I have implemented adding room to a Space.
I needed help from Tobias with it. I wanted to send m.space.child room event to a Space, but didn't know the right way to do it. Tobias suggested that I use the following function
SetRoomStateWithKeyJob* setState(const QString& evtType,
const QString& stateKey,
const QJsonObject& contentJson);
Here, contentJson will have the room event data.
The function call I used for removing a room from Space is
connection->callApi<SetRoomStateWithKeyJob>(spaceId, "m.space.child", roomId, QJsonObject{});
Similarly, the function call I used to add room to Space is
connection->callApi<SetRoomStateWithKeyJob>(spaceId, "m.space.child", roomId, QJsonObject{{"via", {connection->homeserver().toString()}}});
Its all well and good, except that adding room to Space doesn't work!
I asked about this to Tobias on our call. He pointed out that {connection->homeserver().toString()} should be written as QJsonObject{connection->homeserver().toString()}.
So, finally the call becomes connection->callApi<SetRoomStateWithKeyJob>(spaceId, "m.space.child", roomId, QJsonObject{{"via", QJsonArray{connection->homeserver().toString()}}});
With this, one can add or remove rooms. This feature is being added to the Child Rooms editor in Space Settings.