Add activation/inactivation tracking for broadcast channel

[elizarov]

See analysis here: #258 (comment)

TL;DR For various BroadcastChannel implementations it is useful to have ability to track active (at least one subscriber) and inactive (no subscribers) status. We should consider use-cases for it and design a convenient API for them.

Note, that it does not apply to regular channels which do not have a concept of subscriber.

This issue is related to #254

[LouisCAD]

This would be very useful since it'd allow LiveData like channels that could be used in multiplatform projects.

[LouisCAD]

Is there any design path for this? Where the methods would be, their signature, and name ideas?

[elizarov]

@LouisCAD There is no design yet. You are welcome to suggest your ideas here.

[SolomonSun2010]

Can I treat BroadcastChannel as data-flow variables in Oz-Mozart ?https://www.info.ucl.ac.be/~pvr/GeneralOverview.pdf

Data-Driven Concurrency is cool !
see also:
Building Scalable, Highly Concurrent & Fault Tolerant Systems - Lessons Learned
https://www.slideshare.net/jboner/building-scalable-highly-concurrent-fault-tolerant-systems-lessons-learned?from_action=save

Dataflow Concurrency
• Deterministic
• Declarative
• Data-driven
• Threads are suspended until data is available
• Lazy & On-demand
• No difference between:
• Concurrent code
• Sequential code
• Examples: Akka & GPars

[qwwdfsad]

Superseded by the same Flow design problem :)