A closure is, simply put, a prepared function call. It therefore specifies which function to call as well as some of the arguments to call it with.
More specifically, a closure in this context is a function directly callable from C, i.e. from a library, which in turn calls a Lua function that you provide. For example, g_tree_foreach expects a GTraverseFunc, which it calls for each node in the list.
LuaGnome can create such a closure automatically or explicitely, and you must use the correct of these two mechanisms in order to avoid problems. Such a closure prepares the call to your Lua function, calls it, and returns its return value (if any) to the closure's caller.
Note: Another mechanism that allows a library to call a Lua function is to install signal handlers. This is handled by the connect function and does not use the closures described here.
Such closures are create automatically, i.e. you provide a Lua function as an argument to a library function. The closure is created, used as the argument, and dereferenced after the library function returns. A simple example is this:
sel = tree_view:get_selection() sel:selected_foreach(function(model, path, iter, data) print(model:get_value(iter, 0)) end, nil)
In the above code snippet, the gtk_tree_selection_selected_foreach function calls the given function once for each selected row. Before LuaGnome calls the ..._foreach function, a closure is created for the given function, and after the call returns, it is dereferenced and will eventually be garbage collected.
If the function can be called later, the closure must not be freed. To ensure that, you have to create a closure manually, store it in a variable, and not unset the variable until the closure is no longer needed.
function compare_func(a, b) a = a.value b = b.value if a == b then return 0 end return a < b and -1 or 1 end -- WRONG t = glib.tree_new(compare_func) -- CORRECT compare_func_closure = gnome.closure(compare_func) t = glib.tree_new(compare_func_closure)
As you can see, a g_tree requires a comparison function, to order all nodes. It will call this function (at least) every time you insert more data. Therefore, it won't do to provide a Lua function, because the closure created for it will be freed (or could, as soon as garbage collection runs) too soon. To keep such closures around, you could store them in the object itself:
local closure = gnome.closure(compare_func) t = glib.tree_new(closure) t._compare_closure = closure
This way, you won't have to worry about keeping the closure variable around. Note: unless you configured LuaGnome with --disable-debug, it tries hard to detect cases when an automatic closure is given but won't work. You'll receive a runtime error. Explicit closures are always safe to use but require more coding.