final, private, @objc, dynamic — how do they affect dispatch?

Senior PRO

The idea

Modifiers move a method along the scale "static → vtable → objc_msgSend". The more calls are static, the more inlining and the faster the code; dynamism is needed for overriding, KVO and swizzling.

import Foundation

class Base: NSObject {
    func a() {}                 // vtable
    final func b() {}           // static: cannot be overridden
    private func c() {}         // static: override impossible
    @objc func d() {}           // Swift: vtable, Obj-C: objc_msgSend
    @objc dynamic func e() {}   // always objc_msgSend (KVO, swizzling)
    dynamic func f() {}         // native dynamic replacement, no Obj-C
}
extension Base {
    @objc func g() {}           // extensions have no vtable → objc_msgSend
}