Observe datamodel?

[espkk]

Hey
Can we get ObserverPtr or notification mechanism for data models similarly with elements?
Does it even make sense; any other ideas? re: using data model in scripted environment

[mikke89]

Hello,

I don't really follow here, what exactly do you want to be notified of?

[espkk]

I use RmlUi via custom lua bindings (https://github.com/LoneBoco/RmlSolLua), where lua fully drives RmlUi (aside from init + plugin init + destroy).
For datamodel I use a proxy which holds a DataModelHandle, but because of the existing API 1) it cannot own the data model itself 2) it cannot query it's validity
So at an arbitrary point of execution it's impossible to reason if the data model can be accessed or not (it could be destroyed with RemoveDataModel or context destruction).
It can be done by subscribing to RemoveDataModel and content destruction, but it feels to be more intrusive - the notification should be sent from every place where data model becomes invalid after calling RmlUi API, but for the external code it's kind of an implementation detail.

Looking at ObserverPtr that Elements have, if DataModel had it in some form, it sounds like it would be enough to just have a thin non-owning wrapper that could query the validity of the data model and decide whether to access it.

There might be a better solution, I'm curious what do you think in general?

Thanks!

[espkk]

the same can be achivable if the data model could be tracked in a event-subscriber style, so that we can subscribe to its destruction regardless of the source (RemoveDataModel, context destruction, rmlui shutdown, anything else added in the future).

So for the userdata in a pseudocode, it would be something like:

dm = CreateDataModel(); // this is RmlUi datamodel
proxy = std::make_shared<CreateDataModelLuaRepresentation>(lua_table); // this is custom type from plugin - binds lua<->rmlui (see https://github.com/LoneBoco/RmlSolLua/pull/7)
std::weak_ptr weak_proxy = proxy; // this we can expose to lua
dm.AttachUsertype(proxy); // transfer ownership to rmlui

with observer pattern (unrelated to ObserverPtr/control block mechanism), again in pseudocode

dm = CreateDataModel(); // this is RmlUi datamodel
proxy = std::make_shared<CreateDataModelLuaRepresentation>(lua_table); // this is custom type from plugin - binds lua<->rmlui (see https://github.com/LoneBoco/RmlSolLua/pull/7)
std::weak_ptr weak_proxy = proxy; // this we can expose to lua
proxies.emplace(std::move(proxy)); // store the state somewhere that has same or greater lifetime that RmlUi's lifetime
dm.OnDestroy([&proxies](DataModel * dm){ proxies.erase(proxies.find(dm)); });

there still can be a better approach that I just dont see yet

[espkk]

just to pinpoint again the main issue I'm trying to resolve - there's no clean and not implementation-defined way for the external code (rmlui plugin) to detect the end of the data model lifetime

[mikke89]

Hey! Sorry for the delay, I've been out traveling, and this was a bit much to chew over between things.

I could see something like this being added to the Plugin interface. Something like OnDataModelCreate/Destroy functions. Could that be used to solve the issue do you think?

[espkk]

It sounds like it would be enough indeed!
Considering OnDataModelDestroy can be used to flag data model as disposed in some registry introduced in a plugin.

[mikke89]

Great! Would you be willing to make a pull request?

[espkk]

@mikke89 sorry for ping, just making sure this didn’t slip through