The dynamic keyword and message dispatch

WHAT THIS TESTS It probes whether you understand Swift's four dispatch styles and the precise role dynamic plays in Objective-C runtime features.

SWIFT DISPATCH BACKGROUND Swift uses inline or static dispatch for many calls, vtable dispatch for non-final class methods, witness-table dispatch for protocol requirements, and Objective-C message dispatch when needed. The compiler prefers the fastest option that preserves correctness, inlining and devirtualizing aggressively.

WHAT DYNAMIC DOES Marking a member dynamic forces it through the Objective-C messaging path, objc_msgSend, every time, even when the compiler could otherwise resolve it statically. Because objc_msgSend looks the selector up at runtime, the implementation can be replaced or intercepted, which is exactly what enables Key-Value Observing, which swizzles setters, and manual method swizzling. dynamic usually appears together with @objc, since the member must be visible to the Objective-C runtime; @objc alone exposes the member but does not by itself force dynamic dispatch in all cases.

PERFORMANCE TRADE-OFFS Message dispatch cannot be inlined or devirtualized and carries per-call lookup overhead. In a tight loop this is measurable. Static and inline dispatch let the optimizer fold and specialize code. So dynamic is a deliberate trade of speed for runtime flexibility, used sparingly.

COMMON WRONG ANSWERS Saying dynamic is just polymorphism or method overriding, which vtable dispatch already provides. Conflating dynamic with @objc; they are related but not identical. Claiming dynamic improves performance.

LIKELY FOLLOW-UPS How does KVO use dynamic? What is the difference between dynamic and final? When does Swift use witness tables? How does the optimizer devirtualize a non-final method?

ONE CONCRETE EXAMPLE You want KVO on a Swift property progress. You declare @objc dynamic var progress: Double. The dynamic attribute routes the setter through objc_msgSend so the KVO machinery can swizzle it and emit change notifications. If you drop dynamic, observers never fire because the call is dispatched statically and bypasses the runtime hook.