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 that this data was accessed. Later, when the data changes, the computation is rerun automatically. This is how a template knows how to re-render whenever the data in its helpers changes.

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Tracker.autorun allows you to run a function that depends on reactive data sources, in such a way that if there are changes to the data later, 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, calling stop on the computation to prevent further rerunning:

// This computation will wait for a session variable to become true,
// then run exactly once.
Tracker.autorun(function (computation) {
  if (Session.get("shouldAlert")) {
    computation.stop();
    alert("Oh no!");
  }
});

The function is invoked immediately, at which point it may alert and stop right away if shouldAlert is already true. If not, the function is run again when shouldAlert becomes true.

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 complete detail.

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