流星辅助函数与变量？

Question

I was trying to display a count of all online users on my website, and installed the meteor-user-status plugin to help with this. I thought all I had to do was add a template helper

Template.header.helpers({
    numOnline: Meteor.users.find({"status.online":true}).count()
});

like so, and include the {{numOnline}} variable in my header template. But, for some reason, that resulted in always displaying 0 users online. However, when I simply ran the method Meteor.users.find({"status.online":true}).count() in the javascript console, it gave the correct amount of users online.

After messing around with it a bit, I got it to work by wrapping it in a function:

Template.header.helpers({
    numOnline: function(){
        return Meteor.users.find({"status.online":true}).count();
    }
});

This change makes it work perfectly, but I have no clue why. Can someone explain why it was necessary to wrap it in a function?

Answer 1

Adding Christian Fritz, The only reason I think this can be happening is in the first case numOnline: Meteor.users.find({"status.online":true}).count() the collection is not ready at the point of evaluation of the template and assign 0 or empty array [] since is what the subscription return, and in the second case since is a function will react whenever a change occurs in a collection, so that's why will have the value a soon the collection get fill with the subscription and the function will get execute whit the most recent value.Just my two cents. Please correct me if I'm wrong.

Answer 2

Well, that's just what it is (and the documentation tells you so, too). The reason why this need to be a function is reactivity: in order to reevaluate the piece of code at a later point in time (when a reactive datasource has changed value), you need to have a function.