Meteor template autorun (Session variable) - meteor

Question

Imagine I have a session variable that holds an image source. Every second, I want to the helper that contains this session to run.
if (Meteor.isClient) {
Template.TargetTask.helpers({
'imageSrc': function (e, template) {
var clock = setTimeout(function() {
var position = IMAGE_POOL.pop();
Session.set("currentTarget", position);
}, 1000);
var position = Session.get("currentTarget");
var imageSrc = '/' + position + '.bmp';
return imageSrc;
}
});
the image sources are coming from a global IMAGE_POOL. However, it is possible that the pool contains two same images consecutively. In this case, Session.set() will be called with the same argument and the session will remain unchanged.
Q1. When Session variable remains unchanged, does the template helper not autorun even if Session.set() is called?
Q2. If so, how should I make it run every time a new image is popped?

Answer 1

No, Tracker computations are not invalidated if the value doesn't change.
Session.set('test', false);
Tracker.autorun(function() { console.log(Session.get('test')) }); //Logs 'false'
Session.set('test', false); //Nothing
Session.set('test', true); //Logs true
In your case, if you want to preserve this code structure (which seems a bit heavy to me) you could instead store an object with a timestamp:
if (Meteor.isClient) {
Template.TargetTask.helpers({
'imageSrc': function (e, template) {
var clock = setTimeout(function() {
var position = IMAGE_POOL.pop();
Session.set("currentTarget", {
position : position,
timestamp : Date.now()
});
}, 1000);
var position = Session.get("currentTarget").position;
var imageSrc = '/' + position + '.bmp';
return imageSrc;
}
});
}