nelua "Handles"

handles.lua

local Button = @record{
	text: string,
	on_mouse_down: function(btn: *Button, x: integer, y: integer),
}
## Button.value.is_widget = true -- related doc: https://nelua.io/overview/#specializing-concepts-for-records

-- let's create an alias to make this easier
local OnMouseDownCallback = @function(btn: *Button, x: integer, y: integer)

-- note: we don't need the `&` operator when binding a function to a variable/field, because
-- function types and values are references already.
function Button.init(text: string, on_mouse_down: OnMouseDownCallback): Button
	return Button{
		=text, -- syntax sugar to `text=text`
		=on_mouse_down,
	}
end

-- test without handles
-- the function we want to bind
local function btn_click(btn: *Button, x: integer, y: integer)
  print(btn.text, x, y)
end

local btn1 = Button.init('Btn 1', btn_click)
btn1:on_mouse_down(20, 20) -- outputs 	'Btn 1	20	20'

---

-- This is the best `Handles` mimic I can think of,
-- maybe with more metaprogramming an equivalent of VB.NET's it's possible,
-- but I think it would still use the code below.

local a_function = #[concept(function(attr) return attr.type.is_function end)]#
local a_widget_ptr = #[concept(function(attr) return attr.type.is_pointer and attr.type.subtype.is_widget end)]#

-- Note: I could use var args to better mimic Handles, but for simplicity, I'm just using a single
-- parameter.
local function handles(widget: a_widget_ptr, callback: a_function)
	-- search each field of Button type, find the matching type of callback
	## for _, field in ipairs(Button.value.fields) do
		## if field.type == callback.type then 
			-- matching callback found
			widget.#|field.name|# = callback
			## break
		## end
	## end
end

-- test
local btn1 = Button.init('Btn 1')
local btn2 = Button.init('Btn 2')

-- the function we want to bind
local function btn_click_clone(btn: *Button, x: integer, y: integer)
  print(btn.text, x, y)
end

-- the "Handles" after the function declaration
handles(&btn1, btn_click_clone)
handles(&btn2, btn_click_clone)


btn1:on_mouse_down(50, 50) -- outputs 	'Btn 1	50	50'
btn2:on_mouse_down(30, 30) -- outputs 	'Btn 2	30	30'