CODEWITHSUNDEEP
meteor
Jason Ip
Jason Ip

Reputation: 353

Meteor reactivity in functions, return session.get('variable'), runs everytime session.set('variable') is run?

Ok, let's say I have a client side function that returns a Session variable, eg:

Template.hello.random = function() {
  return Session.get('variable');
};

Somewhere else let's say I have a button that does

Session.set('variable', random_number);

Everytime I hit that button, will my Template.hello.random function run? I find it hard to wrap my head around that..

Upvotes: 1

Views: 210

Answers (1)

nathan-m
nathan-m

Reputation: 8865

All the Meteor "Magic" comes from Deps.Dependency and Deps.Computation (http://docs.meteor.com/#deps_dependency)

eg

var weather = "sunny";
var weatherDep = new Deps.Dependency;

var getWeather = function () {
  weatherDep.depend()
  return weather;
};

var setWeather = function (w) {
  weather = w;
  // (could add logic here to only call changed()
  // if the new value is different from the old)
  weatherDep.changed();
};

Session mirror's this pattern, but as a key/value store instead of just one value. (Actually Session is just an instance of the ReactiveDict class)

When you have a computation that calls getWeather() the .depend() call links it to the computation - and the .changed() call invalidates that computation.

eg, getting a computation via Deps.autorun()

computaton = Deps.autorun(function(){
  var localWeather = getWeather();
  console.log("local weather", localWeather);
});
computation.onInvalidate(function(){ 
  console.log("a dependency of this computation changed");
});
console.log("setting  weather");
setWeather("abc");

With this infrastructure - we find out that Template helpers are run in a computation - and when the dependencies of the computation are .changed(), it queues up the template to re-render.

Upvotes: 4

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