When a property cannot be marked with the @objc attribute, it means that the property’s type cannot be represented in Objective-C. The @objc attribute is used to make Swift code accessible and usable in Objective-C code. However, not all types in Swift can be bridged to Objective-C automatically.
For example, Swift’s native structures and enumerations that do not inherit from NSObject cannot be represented in Objective-C, so properties with these types cannot be marked with @objc. Let’s consider the following code snippet:
struct Person {
var name: String
var age: Int
}
@objc class Example {
@objc var person: Person // This will cause an error
@objc var name: String // This is allowed because String is bridgable to Objective-C
}
In the above code, we have a struct “Person” representing a person’s name and age. The “Person” struct cannot be represented in Objective-C, so attempting to mark the “person” property with @objc will result in an error.
On the other hand, the “name” property is of type String, which is natively bridgable to Objective-C. So, marking it with @objc is allowed and the property can be accessed from Objective-C code.
Therefore, the error message “property cannot be marked @objc because its type cannot be represented in objective-c” indicates that the specific type of the property cannot be automatically bridged to Objective-C, and therefore, it cannot be used in Objective-C code directly.