Implement update_anim pass (#539)

This is part of the Pass Specification RFC:
https://github.com/linebender/rfcs/pull/7

---------

Co-authored-by: Daniel McNab <36049421+DJMcNab@users.noreply.github.com>

masonry/src/contexts.rs

@@ -450,6 +450,7 @@ impl_context_method!(MutateCtx<'_>, EventCtx<'_>, LifeCycleCtx<'_>, {
     pub fn request_anim_frame(&mut self) {
         trace!("request_anim_frame");
         self.widget_state.request_anim = true;
+        self.widget_state.needs_anim = true;
     }
 
     /// Indicate that your children have changed.

masonry/src/passes/update.rs

@@ -230,3 +230,60 @@ pub(crate) fn run_update_scroll_pass(root: &mut RenderRoot) {
         });
     }
 }
+
+// ----------------
+
+fn update_anim_for_widget(
+    global_state: &mut RenderRootState,
+    mut widget: ArenaMut<'_, Box<dyn Widget>>,
+    mut state: ArenaMut<'_, WidgetState>,
+    elapsed_ns: u64,
+) {
+    let _span = widget.item.make_trace_span().entered();
+
+    if !state.item.needs_anim {
+        return;
+    }
+    state.item.needs_anim = false;
+
+    // Most passes reset their `needs` and `request` flags after the call to
+    // the widget method, but it's valid and expected for `request_anim` to be
+    // set in response to `AnimFrame`.
+    if state.item.request_anim {
+        state.item.request_anim = false;
+        let mut ctx = LifeCycleCtx {
+            global_state,
+            widget_state: state.item,
+            widget_state_children: state.children.reborrow_mut(),
+            widget_children: widget.children.reborrow_mut(),
+        };
+        widget
+            .item
+            .lifecycle(&mut ctx, &LifeCycle::AnimFrame(elapsed_ns));
+    }
+
+    let id = state.item.id;
+    let parent_state = state.item;
+    recurse_on_children(
+        id,
+        widget.reborrow_mut(),
+        state.children,
+        |widget, mut state| {
+            update_anim_for_widget(global_state, widget, state.reborrow_mut(), elapsed_ns);
+            parent_state.merge_up(state.item);
+        },
+    );
+}
+
+/// Run the animation pass.
+pub(crate) fn run_update_anim_pass(root: &mut RenderRoot, elapsed_ns: u64) {
+    let _span = info_span!("update_anim").entered();
+
+    let (root_widget, mut root_state) = root.widget_arena.get_pair_mut(root.root.id());
+    update_anim_for_widget(
+        &mut root.state,
+        root_widget,
+        root_state.reborrow_mut(),
+        elapsed_ns,
+    );
+}

masonry/src/render_root.rs

@@ -26,7 +26,7 @@ use crate::passes::layout::root_layout;
 use crate::passes::mutate::{mutate_widget, run_mutate_pass};
 use crate::passes::paint::root_paint;
 use crate::passes::update::{
-    run_update_disabled_pass, run_update_pointer_pass, run_update_scroll_pass,
+    run_update_anim_pass, run_update_disabled_pass, run_update_pointer_pass, run_update_scroll_pass,
 };
 use crate::text::TextBrush;
 use crate::tree_arena::TreeArena;
@@ -208,12 +208,17 @@ impl RenderRoot {
                 // See https://github.com/linebender/druid/issues/85 for discussion.
                 let last = self.last_anim.take();
                 let elapsed_ns = last.map(|t| now.duration_since(t).as_nanos()).unwrap_or(0) as u64;
-                let root_state = self.root_state();
-                if root_state.request_anim {
-                    root_state.request_anim = false;
-                    self.root_lifecycle(LifeCycle::AnimFrame(elapsed_ns));
-                    self.last_anim = Some(now);
-                }
+
+                run_update_anim_pass(self, elapsed_ns);
+
+                let mut root_state = self.widget_arena.get_state_mut(self.root.id()).item.clone();
+                self.post_event_processing(&mut root_state);
+
+                // If this animation will continue, store the time.
+                // If a new animation starts, then it will have zero reported elapsed time.
+                let animation_continues = root_state.needs_anim;
+                self.last_anim = animation_continues.then_some(now);
+
                 Handled::Yes
             }
             WindowEvent::RebuildAccessTree => {
@@ -272,8 +277,6 @@ impl RenderRoot {
         // TODO - Xilem's reconciliation logic will have to be called
         // by the function that calls this
 
-        // TODO - if root widget's request_anim is still set by the
-        // time this is called, emit a warning
         if self.root_state().needs_layout {
             self.root_layout();
         }

masonry/src/widget/widget_pod.rs

@@ -333,12 +333,10 @@ impl<W: Widget> WidgetPod<W> {
 
                 true
             }
-            LifeCycle::AnimFrame(_) => {
-                state.request_anim = false;
-                true
-            }
             // Routing DisabledChanged has been moved to the update_disabled pass
             LifeCycle::DisabledChanged(_) => false,
+            // Animations have been moved to the update_anim pass
+            LifeCycle::AnimFrame(_) => false,
             LifeCycle::BuildFocusChain => {
                 if state.update_focus_chain {
                     // Replace has_focus to check if the value changed in the meantime

masonry/src/widget/widget_state.rs

@@ -99,8 +99,10 @@ pub struct WidgetState {
     /// The accessibility method must be called on this widget or a descendant
     pub(crate) needs_accessibility: bool,
 
-    /// Any descendant has requested an animation frame.
+    /// An animation must run on this widget
     pub(crate) request_anim: bool,
+    /// An animation must run on this widget or a descendant
+    pub(crate) needs_anim: bool,
 
     /// This widget or a descendant changed its `explicitly_disabled` value
     pub(crate) needs_update_disabled: bool,
@@ -174,6 +176,7 @@ impl WidgetState {
             needs_accessibility: true,
             has_focus: false,
             request_anim: true,
+            needs_anim: true,
             needs_update_disabled: true,
             focus_chain: Vec::new(),
             children_changed: true,
@@ -197,11 +200,12 @@ impl WidgetState {
             needs_layout: false,
             request_compose: false,
             needs_compose: false,
-            needs_paint: false,
             request_paint: false,
+            needs_paint: false,
             request_accessibility: false,
             needs_accessibility: false,
             request_anim: false,
+            needs_anim: false,
             needs_update_disabled: false,
             children_changed: false,
             update_focus_chain: false,
@@ -231,7 +235,7 @@ impl WidgetState {
         self.needs_layout |= child_state.needs_layout;
         self.needs_compose |= child_state.needs_compose;
         self.needs_paint |= child_state.needs_paint;
-        self.request_anim |= child_state.request_anim;
+        self.needs_anim |= child_state.needs_anim;
         self.needs_accessibility |= child_state.needs_accessibility;
         self.needs_update_disabled |= child_state.needs_update_disabled;
         self.has_focus |= child_state.has_focus;