meteor/docs/client/basic/sections/tracker.md

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Tracker

Meteor has a simple dependency tracking system which allows it to automatically rerun templates and other computations whenever Session variables, database queries, and other data sources change.

Unlike most other systems, you don't have to manually declare these dependencies — it "just works." The mechanism is simple and efficient. Once you've initialized a computation with Tracker.autorun, whenever you call a function that supports reactive updates, Tracker automatically records which data were accessed. Later, when those data change, the computation is rerun automatically. This is how a template knows how to re-render whenever its helper functions have new data to return.

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Tracker.autorun allows you to run a function that depends on reactive data sources. Whenever those data sources are updated with new data, the function will be rerun.

For example, you can monitor one Session variable and set another:

Tracker.autorun(function () {
  var celsius = Session.get("celsius");
  Session.set("fahrenheit",  * 9/5 + 32);
});

Or you can wait for a session variable to have a certain value, and do something the first time it does. If you want to prevent further rerunning of the function, you can call stop on the computation object that is passed as the first parameter to the callback function:

// Initialize a session variable called "counter" to 0
Session.set("counter", 0);

// The autorun function runs but does not alert (counter: 0)
Tracker.autorun(function (computation) {
  if (Session.get("counter") === 2) {
    computation.stop();
    alert("counter reached two");
  }
});

// The autorun function runs but does not alert (counter: 1)
Session.set("counter", Session.get("counter") + 1);

// The autorun function runs and alerts "counter reached two"
Session.set("counter", Session.get("counter") + 1);

// The autorun function no longer runs (counter: 3)
Session.set("counter", Session.get("counter") + 1);

The first time Tracker.autorun is called, the callback function is invoked immediately, at which point it alerts and stops right away if counter === 2 already. In this example, Session.get("counter") === 0 when Tracker.autorun is called, so nothing happens the first time, and the function is run again each time counter changes, until computation.stop() is called after counter reaches 2.

If the initial run of an autorun throws an exception, the computation is automatically stopped and won't be rerun.

To learn more about how Tracker works and to explore advanced ways to use it, visit the Tracker chapter in the Meteor Manual, which describes it in much more detail.

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