Target-Action and the Responder Chain on iOS

Slides to accompany talk from NSManchester, 1st February 2016. With examples in Swift!

Example project at https://github.com/sammeadley/responder-chain

1. Target-Action and the Responder Chain on iOS with examples in Swift! Sam Meadley @sammeadley
2. Overview • Classic UIControl Target-Action • Gesture Recognizer Target-Action • Interface Builder support
3. Why does this matter? • Design (weak coupling) • Safety (strong typing) • Performance (reduces introspection)
4. UIControl Target-Action The Players: • Principal UIApplication class • UIApplication’s keyWindow (UIWindow) • UIControl • The Responder Chain
5. UIControl Target-Action • Principal UIApplication class pops top event from queue and dispatches for handling via sendEvent(_:) Event A Event B Event C Event D UIApplication sendEvent(_:)
6. UIControl Target-Action • Principal UIApplication class pops top event from queue and dispatches for handling via sendEvent(_:) Event B Event C Event D • Forwards the event to the keyWindow, invoking the sendEvent(_:) method of the UIWindow object Event A UIWindow sendEvent(_:) UIApplication sendEvent(_:)
7. UIControl Target-Action • UIWindow object delivers the event to the view where the touch occurred Event A Event B Event C Event D Event A UIControl hitTest(_:withEvent:) UIApplication sendEvent(_:) UIWindow sendEvent(_:)
8. UIControl Target-Action • UIWindow object delivers the event to the view where the touch occurred Event A Event B Event C Event D UIApplication sendEvent(_:) Event A UIWindow sendEvent(_:) UIControl hitTest(_:withEvent:) • If the View is a UIControl, UIKit checks the target and action properties target: AnyObject, action: Selector
9. UIControl Target-Action • The action message is dispatched via UIApplication's sendAction(_:to:from:forEvent:) • The message is forwarded to the control contained in the target property Event B Event C Event D UIApplication sendAction(_:to:from:forEvent:) UIControl doSomething(_:) target: UIControl, action: doSomething
10. UIControl Target-Action • Warning: If the selector is not implemented on the target class, an NSInvalidArgumentException "unrecognized selector sent to instance" is thrown. Event B Event C Event D UIControl target: UIControl, action: doSomething UIApplication sendAction(_:to:from:forEvent:)
11. UIControl Target-Action • Alternatively, passing a nil target hands off the event to the Responder Chain. UIControl target: nil, action: doSomething UIApplication sendAction(_:to:from:forEvent:)
12. UIControl Target-Action • Alternatively, passing a nil target hands off the event to the Responder Chain. UIControl target: nil, action: doSomething UIView UIViewController UIWindow AppDelegate • Starting at the First Responder the chain is traversed until the specified action message is encountered. UIApplication doSomething(_:) UIApplication sendAction(_:to:from:forEvent:)
13. UIControl Target-Action • If no responder implements the action method, the invocation fails gracefully. UIApplication sendAction:to:from:forEvent target: nil, action: doSomething UIControl UIView UIViewController UIWindow AppDelegate UIApplication
14. Demo
15. Gesture Recognizer Target-Action • UIGestureRecognizer uses Target-Action pattern • init(target:action:) • addTarget(_:action:)
16. Gesture Recognizer Target-Action • Special considerations… NULL is not a valid value.
17. Gesture Recognizer Target-Action • Why is NULL not a valid value?
18. Gesture Recognizer Target-Action • Still possible with UIApplication’s sendAction(_:to:from:forEvent:) let application = UIApplication.sharedApplication() application.sendAction(“didReceiveTapGesture:", to: nil, // Send up the responder chain. from: self, // UIKit drops any action sent from UIGestureRecognizer forEvent: nil)
19. Gesture Recognizer Target-Action • Fighting the API • Risks exposing UIGestureRecognizer to access methods/properties let application = UIApplication.sharedApplication() application.sendAction(“didReceiveTapGesture:", to: nil, // Send up the responder chain. from: self, // UIKit drops any action sent from UIGestureRecognizer forEvent: nil)
20. Interface Builder support • Make no mistake, IB loves Target-Action • Support for: • Targeted Actions • Nil-targeted Actions • Typed sender parameters (sometimes)
21. IB: Targeted Actions class ViewController: UIViewController { @IBAction func didSelectButton(sender: AnyObject) { ... } }
22. IB: Nil-targeted Actions class ViewController: UIViewController { @IBAction func didSelectButton(sender: AnyObject) { ... } }
23. Typed sender parameters class ViewController: UIViewController { @IBAction func didSelectButton(sender: UIButton) { ... } }
24. Typed sender parameters class ViewController: UIViewController { @IBAction func didSelectButton(sender: UILabel) { ... } }
25. Typed sender parameters class ViewController: UIViewController { @IBAction func didSelectButton(sender: UILabel) { ... } }
26. Summary • Nil-targeted actions rule, mostly. • Gesture recognizers are weird. • Interface Builder supports you, sometimes.
27. Acknowledgments • UIGestureRecognizer and nil-targeted Actions by Brent Simmons. http://inessential.com/2012/06/22/ uigesturerecognizer_and_nil-targeted_act • Cocoa Design Patterns by Erik M. Buck. Donald A. Yacktman. http://www.goodreads.com/book/show/ 3048265-cocoa-design-patterns • Apple documentation • Paul Goracke (@pgor)