feat(ui): Add the RoomList Input API.

This patch lands the first design of the `Input` API. An `Input` is
something external that can be understood by the `RoomList` state
machine. The first inpuput is `Viewport` to change the “viewport” of the
`RoomList`, which translates to the change of the ranges of one specific
Sliding Sync list.

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

@@ -10,7 +10,8 @@ use futures_util::{pin_mut, Stream, StreamExt};
 use imbl::Vector;
 pub use matrix_sdk::RoomListEntry;
 use matrix_sdk::{
-    Client, Error as SlidingSyncError, SlidingSync, SlidingSyncList, SlidingSyncMode,
+    sliding_sync::Ranges, Client, Error as SlidingSyncError, SlidingSync, SlidingSyncList,
+    SlidingSyncMode,
 };
 use once_cell::sync::Lazy;
 use thiserror::Error;
@@ -115,6 +116,25 @@ impl RoomList {
             .ok_or_else(|| Error::UnknownList(ALL_ROOMS_LIST_NAME.to_string()))
     }
 
+    pub async fn apply_input(&self, input: Input) -> Result<(), Error> {
+        use Input::*;
+
+        match input {
+            Viewport(ranges) => {
+                self.sliding_sync
+                    .on_list(VISIBLE_ROOMS_LIST_NAME, |list| {
+                        ready(list.set_sync_mode(
+                            SlidingSyncMode::new_selective().add_ranges(ranges.clone()),
+                        ))
+                    })
+                    .await
+                    .ok_or_else(|| Error::InputHasNotBeenApplied(Viewport(ranges)))?;
+            }
+        }
+
+        Ok(())
+    }
+
     #[cfg(any(test, feature = "testing"))]
     pub fn sliding_sync(&self) -> &SlidingSync {
         &self.sliding_sync
@@ -136,6 +156,9 @@ pub enum Error {
 
     #[error("Failed to acquire a lock to update the entries filter")]
     CannotUpdateEntriesFilter,
+
+    #[error("The input has been not applied")]
+    InputHasNotBeenApplied(Input),
 }
 
 #[derive(Copy, Clone, Debug, PartialEq)]
@@ -257,6 +280,11 @@ impl Actions {
     }
 }
 
+#[derive(Debug)]
+pub enum Input {
+    Viewport(Ranges),
+}
+
 #[cfg(test)]
 mod tests {
     use matrix_sdk::{config::RequestConfig, Session};

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

@@ -5,7 +5,7 @@ use imbl::vector;
 use matrix_sdk_test::async_test;
 use matrix_sdk_ui::{
     room_list::{
-        Error, RoomListEntry, State, ALL_ROOMS_LIST_NAME as ALL_ROOMS,
+        Error, Input, RoomListEntry, State, ALL_ROOMS_LIST_NAME as ALL_ROOMS,
         VISIBLE_ROOMS_LIST_NAME as VISIBLE_ROOMS,
     },
     RoomList,
@@ -724,3 +724,79 @@ async fn test_entries_stream_with_updated_filter() -> Result<(), Error> {
 
     Ok(())
 }
+
+#[async_test]
+async fn test_input_viewport() -> Result<(), Error> {
+    let (server, room_list) = new_room_list().await?;
+
+    let sync = room_list.sync();
+    pin_mut!(sync);
+
+    // The input cannot be applied because the `VISIBLE_ROOMS_LIST_NAME` list isn't
+    // present.
+    assert_matches!(
+        room_list.apply_input(Input::Viewport(vec![10..=15])).await,
+        Err(Error::InputHasNotBeenApplied(_))
+    );
+
+    sync_then_assert_request_and_fake_response! {
+        [server, room_list, sync]
+        states = Init -> FirstRooms,
+        assert request = {
+            "lists": {
+                ALL_ROOMS: {
+                    "ranges": [[0, 19]],
+                },
+            },
+        },
+        respond with = {
+            "pos": "0",
+            "lists": {},
+            "rooms": {},
+        },
+    };
+
+    sync_then_assert_request_and_fake_response! {
+        [server, room_list, sync]
+        states = FirstRooms -> AllRooms,
+        assert request = {
+            "lists": {
+                ALL_ROOMS: {
+                    "ranges": [[0, 49]],
+                },
+                VISIBLE_ROOMS: {
+                    "ranges": [],
+                }
+            }
+        },
+        respond with = {
+            "pos": "1",
+            "lists": {},
+            "rooms": {},
+        },
+    };
+
+    assert!(room_list.apply_input(Input::Viewport(vec![10..=15, 20..=25])).await.is_ok());
+
+    sync_then_assert_request_and_fake_response! {
+        [server, room_list, sync]
+        states = AllRooms -> Enjoy,
+        assert request = {
+            "lists": {
+                ALL_ROOMS: {
+                    "ranges": [[0, 49]],
+                },
+                VISIBLE_ROOMS: {
+                    "ranges": [[10, 15], [20, 25]],
+                }
+            }
+        },
+        respond with = {
+            "pos": "1",
+            "lists": {},
+            "rooms": {},
+        },
+    };
+
+    Ok(())
+}