Class ReadonlyAtom<T>

There are two ways to use ReadonlyAtom's, the first of which is as a normal atom with a value that will never change. You can do this by using the readonlyAtom function and passing in just it's first argument:

const ten = readonlyAtom(10);

In this example ten will always hold the value 10, since we did not pass a start function that would receive the setter we can be assured that it's value is static for the lifetime of the atom. While it is still possible to subscribe to these static atoms, they are essentially no-op, since there would be no way for the atom's value to change subscribe requests are ignored.

The other, and maybe more useful, method of using a ReadonlyAtom is one that receives a start function in addition to the intitial value that allows it's value to be updated in response to something. These kinds of ReadonlyAtoms are created by providing a start function as the second argument to readonlyAtom, a smiple example would be an atom that acts like a clock:

const clock = readonlyAtom(new Date(), set => {
  set(new Date());

  const interval = setInterval(() => {
    set(new Date());
  }, 1000);

  return () => {
    clearInterval(interval);
  };
});

In the above example we create a ReadonlyAtom that will have it's value updated ever second via an interval. We do this by passing in a start function which receives one argument, set which lets you set the value of the atom and should return a stop function to clean up any intervals or timeouts or anything else that should be stopped.

Since the value will change we can subscribe to listen for the value updates like any other atom. And subscription is key for these atoms. For efficiency sake the start function will not be called until there is at least one subscriber on the atom and it will be halted when the last subscriber unsubscribes. This is why you would typically want to update the value at the beginning of your start function so that subscribers can read and use the latest values.

If you do need your readonly atom to run even without subscribers you can use setLive to switch it to a live atom which means that it will call the start function and will not call stop regardless of how many subscribers it has. This should be used sparingly because any resources will be constantly running, it's better to try and work in using the subscribe functions for the lifetime you use the atom instead of having it constatly live.

internal

DO NOT USE Prefer using the readonlyAtom factory function to manually creating instances of this class.