Detect collisions with both parent and child nodes in Godot

Question

Using Godot 4.2.1, I have a 3D scene with a CharacterBody3D as the root. It has two children; a child CollisionShape3D, and a child CharacterBody3D with its own collision shape.

CharacterBody3D (scene root)
    CollisionShape3D (parent collision shape)
    CharacterBody3D (child)
        CollisionShape3D (child collision shape)

The scene root has a script which calls MoveAndSlide in PhysicsProcess(). It works fine for the parent and its collision shape, if you hit an obstacle, the parent stops moving. However I would like the parent to stop moving also if the child shape hits an obstacle, and that is not happening.

The child is not being moved manually through scripts, it's only being taken along with the parent's movement. I tried changing it to a StaticBody3D too, but the issue persists. The collision layers and masks are the same for both parent and child; if that was wrong, the collision wouldn't work for either.

How can I get the child's collision to be detected? And the movement to stop even when the child collides, same as the parent? Is there a solution that handles this issue even when the child is instantiated at runtime, through scripts?

Answer 1

I'm having a hard time visualizing what you mean.

From what I've seen in Godot, if movement is not controlled by the built in functions (like moveAndSlide()) collisions are not checked. So if the child is being dragged with the parent the child collisions likely aren't checked.

Something you can try to do is to use signals so that when the parent moves, a signal is fired. The signal would contain a vector indicating the movement expected of the child and then the child would respond to the signal by calling moveAndSlide() with the supplied vector.

I think the only challenge would be suppressing the child's native behavior of being dragged with the parent. The easy way to do this is not add it as a child to the parent but rather a sibling.

You can connect signals on dynamically instantiated objects. This is some c# code I use to do this but I believe this can also be done in GDScript:

PackedScene cellResource = ResourceLoader.Load<PackedScene>("res://TicTacToe/Scenes/GameBoard/Cell.tscn");
Godot.StringName signalName = new Godot.StringName("cellClicked");
Godot.Callable signalHandler = new Godot.Callable(this, MethodName.onCellClicked);
Cell cell = cellResource.Instantiate<Cell>();
cell.Connect(signalName, signalHandler);

Here's an answer on StackOverflow that I think does a great job at explaining collision: https://stackoverflow.com/questions/69728827/how-do-i-detect-collisions-in-godot

It does still emphasize the use of collision signals or the use of moveAndSlide/moveAndCollide functions.