OnActionExtension — Specs

Summary

NSControl.onAction is a convenience extension (defined in HelperExtensionsTestable.swift) that replaces the classic target/action pair with a stored closure: assigning control.onAction = { … } wires the control's target/action to invoke that closure, and reading the property back returns the stored closure. It's the backbone of AppearanceTab's Pro-lock wrap pattern — a control's existing action is read, captured as original, and re-wrapped so a Pro-gate check can run first and then optionally fall through to original?(control).

Behavior & edge cases

• The getter returns the same closure that was stored (not nil). This is load-bearing: the wrap pattern reads let original = control.onAction; a broken getter would leave original nil and orphan the underlying action (the regression these tests pin).
• Setting a new closure replaces the previous one — the old closure must not fire afterward.
• Setting onAction = nil clears target, action, and the stored closure.
• The captured original closure must survive later reassignments of onAction (the closure is held by a strong associated object), so a wrapper can still call through after the control's onAction is overwritten.

Test scenarios

Mirrors OnActionExtensionTests.swift 1:1.

A. Getter round-trip

• testGetterReturnsNilWhenUnset — a fresh control reports onAction == nil.
• testGetterReturnsClosureAfterSet — after assigning a closure, the getter is non-nil.
• testGetterReturnsSameClosureThatSetterStored — invoking the read-back closure runs the stored one (once).

B. Target/action plumbing

• testSetterConfiguresTargetAndAction — assigning a closure populates target and action.
• testSetterReplacesPreviousClosure — a second assignment replaces the first; only the new one fires.
• testSettingNilClearsTargetAndAction — assigning nil clears target, action, and the closure.
• testActionFiresClosureViaTargetActionPlumbing — invoking the wired action (the real AppKit call path: target.perform(action, with: control)) reaches the stored closure through SelectorWrapper.callClosure, with the sender forwarded.

C. Wrap pattern (the AppearanceTab use case)

• testWrapPatternInvokesPreviousClosureOnFallthrough — a wrapper that calls original?(c) runs both the wrapper and the captured original.
• testWrapPatternSkipsPreviousClosureOnEarlyReturn — a wrapper that returns early (the Pro-locked branch) does NOT fire the original, so the Pro value is never written to Preferences.
• testOriginalClosureSurvivesReassignment — the captured original still fires after onAction is reassigned many times (no premature release).