP::build(tabs), index),
                     }
                 }
-                TabsContent::Complete { tabs } => {
+                TabsContent::Complete { tabs, index } => {
                     ctx.children_changed();
                     TabsContent::Running {
-                        scope: self.make_scope(tabs),
+                        scope: self.make_scope(tabs, index),
                     }
                 }
                 _ => content,