In Objective-C there is a distinction between atomic and nonatomic properties with properties being atomic by default. Swift has no such specifier. In Objective-C the implementation of an atomic property allows properties to be safely read and written from different threads. For nonatomic properties, the underlying pointer of a read value could be released when a new value is being written at the same time.
Are Swift reference type properties atomic?
I assume the reference types properties in Swift are nonatomic.
I just performed a simple test which I think answers my question. I created a new Swift class inherited from NSObject and added a reference type property. In the generated header for Objective-C it shows that property declared as nonatomic.
Confirming that Swift properties are nonatomic in the ObjC sense. One reason is so you think about whether per-property atomicity is sufficient for your needs.