RxJS: Calling publish with a Selector
The publish and publishReplay operators can be called in two ways: either with or without a selector function. Let’s look at the differences and at why you should almost always pass a selector.
Without a selector
When publish is called without a selector, it returns a particular type of observable: a ConnectableObservable.
A ConnectableObservable is a little unusual. It does not connect — i.e. subscribe — to its source when a subscriber subscribes. Instead, a ConnectableObservable subscribes to its source when its connect method is called.
This behaviour can be used to control the order in which subscriptions occur and to compose observables before a subscription is made to the source.
Let’s look at an example.
I’ve seen a ConnectableObservable used in a Stack Overflow answer on several occasions, so let’s make up a question and then answer it:
How can I compose an observable so that a specified, initial value is emitted if the source observable does not emit a value within a specified duration?
Here’s a solution that uses race:
