Parse iOS SDK: How to set user "online" status?

Question

Scenario = I have an app that allows users to log in and view other users who are also "online". In order to set each user's online status I believe that I should set the code...

[PFUser currentUser] setObject:[NSNumber numberWithBool:YES] forKey:@"isOnline"];
[[PFUser currentUser] saveInBackground];

at certain times in the application when the user uses the app. Probably the appDelegate but I'm not sure.

(if this is not correct then please correct me)

Question = Where should this code be set inside the application so that it always keeps track of when a user is "online" and when they are "offline"?

(please include method names)

Answer 1

The most reliable way to handle this is to create a column named lastActive of type Date, then create a Cloud Code function to update this with the server time (so clock differences isn't an issue).

Then to get "online" users just have another Cloud Function that does a query where lastActive is greater than now - some time window like 2 minutes.

var moment = require("moment");

Parse.Cloud.define("registerActivity", function(request, response) {
    var user = request.user;
    user.set("lastActive", new Date());
    user.save().then(function (user) {
        response.success();
    }, function (error) {
        console.log(error);
        response.error(error);
    });
});

Parse.Cloud.define("getOnlineUsers", function(request, response) {
    var userQuery = new Parse.Query(Parse.User);
    var activeSince = moment().subtract("minutes", 2).toDate();
    userQuery.greaterThan("lastActive", activeSince);
    userQuery.find().then(function (users) {
        response.success(users);
    }, function (error) {
        response.error(error);
    });
});

Your client will want to call the registerActivity Cloud Function every 1.5 minutes to allow some overlap so users don't appear to go offline if their internet is a bit slow.

Of course you can adjust the time windows to suit your needs. You could also add the ability to filter which users are returned (e.g. online friends only).