feat(ui): Use an initial limit in room list dynamic entries.

In `RoomList::entries_with_dynamic_adapters`, we were using the
`Limit::dynamic` constructor. It builds a `Limit` stream adapter where
the limits come from a `Stream`. The immediate impact is that this
constructor does only return a new `Stream`, but it cannot return
the initial items of the vector. Thus, it wasn't possible to reset
the final `Stream` with a `VectorDiff::Reset { values }`. Instead, we
were emitting a `Clear` (from this code) then a `Append` (from the
`Limit` stream). Sadly, for some client apps, like Element X iOS, these
2 diffs (`Clear` and `Append`) result in a “rendering blink”.

Hopefully for us, now there is new constructors for `Limit`! One of them
is `Limit::dynamic_with_initial_limit`, which returns the initial items
along with the new `Stream`. That's perfect, we can reset the final
`Stream` with `Reset { values }` again, thus removing the “UI blinking”
effect in Element X iOS.

This patch switches `Limit::dynamic` to
`Limit::dynamic_with_initial_limit`, and updates/simplifies the tests
accordingly.

crates/matrix-sdk-ui/src/room_list_service/room_list.rs

@@ -132,13 +132,13 @@ impl RoomList {
 
         let filter_fn_cell = AsyncCell::shared();
 
-        let dynamic_limit = SharedObservable::<usize>::new(0);
-        let dynamic_limit_stream = dynamic_limit.subscribe();
+        let limit = SharedObservable::<usize>::new(page_size);
+        let limit_stream = limit.subscribe();
 
         let dynamic_entries_controller = RoomListDynamicEntriesController::new(
             filter_fn_cell.clone(),
             page_size,
-            dynamic_limit.clone(),
+            limit.clone(),
             list.maximum_number_of_rooms_stream(),
         );
 
@@ -146,12 +146,10 @@ impl RoomList {
             loop {
                 let filter_fn = filter_fn_cell.take().await;
                 let (items, stream) = list.room_list_filtered_stream(filter_fn);
-                let stream = Limit::dynamic(items, stream, dynamic_limit_stream.clone());
-
-                dynamic_limit.set(page_size);
+                let (items, stream) = Limit::dynamic_with_initial_limit(items, stream, page_size, limit_stream.clone());
 
                 // Clearing the stream before chaining with the real stream.
-                yield stream::once(ready(vec![VectorDiff::Clear]))
+                yield stream::once(ready(vec![VectorDiff::Reset { values: items }]))
                     .chain(stream);
             }
         }

crates/matrix-sdk-ui/tests/integration/room_list_service.rs

@@ -190,22 +190,6 @@ macro_rules! assert_entries_batch {
         )
     };
 
-    // `clear`
-    ( @_ [ $entries:ident ] [ clear ; $( $rest:tt )* ] [ $( $accumulator:tt )* ] ) => {
-        assert_entries_batch!(
-            @_
-            [ $entries ]
-            [ $( $rest )* ]
-            [
-                $( $accumulator )*
-                assert_eq!(
-                    $entries.next(),
-                    Some(&VectorDiff::Clear),
-                );
-            ]
-        )
-    };
-
     // `truncate [$length]`
     ( @_ [ $entries:ident ] [ truncate [ $length:literal ] ; $( $rest:tt )* ] [ $( $accumulator:tt )* ] ) => {
         assert_entries_batch!(
@@ -1659,14 +1643,8 @@ async fn test_dynamic_entries_stream() -> Result<(), Error> {
     // Assert the dynamic entries.
     assert_entries_batch! {
         [dynamic_entries_stream]
-        // Receive a `clear` because the filter has been reset/set for the first time.
-        clear;
-        end;
-    };
-    assert_entries_batch! {
-        [dynamic_entries_stream]
-        // Receive the initial values.
-        append [ F("!r0:bar.org") ];
+        // Receive a `reset` because the filter has been reset/set for the first time.
+        reset [ F("!r0:bar.org") ];
         end;
     };
     assert_pending!(dynamic_entries_stream);
@@ -1821,16 +1799,10 @@ async fn test_dynamic_entries_stream() -> Result<(), Error> {
     // Assert the dynamic entries.
     assert_entries_batch! {
         [dynamic_entries_stream]
-        // Receive a `clear` again because the filter has been reset.
-        clear;
+        // Receive a `reset` again because the filter has been reset.
+        reset [ F("!r2:bar.org"), F("!r3:bar.org"), F("!r6:bar.org") ];
         end;
     }
-    assert_entries_batch! {
-        [dynamic_entries_stream]
-        // Receive the new initial values.
-        append [ F("!r2:bar.org"), F("!r3:bar.org"), F("!r6:bar.org") ];
-        end;
-    };
     assert_pending!(dynamic_entries_stream);
 
     // Now, let's change again the dynamic filter!
@@ -1839,14 +1811,8 @@ async fn test_dynamic_entries_stream() -> Result<(), Error> {
     // Assert the dynamic entries.
     assert_entries_batch! {
         [dynamic_entries_stream]
-        // Receive a `clear` again because the filter has been reset.
-        clear;
-        end;
-    }
-    assert_entries_batch! {
-        [dynamic_entries_stream]
-        // Receive the new initial values.
-        append [
+        // Receive a `reset` again because the filter has been reset.
+        reset [
             F("!r0:bar.org"),
             F("!r1:bar.org"),
             F("!r2:bar.org"),
@@ -1855,7 +1821,7 @@ async fn test_dynamic_entries_stream() -> Result<(), Error> {
             // Stop! The page is full :-).
         ];
         end;
-    };
+    }
     assert_pending!(dynamic_entries_stream);
 
     // Let's ask one more page.