First cut at request_update

CHANGELOG.md

@@ -17,6 +17,7 @@ You can find its changes [documented below](#060---2020-06-01).
 - `BoxConstraints::UNBOUNDED` constant. ([#1126] by [@danieldulaney]) 
 - 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])
 
 ### Changed
 
@@ -389,6 +390,7 @@ Last release without a changelog :(
 [#1119]: https://github.com/linebender/druid/pull/1119
 [#1120]: https://github.com/linebender/druid/pull/1120
 [#1126]: https://github.com/linebender/druid/pull/1120
+[#1128]: https://github.com/linebender/druid/pull/1128
 [#1133]: https://github.com/linebender/druid/pull/1133
 
 [Unreleased]: https://github.com/linebender/druid/compare/v0.6.0...master

druid/src/contexts.rs

@@ -495,6 +495,19 @@ impl EventCtx<'_, '_> {
             );
         }
     }
+
+    /// Request an update cycle.
+    ///
+    /// After this, `update` will be called on the widget in the next update cycle, even
+    /// if there's not a data change.
+    ///
+    /// The use case for this method is when a container widget synthesizes data for its
+    /// children. This is appropriate in specialized cases, but before reaching for this
+    /// method, consider whether it might be better to refactor to be more idiomatic, in
+    /// particular to make that data available in the app state.
+    pub fn request_update(&mut self) {
+        self.widget_state.request_update = true;
+    }
 }
 
 impl LifeCycleCtx<'_, '_> {

druid/src/core.rs

@@ -108,6 +108,9 @@ pub(crate) struct WidgetState {
     /// Any descendant has requested an animation frame.
     pub(crate) request_anim: bool,
 
+    /// Any descendant has requested update.
+    pub(crate) request_update: bool,
+
     pub(crate) focus_chain: Vec<WidgetId>,
     pub(crate) request_focus: Option<FocusChange>,
     pub(crate) children: Bloom<WidgetId>,
@@ -844,15 +847,17 @@ impl<T: Data, W: Widget<T>> WidgetPod<T, W> {
     ///
     /// [`update`]: trait.Widget.html#tymethod.update
     pub fn update(&mut self, ctx: &mut UpdateCtx, data: &T, env: &Env) {
-        match (self.old_data.as_ref(), self.env.as_ref()) {
-            (Some(d), Some(e)) if d.same(data) && e.same(env) => return,
-            (None, _) => {
-                log::warn!("old_data missing in {:?}, skipping update", self.id());
-                self.old_data = Some(data.clone());
-                self.env = Some(env.clone());
-                return;
+        if !self.state.request_update {
+            match (self.old_data.as_ref(), self.env.as_ref()) {
+                (Some(d), Some(e)) if d.same(data) && e.same(env) => return,
+                (None, _) => {
+                    log::warn!("old_data missing in {:?}, skipping update", self.id());
+                    self.old_data = Some(data.clone());
+                    self.env = Some(env.clone());
+                    return;
+                }
+                _ => (),
             }
-            _ => (),
         }
 
         let mut child_ctx = UpdateCtx {
@@ -865,7 +870,8 @@ impl<T: Data, W: Widget<T>> WidgetPod<T, W> {
         self.old_data = Some(data.clone());
         self.env = Some(env.clone());
 
-        ctx.widget_state.merge_up(&mut self.state)
+        self.state.request_update = false;
+        ctx.widget_state.merge_up(&mut self.state);
     }
 }
 
@@ -893,6 +899,7 @@ impl WidgetState {
             has_active: false,
             has_focus: false,
             request_anim: false,
+            request_update: false,
             request_focus: None,
             focus_chain: Vec::new(),
             children: Bloom::new(),
@@ -925,6 +932,7 @@ impl WidgetState {
         self.has_active |= child_state.has_active;
         self.has_focus |= child_state.has_focus;
         self.children_changed |= child_state.children_changed;
+        self.request_update |= child_state.request_update;
         self.request_focus = child_state.request_focus.take().or(self.request_focus);
         self.timers.extend_drain(&mut child_state.timers);
     }

druid/src/tests/mod.rs

@@ -463,3 +463,27 @@ fn register_after_adding_child() {
         assert_eq!(harness.get_state(id_5).children.entry_count(), 5);
     })
 }
+
+#[test]
+/// Test that request_update actually causes the request.
+fn request_update() {
+    const REQUEST_UPDATE: Selector = Selector::new("druid-tests.request_update");
+    let updated: Rc<Cell<bool>> = Default::default();
+    let updated_clone = updated.clone();
+
+    let widget = ModularWidget::new(())
+        .event_fn(|_, ctx, event, _data, _env| {
+            if matches!(event, Event::Command(cmd) if cmd.is(REQUEST_UPDATE)) {
+                ctx.request_update();
+            }
+        })
+        .update_fn(move |_, _ctx, _old_data, _data, _env| {
+            updated_clone.set(true);
+        });
+    Harness::create_simple((), widget, |harness| {
+        harness.send_initial_events();
+        assert!(!updated.get());
+        harness.submit_command(REQUEST_UPDATE, None);
+        assert!(updated.get());
+    })
+}