Standardise auto-receipt of current context on join/addContextListener in DesktopAgent and Channel

[kriswest]

Enhancement Request

According to the FDC3 API spec:

When an app joins a channel it will automatically recieve the current context for that channel.

https://fdc3.finos.org/docs/api/spec#joining-channels

We take that to mean that the current context of the channel will be auto-broadcast to the app when it joins the channel with context listeners already set (last context for an un-typed listener, last context of the specific type for a type listener), or when it adds a context listener.

n.b. assume a context has already been broadcast to the channel in each case below

fdc3.addContextListener(null, console.log);
fdc3.joinChannel("someSystemChannel");
// last context broadcast to the channel should be logged immediately
fdc3.addContextListener("fdc3.contact", console.log);
// last fdc3.contact context broadcast to the channel should be logged immediately

This is not the case for Channels, where you have to manually retrieve the context

let channel = await fdc3.getOrCreateChannel("someChannel");
channel.addContextListener(null, console.log);
//nothing is received initially until a new context is broadcast
let context = await channel.getCurrentContext();

We've found that this difference can be confusing for developers using the API. It also introduces complexity in implementing the DesktopAgent/Channel API that may be unnecessary (and the requirement is easy to miss in the spec).

To resolve, we suggest that the current context of a channel should be received automatically when adding a context listener and that this detail should be added to the documentation of the addContextListener functions (in both APIs).

[kriswest]

Agreed to close at meeting:

• Standard WG Meeting - April 28, 2022 #683
  While inconsistency is bad, auto-broadcast on user channels is popular there are use cases for not receiving the current context on adding a listener to an app channel (and while an app can retrieve current context it can't stop itself doing so if we make this change, preventing implementation of those use cases).