Swift Bits: @dynamic vs @objc

Dispatch caveats

You have probably, at some point, become aware of dispatch methods:

• Static

• Dynamic

• Message dispatch

Explaining each of them has been done many times and continues to bring new insights. Here are some valuable links to help you get the full picture:

• Method Dispatch in Swift

• The Swift Method Dispatch Deep Dive

• Understanding method dispatch in Swift

What’s the Difference?

Both of them come from the Objective-C world when we had only Message Dispatch using objc_msgSend.

@objc exposes things to the Objective-C runtime, but by itself does not always force Objective-C message dispatch for Swift callers.

Swift adds @objc implicitly when:

• You subclass NSObject

• You override an Objective-C method

• You conform to an @objc protocol

In other cases, behavior varies depending on where it’s added.

---

For a property:

class Person: NSObject {
    @objc var name: String = “”
}

What this does:

1. The getter and setter are exposed as Objective-C methods:

   • - (NSString *)name

   • - (void)setName:(NSString *)name

2. The property becomes visible to:

   • Objective-C code (normal property access)

   • KVC (value(forKey: “name”), setValue(_:forKey:))

---

For a class:

@objc class MyController: NSObject {
    @objc func doStuff() { }
}

What this does:

1. Registers the class with the Objective-C runtime (as long as it’s Obj-C compatible, typically via NSObject).

2. The class is now visible in Objective-C:

   • MyController *c = [[MyController alloc] init];

   • Can be used in Objective-C APIs, storyboards, etc.

Important points:

• Marking a class with @objc does not automatically make all members @objc. You must either:

  • Mark members individually with @objc, or

  • Use @objcMembers on the class:

    @objcMembers
    class MyController: NSObject {
        func foo() { }  // implicitly @objc
    }

---

dynamic (usually used as @objc dynamic) is what forces Objective-C–style message dispatch even from Swift.

In Swift, this forces dynamic dispatch. And that’s something confusing for Objective-C old-timers.

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Tricky Story

If you remember @dynamic from Objective-C, it will NOT generate getter/setter and it says: “You will find out the implementation later, at runtime“.

In Swift, @NSManaged is used to postpone getter/setter generation:

class Person: NSManagedObject {
    @NSManaged var name: String
}

---

Summary

• @objc:

  • Makes the symbol participate in the Objective-C runtime (gives it a selector, etc.).

  • Ensures Objective-C callers use message dispatch.

  • Does not forbid the Swift compiler from using static/vtable dispatch for Swift call sites.

• dynamic (which implies @objc):

  • Does force the use of Objective-C dynamic dispatch (Message Dispatch) (via objc_msgSend) for all calls, including from Swift.

  • This is what you need for KVO, method swizzling, and other runtime features.

If you want a simple rule of thumb:

• Need Obj-C interop only? → @objc

• Need KVO / swizzling? → @objc dynamic