# Subscribable

A subscribable is a common interface for all of the primitives provided by this library.

Each subscribable has the following methods:

## subscribe(callback)

`subscribe` takes a _callback_ function that will be called when the primitive's value changes.
`subscribe` will return an _unsubscribe_ function that will unsubscribe the _callback_ from the
primitive when called. The returned _unsubscribe_ function will also return the remaining number of
subscriptions for that primitive.

Example:

```js
const p = prop(false);
const unsubscribe = p.subscribe(val => console.log(val));

p(true); // Logs "true" to the console
p(true); // Nothing logged because the value of p did not change.

const remainingSubscriptionCount = unsubscribe();
p(false); // Nothing logged because the subscription was revoked.

if (remainingSubscriptionCount === 0) {
	// Run any necessary cleanup
}
```

## unsubscribeAll()

`unsubscribeAll` simply disconnects all subscribers from the primitive.