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Tracing subscribers must be set up before `EventLoop::new()`.
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"
```
Spans are more powerful, and can even optionally be emitted as events by using `.with_span_events(tracing_subscriber::fmt::format::FmtSpan::*)`.
This allows the examples to work a bit better in WASM and on iOS.
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Most events in AppKit go through `sendEvent:`, and they contain a lot of information, so it's nice to surface this when debugging. We could override `sendEvent:` in UIKit and track this in there too, but that's much less important, since there the relevant events are fairly narrowly scoped, see the link below, other events go through CFRunLoop. https://developer.apple.com/documentation/uikit/uievent/eventtype
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kchibisov
requested changes
Feb 24, 2026
| // FIXME(madsmtm): Use `std::ptr::fn_addr_eq` (Rust 1.85) once available in MSRV. | ||
| #[allow(unknown_lints, unpredictable_function_pointer_comparisons)] | ||
| if overridden == method.implementation() { | ||
| if ptr::fn_addr_eq(overridden, method.implementation()) { |
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I think this should be sent as a separate patch? Wouldn't mind that much though.
| pub fn flush_requests(&self) -> Result<(), XError> { | ||
| unsafe { (self.xlib.XFlush)(self.display) }; | ||
| // println!("XFlush"); | ||
| // tracing::trace!("XFlush"); |
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Suggested change
| // tracing::trace!("XFlush"); |
| pub fn sync_with_server(&self) -> Result<(), XError> { | ||
| unsafe { (self.xlib.XSync)(self.display, ffi::False) }; | ||
| // println!("XSync"); | ||
| // tracing::trace!("XSync"); |
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| // tracing::trace!("XSync"); |
| ToUnicodeResult::Dead(dead_char) => { | ||
| // println!("{:?} - {:?} produced dead {:?}", key_code, mod_state, | ||
| // dead_char); | ||
| // trace!("{:?} - {:?} produced dead {:?}", key_code, mod_state, dead_char); |
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| // trace!("{:?} - {:?} produced dead {:?}", key_code, mod_state, dead_char); |
| pub fn tracing_observers( | ||
| mtm: MainThreadMarker, | ||
| ) -> Option<(MainRunLoopObserver, MainRunLoopObserver)> { | ||
| // HINT: You can use something like the following to emit relevant events: |
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This thing looks out of place?
| }; | ||
|
|
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| extern "C-unwind" fn send_event(app: &NSApplication, sel: Sel, event: &NSEvent) { | ||
| // Pass `RUST_LOG='trace,winit_appkit::app=warn'` if you want TRACE logs but not this. |
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Not sure who'll read this, maybe it should be somewhere more visible or do you left it for yourself?
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Use
tracing::spanin clever ways to improve the logging infrastructure in the AppKit and UIKit backends. See the commits for details.Running
RUST_LOG='trace,winit_appkit::app=off' cargo run --example windowand resizing now produces traces like:With this, one can glance that
RedrawRequestedis called insidedrawRect:, and that it is in turn run insideNSEventTrackingRunLoopModeas a timer - which should in turn help debug why we're issuingRedrawRequestedtwice.Tested AppKit backend on macOS 15.7.3 and UIKit briefly with Mac Catalyst.
I didn't include a changelog entry since these are meant to be internal.