template_child: Implement PropertyGet for TemplateChild

[felinira]

This circumvents the drop issue for now by just not allowing property setters at all. Then we can have the field be transparent again. :)

As we don't have public setters on TemplateChild it creates an inconsistency where setting via Property would work, but setting the field directly wouldn't.

I think we should either:

• Generally allow setting the value of TemplateChild via a public set method
• Not allow it at all (which this opts to do)

This is technically breaking, because it unimplements HasParamSpec, although I can't really see any reason to rely on this in application code. 🤷‍♀️

[bilelmoussaoui]

As I mentioned on the Matrix channel, I don't use this feature on any of the apps I maintain, nor have an idea about the expectations from GTK side on whether you can modify the pointer that gets initialized with gtk_init_template.

So I am fine with merging this change. It would be nice to confirm if template children widgets as properties can not be writable before doing so though.

[sdroege]

Your call but I think this makes sense. You could implement PropertySet if you want writable properties at a later time

[bilelmoussaoui]

Let us go with this, if someone complains with lack of PropertySet, then we can figure out how/why later