The `Meteor.depends()` function

I'm currently working my way through the Distinctive Meteor manual and I've hit a roadblock with the illustration found on page 137

var _currentLikeCount = 0;
var _currentLikeCountListeners = new Deps.Dependency();


currentLikeCount = function() {
_currentLikeCountListeners.depend();
return currentLikeCount;
}

Meteor.setInterval(function() {
 var postId;
  if (Meteor.user() && postId = Session.get('currentPostId')) {
   getFacebookLikeCount(Meteor.user(), Posts.find(postId),
    function(err, count) {
  if (!err && count !== _currentLikeCount) {
   _currentLikeCount = count;
   _currentLikeCountListeners.changed();
 }
});
}
}, 5 * 1000);

I am finding it challenging to grasp the purpose of "Deps.Dependency()" and "depend()" in this code. What functionality is being demonstrated here? This aspect was briefly covered in the book and I am struggling to find a clear explanation in the documentation.

javascriptmeteor

Answer №1

Meteor has a unique feature where things are reactive - this means that when a value changes in the data, all dependent elements will automatically recalculate themselves.

In order for this to happen, each element needs to know which value it depends on. This dependency is tracked by setting Dependencies. Therefore, Dependencies act as the underlying mechanism necessary for automatic recalculations.

Within the code snippet you provided, there are three key lines:

var _currentLikeCountListeners = new Deps.Dependency();

This line creates a new dependency object specifically designed to monitor changes in `_currentLikeCount`.

_currentLikeCountListeners.depend();

When used within a reactive function, this method allows the function to listen to the dependency. As a result, whenever the dependency experiences a change, the function will be recalculated. It's important to note that not all functions are reactive - they need to be categorized as "reactive computations". Beginners shouldn't worry about this initially; just keep in mind that template helpers fulfill this requirement.

_currentLikeCountListeners.changed();

Finally, this last line actually triggers the recalculation process.

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