Cannot call Callable in a draw method (because no way to set_object on Callable?)

[OmarShehata]

Godot version

v4.0.1.stable.official [cacf499]

System information

Windows 11

Issue description

If you try to pass a lambda/callable to be used inside a draw function, you get the error below. As far as I can tell, this is because the object that the callable is attached to is NOT the class that is calling the _draw function, so Godot doesn't recognize that it's actually being called inside _draw ?

 Drawing is only allowed inside NOTIFICATION_DRAW, _draw() function or 'draw' signal.
  <C++ Error>    Condition "!drawing" is true.
  <C++ Source>   scene/main/canvas_item.cpp:599 @ draw_rect()

Steps to reproduce

class CustomDrawNode extends Node2D:
	var draw_func:Callable
	
	func _init(custom_draw_func:Callable):
		draw_func = custom_draw_func

	func _draw():
		draw_func.call()

# Then instantiate this as follows
var custom_draw_node = CustomDrawNode.new(func(): draw_rect(Rect2(0.0, 0.0, 100.0, 100.0), Color.GREEN)
	)
add_child(custom_draw_node)

I think to fix this I would want to do something like

draw_func.set_object(self)

so that the passed in custom_draw_func can be called with the CustomDrawNode as its self?

Minimal reproduction project

N/A

Can be reproduced completely with the snippet above.

[kleonc]

If you try to pass a lambda/callable to be used inside a draw function, you get the error below. As far as I can tell, this is because the object that the callable is attached to is NOT the class that is calling the _draw function, so Godot doesn't recognize that it's actually being called inside _draw ?

You're kinda right. This is because the object that the callable is attached to is NOT the object (a CanvasItem) that is currently within its drawing routine (NOTIFICATION_DRAW / draw signal / _draw method). So the error itself is correct, the error message not really (as it's not just about being called inside _draw, but rather about being called inside _draw of the specific CanavasItem, the one currently being drawn (the current_item_drawn)).

The error messages like:

godot/scene/main/canvas_item.cpp

Line 599 in 9b0bee8

ERR_FAIL_COND_MSG(!drawing, "Drawing is only allowed inside NOTIFICATION_DRAW, _draw() function or 'draw' signal.");

can be improved for cases when current_item_drawn != nullptr && current_item_drawn != this as for them the execution is actually "inside NOTIFICATION_DRAW, _draw() function or 'draw' signal", just not of the proper CanvasItem.

---

Steps to reproduce

class CustomDrawNode extends Node2D:
	var draw_func:Callable
	
	func _init(custom_draw_func:Callable):
		draw_func = custom_draw_func

	func _draw():
		draw_func.call()

# Then instantiate this as follows
var custom_draw_node = CustomDrawNode.new(func(): draw_rect(Rect2(0.0, 0.0, 100.0, 100.0), Color.GREEN)
	)
add_child(custom_draw_node)

Currently you'd need to pass the drawing CanvasItem as the Callable's argument. Like so:

class CustomDrawNode extends Node2D:
	var draw_func: Callable
	
	func _init(custom_draw_func: Callable):
		draw_func = custom_draw_func

	func _draw():
		draw_func.call(self)

# Then instantiate this as follows
var custom_draw_node = CustomDrawNode.new(
	func(ci: CanvasItem):
		ci.draw_rect(Rect2(0.0, 0.0, 100.0, 100.0), Color.GREEN)
)
add_child(custom_draw_node)

I'd say this is expected.

---

I think to fix this I would want to do something like

draw_func.set_object(self)

so that the passed in custom_draw_func can be called with the CustomDrawNode as its self?

To me this sounds more like a material for a proposal.

[kleonc]

Marking as bug because of the incorrect / confusing error message.

[OmarShehata]

draw_func.call(self)

Ah yeah, that works perfectly well for my use case! I was kind of stuck on this pattern because in JavaScript, given a function you can say "call it with this object as the 'self' (or this) param" (link) but didn't think to just explicitly pass the obj I want to apply it to.

Thank you very much for the explanation @kleonc !

[AThousandShips]

To replace the owner of a callable you can also do (sorry missed this was a lambda for which this doesn't work) (old and new can be the same, just making clere here):

callable_new = Callable(target, callable_old.get_method()).bindv(callable_old.get_bound_arguments())

[kleonc]

To replace the owner of a callable you can also do:

callable_new = Callable(target, callable_old.get_method()).bindv(callable_old.get_bound_arguments())

This won't always work. It should work e.g. when callable_old is not a lambda and the type (including scripts) of target and callable_old.get_object() match (so the given method could be found by name both in target and in callable_old.get_object()). But won't work in general case, e.g. for the use case mentioned by the OP.

[AThousandShips]

True! Forgot about lambdas