Recalculate the signal's current value. Typically this asks the parent signal for its current value, and does the math from there, according to the particular signal's logic (e.g. MapSignal would apply the project function).

This method is used to calculate the signal's initial value, but also to recalculate, this signal's value, to re-sync it with the parent, when this signal is restarted after being stopped. See https://github.com/raquo/Airstream/issues/43

Note: Use Protected.topoRank(observable) to read another observable's topoRank if needed

Subscribe an external observer to this observable

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

A stream of all values in this signal, excluding the initial value.

When re-starting this stream, it emits the signal's new current value if and only if something has caused the signal's value to be updated or re-evaluated while the changes stream was stopped. This way the changes stream stays in sync with the signal even after restarting.

See also debug methods in com.raquo.airstream.debug.DebuggableObservable

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

Distinct events (but keep all errors) by == (equals) comparison

Distinct events (but keep all errors) by matching key Note: key(event) might be evaluated more than once for each event

Distinct events (but keep all errors) using a comparison function

Distinct events (but keep all errors) by reference equality (eq)

Distinct errors only (but keep all events) using a comparison function

Distinct all values (both events and errors) using a comparison function

Airstream may internally use Scala library functions which use == or hashCode for equality, for example List.contains. Comparing observables by structural equality pretty much never makes sense, yet it's not that hard to run into that, all you need is to create a case class subclass, and the Scala compiler will generate a structural-equality equals and hashCode methods for you behind the scenes.

To prevent that, we make equals and hashCode methods final, using the default implementation (which is reference equality).

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

Force reference equality checks. See comment for equals.

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.

Observer removals scheduled to run as soon as this observable's event propagation finishes. Only put calls to removeInternalObserverNow and removeExternalObserverNow here, no custom logic.

Get the signal's current value

Total number of internal and external observers

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.

Must not throw

Must not throw

Here we need to ensure that Signal's default value has been evaluated at least once. 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).

Must not throw

Note: this is overriden in: - com.raquo.airstream.misc.SignalFromStream because parent can be stream, and it has cacheInitialValue logic

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

Note: do not expose this to end users. See https://github.com/raquo/Airstream/issues/10

Removal still happens synchronously, just after we're done iterating over this observable's observers, to avoid interference with that logic.

Note: The delay is necessary not just because of interference with actual while(index < observers.length) iteration, but also because on a high level it is too risky to remove observers from arbitrary observables while the propagation is running. This would mean that some graphs would not propagate fully, which would break very basic expectations of end users.

UPDATE: In 15.0.0 we made the observable removal delay more fine grained – previously we would wait until the whole transaction completed before removing observers, now we only wait until this observable's iteration is done. This helped us fix https://github.com/raquo/Airstream/issues/95

Note: To completely unsubscribe an Observer from this Observable, you need to remove it as many times as you added it to this Observable.

Safely remove internal observer (such that it doesn't interfere with iteration over the list of observers). Removal still happens synchronously, just after we're done iterating over this observable's observers.

Child observable should call this method on its parents when it is stopped.

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

A signal that emits the accumulated value every time that the parent signal emits.

A signal that emits the accumulated value every time that the parent signal emits.

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

Unwrap Try to "undo" recoverToTry – Encode Failure(err) as observable errors, and Success(v) as events

Override defaultDisplayName instead of this, if you need to.

Convert this observable to a signal of Option[A]. If it is a stream, set initial value to None.

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

Note: this is overridden in:

• com.raquo.airstream.distinct.DistinctSignal