Hi,
The Rigidbody2D acts as a compound collider whose shape matches the shape of all colliders attached to this game object and its children. If you just want to check if a specific collider is touching a layer, you must call the IsTouchingLayers method on the collider. If it does not matter or if you want to to check if the compound collider is touching a layer, you call the method on the rigidbody.
Use the version which solves the problem for you. If both solutions solve it, use the one you prefer.
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See also:
- Unity Manual: Rigidbody
- Forum User Guides : How to mark a topic as solved