From a1d88722aa061a4c8683a37f4eec32dc5430eb37 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Thu, 10 Dec 2015 20:53:21 +0000
Subject: [PATCH] Avoid kicking off two pagination requests at once

Make sure that we don't end up with two concurrent pagination requests by
firing off the second from the completion handler of the first. This ends up
making the code a bit simpler.
---
 src/components/structures/RoomView.js | 41 +++++++++++++++------------
 1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/src/components/structures/RoomView.js b/src/components/structures/RoomView.js
index 46258f9580..d438e0cbbe 100644
--- a/src/components/structures/RoomView.js
+++ b/src/components/structures/RoomView.js
@@ -288,9 +288,6 @@ module.exports = React.createClass({
             var heightGained = messageWrapperScroll.scrollHeight - this.oldScrollHeight;
             messageWrapperScroll.scrollTop += heightGained;
             this.oldScrollHeight = undefined;
-            if (!this.fillSpace()) {
-                this.setState({paginating: false});
-            }
         } else if (this.atBottom) {
             messageWrapperScroll.scrollTop = messageWrapperScroll.scrollHeight;
             if (this.state.numUnreadMessages !== 0) {
@@ -299,36 +296,44 @@ module.exports = React.createClass({
         }
     },
 
+    _paginateCompleted: function() {
+        this.waiting_for_paginate = false;
+        this.setState({
+            room: MatrixClientPeg.get().getRoom(this.props.roomId)
+        });
+
+        // we might not have got enough results from the pagination
+        // request, so give fillSpace() a chance to set off another.
+        if (!this.fillSpace()) {
+            this.setState({paginating: false});
+        }
+    },
+
+    // check the scroll position, and if we need to, set off a pagination
+    // request.
+    //
+    // returns true if a pagination request was started (or is still in progress)
     fillSpace: function() {
         if (!this.refs.messagePanel) return;
         if (this.state.searchResults) return; // TODO: paginate search results
         var messageWrapperScroll = this._getScrollNode();
         if (messageWrapperScroll.scrollTop < messageWrapperScroll.clientHeight && this.state.room.oldState.paginationToken) {
-            this.setState({paginating: true});
+            // there's less than a screenful of messages left. Either wind back
+            // the message cap (if there are enough events in the timeline to
+            // do so), or fire off a pagination request.
 
             this.oldScrollHeight = messageWrapperScroll.scrollHeight;
 
             if (this.state.messageCap < this.state.room.timeline.length) {
-                this.waiting_for_paginate = false;
                 var cap = Math.min(this.state.messageCap + PAGINATE_SIZE, this.state.room.timeline.length);
-                this.setState({messageCap: cap, paginating: true});
+                this.setState({messageCap: cap});
             } else {
                 this.waiting_for_paginate = true;
                 var cap = this.state.messageCap + PAGINATE_SIZE;
                 this.setState({messageCap: cap, paginating: true});
-                var self = this;
-                MatrixClientPeg.get().scrollback(this.state.room, PAGINATE_SIZE).finally(function() {
-                    self.waiting_for_paginate = false;
-                    if (self.isMounted()) {
-                        self.setState({
-                            room: MatrixClientPeg.get().getRoom(self.props.roomId)
-                        });
-                    }
-                    // wait and set paginating to false when the component updates
-                });
+                MatrixClientPeg.get().scrollback(this.state.room, PAGINATE_SIZE).finally(this._paginateCompleted).done();
+                return true;
             }
-
-            return true;
         }
         return false;
     },