Refactor thread model to be created from the root event (#2142)

Author: Germain

src/@types/requests.ts

@@ -19,6 +19,7 @@ import { IContent, IEvent } from "../models/event";
 import { Preset, Visibility } from "./partials";
 import { SearchKey } from "./search";
 import { IRoomEventFilter } from "../filter";
+import { Direction } from "../models/event-timeline";
 
 // allow camelcase as these are things that go onto the wire
 /* eslint-disable camelcase */
@@ -144,6 +145,7 @@ export interface IRelationsRequestOpts {
     from?: string;
     to?: string;
     limit?: number;
+    direction?: Direction;
 }
 
 export interface IRelationsResponse {

src/models/event.ts

@@ -277,7 +277,7 @@ export class MatrixEvent extends EventEmitter {
      * it to us and the time we're now constructing this event, but that's better
      * than assuming the local clock is in sync with the origin HS's clock.
      */
-    private readonly localTimestamp: number;
+    public readonly localTimestamp: number;
 
     // XXX: these should be read-only
     public sender: RoomMember = null;

src/models/room.ts

@@ -1377,8 +1377,7 @@ export class Room extends EventEmitter {
             } else {
                 rootEvent.setUnsigned(eventData.unsigned);
             }
-            events.unshift(rootEvent);
-            thread = this.createThread(events);
+            thread = this.createThread(rootEvent, events);
         }
 
         if (event.getUnsigned().transaction_id) {
@@ -1393,8 +1392,12 @@ export class Room extends EventEmitter {
         this.emit(ThreadEvent.Update, thread);
     }
 
-    public createThread(events: MatrixEvent[]): Thread {
-        const thread = new Thread(events, this, this.client);
+    public createThread(rootEvent: MatrixEvent, events?: MatrixEvent[]): Thread {
+        const thread = new Thread(rootEvent, {
+            initialEvents: events,
+            room: this,
+            client: this.client,
+        });
         this.threads.set(thread.id, thread);
         this.reEmitter.reEmit(thread, [
             ThreadEvent.Update,

src/models/thread.ts

@@ -17,11 +17,13 @@ limitations under the License.
 import { MatrixClient } from "../matrix";
 import { ReEmitter } from "../ReEmitter";
 import { RelationType } from "../@types/event";
+import { IRelationsRequestOpts } from "../@types/requests";
 import { MatrixEvent, IThreadBundledRelationship } from "./event";
-import { EventTimeline } from "./event-timeline";
+import { Direction, EventTimeline } from "./event-timeline";
 import { EventTimelineSet } from './event-timeline-set';
 import { Room } from './room';
 import { TypedEventEmitter } from "./typed-event-emitter";
+import { RoomState } from "./room-state";
 
 export enum ThreadEvent {
     New = "Thread.new",
@@ -31,14 +33,16 @@ export enum ThreadEvent {
     ViewThread = "Thred.viewThread",
 }
 
+interface IThreadOpts {
+    initialEvents?: MatrixEvent[];
+    room: Room;
+    client: MatrixClient;
+}
+
 /**
  * @experimental
  */
 export class Thread extends TypedEventEmitter<ThreadEvent> {
-    /**
-     * A reference to the event ID at the top of the thread
-     */
-    private root: string;
     /**
      * A reference to all the events ID at the bottom of the threads
      */
@@ -51,33 +55,37 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
     private lastEvent: MatrixEvent;
     private replyCount = 0;
 
+    public readonly room: Room;
+    public readonly client: MatrixClient;
+
+    public initialEventsFetched = false;
+
     constructor(
-        events: MatrixEvent[] = [],
-        public readonly room: Room,
-        public readonly client: MatrixClient,
+        public readonly rootEvent: MatrixEvent,
+        opts: IThreadOpts,
     ) {
         super();
-        if (events.length === 0) {
-            throw new Error("Can't create an empty thread");
-        }
-
-        this.reEmitter = new ReEmitter(this);
 
+        this.room = opts.room;
+        this.client = opts.client;
         this.timelineSet = new EventTimelineSet(this.room, {
             unstableClientRelationAggregation: true,
             timelineSupport: true,
             pendingEvents: true,
         });
+        this.reEmitter = new ReEmitter(this);
+
+        this.initialiseThread(this.rootEvent);
 
         this.reEmitter.reEmit(this.timelineSet, [
             "Room.timeline",
             "Room.timelineReset",
         ]);
 
-        events.forEach(event => this.addEvent(event));
+        opts?.initialEvents.forEach(event => this.addEvent(event));
 
-        room.on("Room.localEchoUpdated", this.onEcho);
-        room.on("Room.timeline", this.onEcho);
+        this.room.on("Room.localEchoUpdated", this.onEcho);
+        this.room.on("Room.timeline", this.onEcho);
     }
 
     public get hasServerSideSupport(): boolean {
@@ -91,85 +99,115 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
         }
     };
 
+    private get roomState(): RoomState {
+        return this.room.getLiveTimeline().getState(EventTimeline.FORWARDS);
+    }
+
     /**
      * Add an event to the thread and updates
      * the tail/root references if needed
      * Will fire "Thread.update"
      * @param event The event to add
      */
     public async addEvent(event: MatrixEvent, toStartOfTimeline = false): Promise<void> {
-        if (this.timelineSet.findEventById(event.getId())) {
-            return;
+        // Add all incoming events to the thread's timeline set when there's
+        // no server support
+        if (!this.hasServerSideSupport) {
+            if (this.timelineSet.findEventById(event.getId())) {
+                return;
+            }
+
+            // all the relevant membership info to hydrate events with a sender
+            // is held in the main room timeline
+            // We want to fetch the room state from there and pass it down to this thread
+            // timeline set to let it reconcile an event with its relevant RoomMember
+
+            event.setThread(this);
+            this.timelineSet.addEventToTimeline(
+                event,
+                this.liveTimeline,
+                toStartOfTimeline,
+                false,
+                this.roomState,
+            );
+
+            await this.client.decryptEventIfNeeded(event, {});
         }
 
-        if (!this.root) {
-            if (event.isThreadRelation) {
-                this.root = event.threadRootId;
-            } else {
-                this.root = event.getId();
+        if (this.hasServerSideSupport && this.initialEventsFetched) {
+            if (event.localTimestamp > this.lastReply().localTimestamp && !this.findEventById(event.getId())) {
+                this.timelineSet.addEventToTimeline(
+                    event,
+                    this.liveTimeline,
+                    false,
+                    false,
+                    this.roomState,
+                );
             }
         }
 
-        // all the relevant membership info to hydrate events with a sender
-        // is held in the main room timeline
-        // We want to fetch the room state from there and pass it down to this thread
-        // timeline set to let it reconcile an event with its relevant RoomMember
-        const roomState = this.room.getLiveTimeline().getState(EventTimeline.FORWARDS);
-
-        event.setThread(this);
-        this.timelineSet.addEventToTimeline(
-            event,
-            this.timelineSet.getLiveTimeline(),
-            toStartOfTimeline,
-            false,
-            roomState,
-        );
-
         if (!this._currentUserParticipated && event.getSender() === this.client.getUserId()) {
             this._currentUserParticipated = true;
         }
 
-        await this.client.decryptEventIfNeeded(event, {});
-
         const isThreadReply = event.getRelation()?.rel_type === RelationType.Thread;
         // If no thread support exists we want to count all thread relation
         // added as a reply. We can't rely on the bundled relationships count
         if (!this.hasServerSideSupport && isThreadReply) {
             this.replyCount++;
         }
 
-        if (!this.lastEvent || (isThreadReply && event.getTs() > this.lastEvent.getTs())) {
+        // There is a risk that the `localTimestamp` approximation will not be accurate
+        // when threads are used over federation. That could results in the reply
+        // count value drifting away from the value returned by the server
+        if (!this.lastEvent || (isThreadReply && event.localTimestamp > this.replyToEvent.localTimestamp)) {
             this.lastEvent = event;
-            if (this.lastEvent.getId() !== this.root) {
+            if (this.lastEvent.getId() !== this.id) {
                 // This counting only works when server side support is enabled
                 // as we started the counting from the value returned in the
                 // bundled relationship
                 if (this.hasServerSideSupport) {
                     this.replyCount++;
                 }
+
                 this.emit(ThreadEvent.NewReply, this, event);
             }
         }
 
-        if (event.getId() === this.root) {
-            const bundledRelationship = event
-                .getServerAggregatedRelation<IThreadBundledRelationship>(RelationType.Thread);
+        this.emit(ThreadEvent.Update, this);
+    }
 
-            if (this.hasServerSideSupport && bundledRelationship) {
-                this.replyCount = bundledRelationship.count;
-                this._currentUserParticipated = bundledRelationship.current_user_participated;
+    private initialiseThread(rootEvent: MatrixEvent): void {
+        const bundledRelationship = rootEvent
+            .getServerAggregatedRelation<IThreadBundledRelationship>(RelationType.Thread);
 
-                const lastReply = this.findEventById(bundledRelationship.latest_event.event_id);
-                if (lastReply) {
-                    this.lastEvent = lastReply;
-                } else {
-                    const event = new MatrixEvent(bundledRelationship.latest_event);
-                    this.lastEvent = event;
-                }
-            }
+        if (this.hasServerSideSupport && bundledRelationship) {
+            this.replyCount = bundledRelationship.count;
+            this._currentUserParticipated = bundledRelationship.current_user_participated;
+
+            const event = new MatrixEvent(bundledRelationship.latest_event);
+            this.setEventMetadata(event);
+            this.lastEvent = event;
         }
 
-        this.emit(ThreadEvent.Update, this);
+        if (!bundledRelationship) {
+            this.addEvent(rootEvent);
+        }
+    }
+
+    public async fetchInitialEvents(): Promise<boolean> {
+        try {
+            await this.fetchEvents();
+            this.initialEventsFetched = true;
+            return true;
+        } catch (e) {
+            return false;
+        }
+    }
+
+    private setEventMetadata(event: MatrixEvent): void {
+        EventTimeline.setEventMetadata(event, this.roomState, false);
+        event.setThread(this);
     }
 
     /**
@@ -185,7 +223,7 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
     public lastReply(matches: (ev: MatrixEvent) => boolean = () => true): MatrixEvent {
         for (let i = this.events.length - 1; i >= 0; i--) {
             const event = this.events[i];
-            if (event.isThreadRelation && matches(event)) {
+            if (matches(event)) {
                 return event;
             }
         }
@@ -195,14 +233,7 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
      * The thread ID, which is the same as the root event ID
      */
     public get id(): string {
-        return this.root;
-    }
-
-    /**
-     * The thread root event
-     */
-    public get rootEvent(): MatrixEvent {
-        return this.findEventById(this.root);
+        return this.rootEvent.getId();
     }
 
     public get roomId(): string {
@@ -226,14 +257,7 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
     }
 
     public get events(): MatrixEvent[] {
-        return this.timelineSet.getLiveTimeline().getEvents();
-    }
-
-    public merge(thread: Thread): void {
-        thread.events.forEach(event => {
-            this.addEvent(event);
-        });
-        this.events.forEach(event => event.setThread(this));
+        return this.liveTimeline.getEvents();
     }
 
     public has(eventId: string): boolean {
@@ -243,4 +267,55 @@ export class Thread extends TypedEventEmitter<ThreadEvent> {
     public get hasCurrentUserParticipated(): boolean {
         return this._currentUserParticipated;
     }
+
+    public get liveTimeline(): EventTimeline {
+        return this.timelineSet.getLiveTimeline();
+    }
+
+    public async fetchEvents(opts: IRelationsRequestOpts = { limit: 20 }): Promise<{
+        originalEvent: MatrixEvent;
+        events: MatrixEvent[];
+        nextBatch?: string;
+        prevBatch?: string;
+    }> {
+        let {
+            originalEvent,
+            events,
+            prevBatch,
+            nextBatch,
+        } = await this.client.relations(
+            this.room.roomId,
+            this.id,
+            RelationType.Thread,
+            null,
+            opts,
+        );
+
+        // When there's no nextBatch returned with a `from` request we have reached
+        // the end of the thread, and therefore want to return an empty one
+        if (!opts.to && !nextBatch) {
+            events = [originalEvent, ...events];
+        }
+
+        for (const event of events) {
+            await this.client.decryptEventIfNeeded(event);
+            this.setEventMetadata(event);
+        }
+
+        const prependEvents = !opts.direction || opts.direction === Direction.Backward;
+
+        this.timelineSet.addEventsToTimeline(
+            events,
+            prependEvents,
+            this.liveTimeline,
+            prependEvents ? nextBatch : prevBatch,
+        );
+
+        return {
+            originalEvent,
+            events,
+            prevBatch,
+            nextBatch,
+        };
+    }
 }