Allow disk-based diagnostic progress begin/end events to interleave

When multiple saves occur, we can have multiple start events followed by multiple end events. We don't want to update our project diagnostics view until all pending progress is finished.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/gpui/src/app.rs

@@ -992,7 +992,7 @@ impl MutableAppContext {
         })
     }
 
-    fn observe<E, H, F>(&mut self, handle: &H, mut callback: F) -> Subscription
+    pub fn observe<E, H, F>(&mut self, handle: &H, mut callback: F) -> Subscription
     where
         E: Entity,
         E::Event: 'static,

crates/gpui/src/test.rs

@@ -7,7 +7,13 @@ use std::{
     },
 };
 
-use crate::{executor, platform, FontCache, MutableAppContext, Platform, TestAppContext};
+use futures::StreamExt;
+use smol::channel;
+
+use crate::{
+    executor, platform, Entity, FontCache, Handle, MutableAppContext, Platform, Subscription,
+    TestAppContext,
+};
 
 #[cfg(test)]
 #[ctor::ctor]
@@ -87,3 +93,47 @@ pub fn run_test(
         }
     }
 }
+
+pub struct Observation<T> {
+    rx: channel::Receiver<T>,
+    _subscription: Subscription,
+}
+
+impl<T> futures::Stream for Observation<T> {
+    type Item = T;
+
+    fn poll_next(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Option<Self::Item>> {
+        self.rx.poll_next_unpin(cx)
+    }
+}
+
+pub fn observe<T: Entity>(entity: &impl Handle<T>, cx: &mut TestAppContext) -> Observation<()> {
+    let (tx, rx) = smol::channel::unbounded();
+    let _subscription = cx.update(|cx| {
+        cx.observe(entity, move |_, _| {
+            let _ = smol::block_on(tx.send(()));
+        })
+    });
+
+    Observation { rx, _subscription }
+}
+
+pub fn subscribe<T: Entity>(
+    entity: &impl Handle<T>,
+    cx: &mut TestAppContext,
+) -> Observation<T::Event>
+where
+    T::Event: Clone,
+{
+    let (tx, rx) = smol::channel::unbounded();
+    let _subscription = cx.update(|cx| {
+        cx.subscribe(entity, move |_, event, _| {
+            let _ = smol::block_on(tx.send(event.clone()));
+        })
+    });
+
+    Observation { rx, _subscription }
+}