model diff representation

[Ichthyostega]

As an intermediary step to define a backbone for GUI ⟺ Steam communication, we need a mechanism to represent model changes. This needs to be combined in a sensible way with the placement-IDs used in Steam-Layer to denote the relevant model elements.

• find a way to get this update notification handshake straight.
  • please note, the update is initiated from the builder, which does not run in the event thread
  • there are strict conventions how to use threads in GTK (in fact, in any modern GUI toolkit)
• find a way to push model updates while avoiding duplicated state representations in need to be synched
• structure the updates in a way to avoid spurious GUI updates for unchanged parts of the model. There will be lots of change notifications

Design Draft

The idea is to build the model updates on top of diff messages.

• each such message holds a tree diff
• using diffs, we get a loose coupling of similarly structured data
• this leads to recursive breakdown and forwarding of messages
• we define a diff language based on tokens of constant size
• this allows lazy (demand driven) processing of diff detection and application

[Ichthyostega]

As a first milestone, I've now drafted the design of a list diff language, given as sequence of diff verbs. Implemented the simple case, where a given diff
gets applied to a target sequence contained in a vector. Unit test pass!

[Ichthyostega]

Next milestone reached, which is to generate/detect differences between arbitrary sequences. The unit test indeed generates the expected sequence of diff verbs in a simple, hard coded test scenario.

[Ichthyostega]

meanwhile, I've drafted an implementation approach to deal with the handling of generic tree changes

[Ichthyostega]

this was a huge task, and is mostly settled by now

• we can represent and apply tree changes by diff
• handling of such diff messages was integrated into the UI-Element / UI-Bus protocol

What remainst to be done is to find a proper integration into the session/update mechanics; these are themselves still rather unclear, thus I'm leaving this task for now

[Ichthyostega]

So this can really be considered as settled by now. This Diff-Framework is in use, and there is enough experience to see how model elements are to be represented and altered. Last year, just addint a test diff to the DummySessionConnection was enough to trigger the buildup of a complex timelilne structure, to be elaborated on the implementation side.

[Undercover Agent]

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