Data bindings for lua plugin

[cloudwu]

It may be a good start for data binding lua plugin. (#145)

It only support string/bool/int/double data binding now, but I think it's easy to extend.

About lifespan mangement :

context:OpenLuaDataModel will create a DataModel userdata in lua, and we can set and get scalar by it . See Samples/luainvaders/lua/start.lua

We should ensure it lives before C++ DataModel object, and should call CloseLuaDataModel after C++ DataModel released.

btw, PushVariant and GetVariant may have similar implementation in offical lua plugin.

[mikke89]

This is very cool! Very much appreciate the contribution.

The lifetime sounds a bit like an issue. Perhaps hooking into Plugin::OnContextDestroy could be helpful, and then close all the LuaDataModels, including their underlying data models. Or perhaps we could make a new plugin API to capture destruction of data models. I'll merge this for now, and then we can think about a solution for later.

Thanks again!

[cloudwu]

For the lifetime issue, we can create a table associate to LuaContext object , and putting every LuaDataModels into the table.

Another issue is that Context:OpenLuaDataModel can open one DataModel object multiple times now, it can be avoid by this table.

When the Context destroy, we can simply close all the LuaDataModels.

I don’t implement these yet, because I tend to put these into lua side.It’s easier but at first we need a lua layer for the plugin framework rather than putting most things in C++ side. That is to say , the api context:OpenLuaDataModel could be a Lua function rather than a C function.