Evaluate initial value of this Signal. This method must only be called once, when this value is first needed. You should override this method as def (no val or lazy val) to avoid holding a reference to the initial value beyond the duration of its relevance.

When subclassing Observable **outside of com.raquo.airstream package**, just make this field public:

override val topoRank: Int = ???

"protected[airstream]" will allow this. See https://github.com/raquo/Airstream/issues/37

Child observable should call this method on its parents when it is started. This observable calls onStart if this action has given it its first observer (internal or external).

Subscribe an external observer to this observable

See also debug convenience method in Observable

This is the method that subclasses override to preserve the user's ability to set custom display names.

Note: Observer can be added more than once to an Observable. If so, it will observe each event as many times as it was added.

Signal propagates only if its value has changed

Create an external observer from a function and subscribe it to this observable.

Note: since you won't have a reference to the observer, you will need to call Subscription.kill() to unsubscribe

Note: This is enforced to be a Set outside of the type system #performance

value is passed by name, so it will be evaluated whenever the Observable fires. Use it to sample mutable values (e.g. myInput.ref.value in Laminar).

See also: mapToStrict

value is evaluated strictly, only once, when this method is called.

See also: mapTo

This name should identify the instance (observable or observer) uniquely enough for your purposes. You can read / write it to simplify debugging. Airstream uses this in debugLog* methods. In the future, we will expand on this. #TODO[Debug] We don't use this to its full potential yet.

See comment for tryNow right above

Add a noop observer to this signal to ensure that it's started. This lets you access .now and .tryNow on the resulting StrictSignal.

You can use myStream.toWeakSignal.observe.tryNow() to read the last emitted value from event streams just as well.

Here we need to ensure that Signal's default value has been evaluated. It is important because if a Signal gets started by means of its .changes stream acquiring an observer, nothing else would trigger this evaluation because initialValue is not directly used by the .changes stream. However, when the events start coming in, we will need this initialValue because Signal needs to know when its current value has changed.

This method is fired when this observable stops working (listening for parent events and/or firing its own events), that is, when it loses its last Observer (internal or external).

onStop can potentially be called multiple times, the second time being after it has started again (see onStart).

Convert this to an observable that emits Failure(err) instead of erroring

Child observable should call Transaction.removeInternalObserver(parent, childInternalObserver) when it is stopped. This observable calls onStop if this action has removed its last observer (internal or external).

Set the display name for this instance (observable or observer). - This method modifies the instance and returns this. It does not create a new instance. - New name you set will override the previous name, if any. This might change in the future. For the sake of sanity, don't call this more than once for the same instance. - If display name is set, toString will output it instead of the standard type@hashcode string

Override defaultDisplayName instead of this, if you need to.

Initial value is only evaluated if/when needed (when there are observers)