im: Only emit VectorDiff::Clear from ObservableVector::clear when…

… necessary

The idea behind this patch is to avoid emitting a `VectorDiff::Clear`
when an `ObservableVector` is already empty.

eyeball-im/src/vector.rs

@@ -80,11 +80,19 @@ impl<T: Clone + Send + Sync + 'static> ObservableVector<T> {
 
     /// Clear out all of the elements in this `Vector` and notify subscribers.
     pub fn clear(&mut self) {
-        #[cfg(feature = "tracing")]
-        tracing::debug!(target: "eyeball_im::vector::update", "clear");
+        let already_empty = self.values.is_empty();
 
-        self.values.clear();
-        self.broadcast_diff(VectorDiff::Clear);
+        #[cfg(feature = "tracing")]
+        tracing::debug!(
+            target: "eyeball_im::vector::update",
+            nop = already_empty.then_some(true),
+            "clear"
+        );
+
+        if !already_empty {
+            self.values.clear();
+            self.broadcast_diff(VectorDiff::Clear);
+        }
     }
 
     /// Add an element at the front of the list and notify subscribers.

eyeball-im/tests/it/main.rs

@@ -1,5 +1,5 @@
 use imbl::{vector, Vector};
-use stream_assert::{assert_next_eq, assert_pending};
+use stream_assert::{assert_closed, assert_next_eq, assert_pending};
 
 use eyeball_im::{ObservableVector, ObservableVectorEntry, VectorDiff};
 
@@ -58,6 +58,25 @@ fn truncate() {
     assert!(ob.is_empty());
 }
 
+#[test]
+fn clear() {
+    let mut ob: ObservableVector<i32> = ObservableVector::from(vector![1, 2]);
+    let mut sub = ob.subscribe().into_stream();
+    assert_pending!(sub);
+
+    ob.clear();
+    assert_next_eq!(sub, VectorDiff::Clear);
+    assert!(ob.is_empty());
+
+    // Clearing again. The vector is empty now. We don't expect a
+    // `VectorDiff::Clear`.
+    ob.clear();
+    assert_pending!(sub);
+
+    drop(ob);
+    assert_closed!(sub);
+}
+
 #[test]
 fn for_each() {
     let mut ob: ObservableVector<i32> = ObservableVector::from(vector![0, 10, 1, 2, 4, 33, 5]);