Apple: Trace CFRunLoop activities

Add two run loop observers that:
- Create a TRACE-level span when the run loop enters a new state.
- Drops the span when the run loop exits that state.

These spans attach information to events, such that e.g. resizing a view
produces messages like:
```
TRACE inside runloop{mode=NSEventTrackingRunLoopMode}:timers:
  winit_appkit::util: Triggered `drawRect:` target="winit_appkit::view"
TRACE inside runloop{mode=NSEventTrackingRunLoopMode}:timers:
  winit_appkit::util: Completed `drawRect:` target="winit_appkit::view"
```

winit-common/src/core_foundation/mod.rs

@@ -6,7 +6,9 @@
 mod event_loop_proxy;
 mod main_run_loop;
 mod main_run_loop_observer;
+mod tracing_observers;
 
 pub use self::event_loop_proxy::*;
 pub use self::main_run_loop::*;
 pub use self::main_run_loop_observer::*;
+pub use self::tracing_observers::*;

winit-common/src/core_foundation/tracing_observers.rs

@@ -0,0 +1,146 @@
+use std::cell::RefCell;
+use std::rc::Rc;
+
+use objc2::MainThreadMarker;
+use objc2_core_foundation::{CFIndex, CFRunLoop, CFRunLoopActivity, kCFRunLoopDefaultMode};
+use tracing::span::EnteredSpan;
+use tracing::{Level, error, field, span_enabled, trace_span};
+
+use crate::core_foundation::MainRunLoopObserver;
+
+/// Create two run loop observers that add TRACE-level [spans][tracing::span].
+///
+/// This is useful when debugging run loops, it makes it easier to see in which run loop activity an
+/// event is triggered inside (if any).
+///
+/// When debugging these interactions, it can also be useful to configure your tracing subscriber
+/// with `.with_span_events(tracing_subscriber::fmt::format::FmtSpan::ACTIVE)` to emit events upon
+/// entering and exiting these stages.
+pub fn tracing_observers(
+    mtm: MainThreadMarker,
+) -> Option<(MainRunLoopObserver, MainRunLoopObserver)> {
+    // Observers are a bit costly, so don't create them if the tracing-level for this module is
+    // configured to disable them.
+    if !span_enabled!(Level::TRACE) {
+        return None;
+    }
+
+    /// The state that we think the runloop is currently in.
+    ///
+    /// The order of activities we observe if waiting twice looks something like:
+    /// - CFRunLoopActivity::Entry
+    /// - CFRunLoopActivity::BeforeTimers
+    /// - CFRunLoopActivity::BeforeSources
+    /// - CFRunLoopActivity::BeforeWaiting
+    /// - CFRunLoopActivity::AfterWaiting
+    /// - CFRunLoopActivity::BeforeTimers
+    /// - CFRunLoopActivity::BeforeSources
+    /// - CFRunLoopActivity::BeforeWaiting
+    /// - CFRunLoopActivity::AfterWaiting
+    /// - CFRunLoopActivity::Exit
+    ///
+    /// And if not waiting, it looks something like:
+    /// - CFRunLoopActivity::Entry
+    /// - CFRunLoopActivity::BeforeTimers
+    /// - CFRunLoopActivity::BeforeSources
+    /// - CFRunLoopActivity::Exit
+    #[derive(Default)]
+    #[allow(unused)] // EnteredSpans are kept around
+    enum RunLoopState {
+        /// Currently processing `Entry`/`Exit` observers.
+        #[default]
+        Entered,
+        /// Currently processing timers or `BeforeTimers` observers.
+        Timers(EnteredSpan),
+        /// Currently processing sources or `BeforeSources` observers.
+        Sources(EnteredSpan),
+        /// Currently waiting or processing `BeforeWaiting`/`AfterWaiting` observers.
+        Waiting(EnteredSpan),
+    }
+
+    // A list of currently entered (outer) spans and their state.
+    //
+    // This is a list because runloops can be run recursively.
+    let spans: Rc<RefCell<Vec<(EnteredSpan, RunLoopState)>>> = Rc::new(RefCell::new(Vec::new()));
+    let spans_clone = Rc::clone(&spans);
+
+    // An observer at the start of run loop activities.
+    let activities = CFRunLoopActivity::Entry
+        | CFRunLoopActivity::BeforeTimers
+        | CFRunLoopActivity::BeforeSources
+        | CFRunLoopActivity::BeforeWaiting;
+    let start = MainRunLoopObserver::new(mtm, activities, true, CFIndex::MIN, move |activity| {
+        match activity {
+            // Add an outer span for each runloop iteration.
+            CFRunLoopActivity::Entry => {
+                let span = trace_span!("inside runloop", mode = field::Empty);
+
+                // Get the mode dynamically, the observer may added to multiple different modes.
+                let mode = CFRunLoop::current().unwrap().current_mode().unwrap();
+                // Mode isn't interesting if it's the default mode.
+                if &*mode != unsafe { kCFRunLoopDefaultMode }.unwrap() {
+                    span.record("mode", field::display(mode));
+                }
+
+                let entered = span.entered();
+                spans.borrow_mut().push((entered, RunLoopState::Entered));
+            },
+
+            // Add inner spans that help inspecting the state the runloop is in.
+            CFRunLoopActivity::BeforeTimers => {
+                if let Some((_, state)) = spans.borrow_mut().last_mut() {
+                    *state = RunLoopState::Entered; // Drop any previous spans.
+                    *state = RunLoopState::Timers(trace_span!("processing timers").entered());
+                } else {
+                    error!("unbalanced observer invocations");
+                }
+            },
+            CFRunLoopActivity::BeforeSources => {
+                if let Some((_, state)) = spans.borrow_mut().last_mut() {
+                    *state = RunLoopState::Entered; // Drop any previous spans.
+                    *state = RunLoopState::Sources(trace_span!("processing sources").entered());
+                } else {
+                    error!("unbalanced observer invocations");
+                }
+            },
+            CFRunLoopActivity::BeforeWaiting => {
+                if let Some((_, state)) = spans.borrow_mut().last_mut() {
+                    *state = RunLoopState::Entered; // Drop any previous spans.
+                    *state = RunLoopState::Waiting(trace_span!("waiting").entered());
+                } else {
+                    error!("unbalanced observer invocations");
+                }
+            },
+
+            activity => unreachable!("unexpected activity: {activity:?}"),
+        }
+    });
+
+    // An observer at the end of run loop activities.
+    let activities = CFRunLoopActivity::AfterWaiting | CFRunLoopActivity::Exit;
+    let end = MainRunLoopObserver::new(mtm, activities, true, CFIndex::MAX, move |activity| {
+        match activity {
+            CFRunLoopActivity::AfterWaiting => {
+                if let Some((_, state)) = spans_clone.borrow_mut().last_mut() {
+                    // Transition from the waiting state to the initial state.
+                    *state = RunLoopState::Entered;
+                } else {
+                    error!("unbalanced observer invocations");
+                }
+            },
+
+            CFRunLoopActivity::Exit => {
+                if let Some((span, state)) = spans_clone.borrow_mut().pop() {
+                    drop(state); // Explicitly exit and drop inner span.
+                    drop(span); // Explicitly exit and drop outer span.
+                } else {
+                    error!("unbalanced observer invocations");
+                }
+            },
+
+            activity => unreachable!("unexpected activity: {activity:?}"),
+        }
+    });
+
+    Some((start, end))
+}