support paginating relations

src/base-apis.js

@@ -440,14 +440,20 @@ MatrixBaseApis.prototype.createRoom = function(options, callback) {
 };
 
 MatrixBaseApis.prototype.fetchRelations =
-    async function(roomId, eventId, relationType, eventType) {
+    async function(roomId, eventId, relationType, eventType, opts) {
+    const queryParams = {};
+    if (opts.token) {
+        queryParams.from = opts.token;
+    }
+    const queryString = utils.encodeParams(queryParams);
     // valid options include: room_alias_name, visibility, invite
     const response = await this._http.authedRequestWithPrefix(
         undefined,
-        "GET", `/rooms/${roomId}/relations/${eventId}/${relationType}/${eventType}`,
+        "GET",
+        `/rooms/${roomId}/relations/${eventId}/${relationType}/${eventType}?` + queryString,
         null, null, httpApi.PREFIX_UNSTABLE,
     );
-    return response.chunk;
+    return response;
 };
 
 /**

src/client.js

@@ -3966,9 +3966,17 @@ MatrixClient.prototype.getCanResetTimelineCallback = function() {
 };
 
 MatrixClient.prototype.relations =
-async function(roomId, eventId, relationType, eventType) {
-    const events = await this.fetchRelations(roomId, eventId, relationType, eventType);
-    return events.map(this.getEventMapper());
+async function(roomId, eventId, relationType, eventType, opts = {}) {
+    const result = await this.fetchRelations(
+        roomId,
+        eventId,
+        relationType,
+        eventType,
+        opts);
+    return {
+        events: result.chunk.map(this.getEventMapper()),
+        nextBatch: result.next_batch,
+    };
 };
 
 function setupCallEventHandler(client) {