How does `useTracker` know what to subscribe to?

Going through the Meteor basic tutorial, I am having some issues getting the "task list" to update automatically when I insert an item, so wanted to debug it.

The main app component uses the useTracker function hook to "subscribe" to changes in the TasksCollection:

const tasks = useTracker(() => TasksCollection.find({}).fetch());

return (
  <div>
    <ul>{ tasks.map(task => <Task key={ task._id } task={ task }/>) }</ul>
  </div>
);

I am having a really hard time understanding how useTracker works under the hood.

Isn't the result of fetch() a plain array? How does useTracker know that it needs to subscribe to anything? I doubt that it polls the collection randomly.

I am not sure if this is even related to Meteor, or is it just some React Hooks magic.

Solution

You are actually wrong. useTracker doesn't subscribe to anything. You are responsible for subscribing yourself. The only thing useTracker does is connect updates to your collections (a "reactive data-source" in Meteor nomenclature) to a reactive event in react (similar to useState).

Check your code for a call to Meteor.subscribe('tasks') (or similar).