RFE: MX support to simplify Dart Stream processing

[kennytilton]

Background

Currently, a widget can fairly elegantly handle a stream with a pair of cells:

{:msg-source (cF+ [:watch (fn [_ me ^#/(async/StreamController String) new-source old-source _]
                            (.listen (.-stream ^#/(async/StreamController String) new-source)
                              (fn [^String msg]
                                (mset! me :msg-rcvd msg))))]
               (mget (fm* :msg-sender) :msg-stream))
 :msg-rcvd   (cI "<none>" :ephemeral? true)}

Nice, but Streams are common in Flutter.

The RFE: support streams with one cell

Because Streams are fairly common in Dart/Flutter, we would like to handle a stream with a single cell, perhaps:

{:msg-rcvd (cF+ [:sink (fn [_ me ^#/(async/StreamController String) new-source old-source _]
                         (.listen (.-stream ^#/(async/StreamController String) new-source)
                           (fn [^String msg]
                             (mset! me :msg-rcvd msg))))]
             (mget (fm* :msg-sender) :msg-stream))}

Notes:

• if the formula has a dependency, meaning it could end up seeing a new stream, we should seamlessly change over to that;
• the :async? option starts at nil pending the result. Do we need to make that a specifiable value?
• should this cell default to :ephemeral true?

Also, a stream can have a lot of callbacks:

  onPause: () => print('Paused'),
  onResume: () => print('Resumed'),
  onCancel: () => print('Cancelled'),
  onListen: () => print('Listens'),

Do we have to address those? One solution might be to handle elaborate stream processing with the current 2-cell approach, and reserve the 1-cell approach for simple cases. The 2-cell approach is not all that bad.

Comments? Questions?