Meteor Deep Dive — Reactive Programming With Tracker

Introduction

As a Meteor developer, I believe that everyone who has worked with Meteor all had experience with Tracker, or at least used it through some kinds of interfaces like getMeteordata or createContainer which come with the react-meteor-data package.

However, not all people know how Tracker works its magic. In this post, I’m going to dig into every facet of this case as well as bring to you a little bit of background knowledge.

What Is Tracker?

Tracker is a Meteor’s core package, it is small but incredibly powerful library for transparent reactive programming in Meteor.

Using Tracker you have much of the power of Functional Reactive Programming FRP system without following FRP’s principles when implementing your application. Combined with Tracker-aware libraries, like Session/ReactiveVar/ReactiveDict, this lets you build complex event-driven programs without writing a lot of boilerplate event-handling code.

What Make It Great?

In a nutshell, it is reactivity. In my opinion, it is the strongest aspect of Meteor. Tracker helps us make our system work reactively both on client and server with ease. We do not have to learn any extra stuffs about reactive programming or functional programming to get started.

Just read the Tracker api and do the work then the magic happens. Or even some Meteor-novice who do not know a thing about Tracker, their code still work reactively. Do you know what am I talking about? It is good old Blaze (I say it’s old because I already moved to React for all new projects).

Blaze’s helpers are reactive natively because they use Tracker inside, if you put inside them a reactive data source then whenever that source changes those helpers will be recomputed and you get new data on the screen.

Let’s read some code and behold what I am talking about, if you are already familiar with Tracker, skip this part and move to the next section to inspect the magic.

// Set up the reactive code
const counter1 = new ReactiveVar(0);
const counter2 = new ReactiveVar(0);
const observeCounter = function() {
  Tracker.autorun(function() {
    const text = `Counter1 is now: ${counter1.get()}`;
    console.warn(text);
  });
  console.warn(`Counter2 is now: ${counter2.get()}`);
};
const computation = Tracker.autorun(observeCounter);
/*
Message on the console:
Counter1 is now: 0
Counter2 is now: 0
*/
// and now change the counter1's value
counter1.set(1);
/*
Message on the console:
Counter1 is now: 1
*/
counter2.set(3);
/*
Message on the console:
Counter1 is now: 1
Counter2 is now: 3
*/
counter1.set(7);
/*
Message on the console:
Counter1 is now: 7
*/

In reality, it happens as shown below:

How Does Tracker Work?

Basically Tracker is a simple interface that lets reactive data sources (like counter in the example above) talk to reactive data consumers (the observeCounter function). Below is the flow of Tracker (I ignore some good parts to make it as simple as possible)

• Call Tracker.autorun with function F
• If inside F, there is a reactive data source named R, then that source will add F to its dependence list
• Then whenever the value of R changes, R will retrieve all its dependence from the dependence list and run them.

Everything is easier said than done. So, let’s take a look at the code:

const counter = new ReactiveVar(1);
  const f = function() {
    console.log(counter.get());
  };
  Tracker.autorun(f);

In the above code: counter is our reactive data source. It raises a question is that how can it know what function used inside to add that function as its dependence?

This is where the Meteor team does their trick to make this flow transparent. In fact, Tracker is an implementation of the Observer pattern or at least an Observer-liked pattern. Observer pattern can be used to create a reactive library like Tracker.

In an traditional implementation of Observer, we can think of F as an observer and R as a subject. R must have an interface to add F as its observer and notify/run F when its value changes. Something like this:

const counter = new Source();
  const f = function(val) {
    console.log(val);
  };
  counter.addObserver(f);

To imitate this, Tracker provides us these interface:

• Tracker.autorun
• Tracker.Dependency
• Tracker.Dependency.prototype.depend
• Tracker.Dependency.prototype.changed

Tracker.Dependency is the implementation of Subject in traditional Observer. All of reactive data source to use with Tracker use this object inside.

Let’s look at the basic implementation of ReactiveVar I use for examples above:

ReactiveVar = function (initialValue, equalsFunc) {
    if (! (this instanceof ReactiveVar))
      // called without `new`
      return new ReactiveVar(initialValue, equalsFunc);
    this.curValue = initialValue;
    this.equalsFunc = equalsFunc;
    this.dep = new Tracker.Dependency;
  };
  ReactiveVar.prototype.get = function () {
    if (Tracker.active)
      this.dep.depend();
    return this.curValue;
  };
  ReactiveVar.prototype.set = function (newValue) {
    var oldValue = this.curValue;
    if ((this.equalsFunc || ReactiveVar._isEqual)(oldValue, newValue))
      // value is same as last time
      return;
    this.curValue = newValue;
    this.dep.changed();
  };

Read full article: Meteor Deep Dive: Reactive Programming With Tracker