Change to event-level beforeRedaction event for efficiency

To avoid an O(n^2) situation with every relations container trying to process
every redaction that may occur in a room, this switches to an event-level
notification, so that the specific relations container who cares can listen to
just the events it wants to know about.

src/models/event.js

@@ -661,6 +661,8 @@ utils.extend(module.exports.MatrixEvent.prototype, {
             throw new Error("invalid redaction_event in makeRedacted");
         }
 
+        this.emit("Event.beforeRedaction", this, redaction_event);
+
         // we attempt to replicate what we would see from the server if
         // the event had been redacted before we saw it.
         //

src/models/relations.js

@@ -43,10 +43,6 @@ export default class Relations extends EventEmitter {
         this._annotationsByKey = {};
         this._annotationsBySender = {};
         this._sortedAnnotationsByKey = [];
-
-        if (room) {
-            room.on("Room.beforeRedaction", this._onBeforeRedaction);
-        }
     }
 
     /**
@@ -78,6 +74,8 @@ export default class Relations extends EventEmitter {
 
         this._relations.add(event);
 
+        event.on("Event.beforeRedaction", this._onBeforeRedaction);
+
         this.emit("Relations.add", event);
     }
 
@@ -127,7 +125,7 @@ export default class Relations extends EventEmitter {
      * For relations that have been redacted, we want to remove them from
      * aggregation data sets and emit an update event.
      *
-     * To do so, we listen for `Room.beforeRedaction`, which happens:
+     * To do so, we listen for `Event.beforeRedaction`, which happens:
      *   - after the server accepted the redaction and remote echoed back to us
      *   - before the original event has been marked redacted in the client
      *
@@ -162,6 +160,8 @@ export default class Relations extends EventEmitter {
             });
         }
 
+        redactedEvent.removeListener("Event.beforeRedaction", this._onBeforeRedaction);
+
         // Dispatch a redaction event on this collection. `setTimeout` is used
         // to wait until the next event loop iteration by which time the event
         // has actually been marked as redacted.

src/models/room.js

@@ -1011,7 +1011,6 @@ Room.prototype._addLiveEvent = function(event, duplicateStrategy) {
         // if we know about this event, redact its contents now.
         const redactedEvent = this.getUnfilteredTimelineSet().findEventById(redactId);
         if (redactedEvent) {
-            this.emit("Room.beforeRedaction", redactedEvent, event, this);
             redactedEvent.makeRedacted(event);
             this.emit("Room.redaction", event, this);