FunctionalHandler API Design

[maximkrouk]

A place to discuss FunctionalHandler API

[maximkrouk]

For now, the main way to use FunctionalHanlder is this:

// Internal site

public class TapGestureRecognizer: UITapGestureRecognizer {
    @FunctionalHandler<TapGestureRecognizer, Void>
    var onTapGesture
    
    init() {
        super.init(target: nil, action: nil)
        commonInit()
    }
    
    override public init(target: Any?, action: Selector?) {
        super.init(target: target, action: action)
        commonInit()
    }
    
    private func commonInit() {
        self.addTarget(self, action: #selector(handleTap))
    }
    
    @objc private func handleTap(_ recognizer: TapGestureRecognizer) {
        $onTapGesture?(recognizer)
    }
}

// Public site

func example() {
    let tapRecognizer = TapGestureRecognizer()
    tapRecognizer.onTapGesture { recognizer in
        guard recognizer.state == .ended else { return }
        print("tapped")
    }
}

But also I'd like to add scope method to the public site

// Public site

func example() {
    let tapRecognizer = TapGestureRecognizer()
    tapRecognizer.onTapGesture.scope(\.state) { state in
        guard state == .ended else { return }
        print("tapped")
    }
}

And the main reason for the discussion is the map method. It may be convenient for data sources:

@FunctionalHandler<Void, Int>
var getNumberOfCells

func hideHalfOfCells() {
    getNumberOfCells.map { $0 / 2 }
}

But map is rarely used and its semantics probably shouldn't be mutating.

[maximkrouk]

FunctionalHandler probably should be renamed to FunctionalDataSource, and FunctionalHandler will be a new type with (Input) -> Void action. Handlers should not have outputs, now it is a generic pattern, but handlers are specific, that is why it should be reimplemented. Generic equivalent's name is still a thing to discuss.

[maximkrouk]

Also scope here has map semantics, so the map is definitely a thing to remove, it even doesn't seem to be needed...

[maximkrouk]

Functional prefix will be removed for Handler & DataSource types

[maximkrouk]

Swift 5.9 introduced variadic generics, which may allow us to get rid of generic type overloads, but currently it's only supported on iOS17+ for types, maybe we may consider using macros to support it across all oss 🤔