SwiftUI: Multitouch gesture / Multiple Gestures

Question

Is there a way in SwiftUI to track multiple gestures at once? I want my one main view to be able to keep track of multiple fingers dragging at once.

ZStack {
    Color.black
      .edgesIgnoringSafeArea(.all)
      .gesture(DragGesture(minimumDistance: 0)
               .onChanged { (value) in
                 //some logic
               }.onEnded { (value) in
                  //more logic         
               })
       //other code
}

I have this code however I can only ever have one drag gesture being processed at a time. If one finger is dragging and then I try to add another one the first one stops.

I am trying to achieve an effect where multiple fingers are on screen at once. Each finger is dragging a circle simultaneously (one circle is following each finger).

I see simultaneous gestures on Apple's documentation, but this is referring to have one gesture trigger multiple blocks.

Answer 1

the solution (How to detect a tap gesture location in SwiftUI?) that uses UIViewRepresentable would work. Although its not the pure swiftUI solution (I've looked for a pure swiftUI solution but cannot find one and would be interested to see one). I've updated code to the below code, you would need to changed the numberOfTouchPoints = 5. and then use call back to get value into view.

struct TapView: UIViewRepresentable {
    var tappedCallback: (() -> Void)

    func makeUIView(context: UIViewRepresentableContext<TapView>) -> TapView.UIViewType {
        let v = UIView(frame: .zero)
        let gesture = NFingerGestureRecognizer(target: context.coordinator,
                                                       action: #selector(Coordinator.tapped))
        v.addGestureRecognizer(gesture)
        return v
    }

    class Coordinator: NSObject {
        var tappedCallback: (() -> Void)

        init(tappedCallback: @escaping (() -> Void)) {
            self.tappedCallback = tappedCallback
        }

        @objc func tapped(gesture:NFingerGestureRecognizer) {
            for i in 0..<gesture.numberOfTouches{
                print(gesture.location(ofTouch: i, in: gesture. view))
            }
            self.tappedCallback()
        }
    }

    func makeCoordinator() -> TapView.Coordinator {
        return Coordinator(tappedCallback:self.tappedCallback)
    }

    func updateUIView(_ uiView: UIView,
                      context: UIViewRepresentableContext<TapView>) {
    }
}

class NFingerGestureRecognizer: UIGestureRecognizer {
    var numberOfTouchPoints: Int = 2
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
        if self.state == .possible {
            if (self.numberOfTouches == numberOfTouchPoints){ // we have a two finger interaction starting
                self.state = .began
                print("two finger drag - Start")
            }
        } else {        // check to see if there are more touchs
            if (self.numberOfTouches > numberOfTouchPoints){ // we have a two finger interaction starting
                self.state = .failed
                print("two finger drag - failed")
            }
        }
        //printInternalTouches()
    }
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        if (self.state != .possible){
            self.state = .changed        // we are looking for two finger interaction
            //printInternalTouches()
            print("changed")
        }
    }
    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
        if numberOfTouches == 2 {
            state = .ended
        }
        else {
            state = .failed
        }
        //printInternalTouches()
        print("ended")
    }
    
    private func printInternalTouches(){
        // now print the internal touchUI elements - should be the better solution
        print("Internal Locations")
        for i in 0..<self.numberOfTouches{
            print(self.location(ofTouch: i, in: self.view))
        }
    }
}

Answer 2

I combined Paul's answer with the outdated example from Apple to create a way to detect as many touches as you like.

Update 3: (commented code is for debug output)

import Foundation
import UIKit

class NFingerGestureRecognizer: UIGestureRecognizer {

    var tappedCallback: (UITouch, CGPoint?) -> Void

    var touchViews = [UITouch:CGPoint]()

    init(target: Any?, tappedCallback: @escaping (UITouch, CGPoint?) -> ()) {
        self.tappedCallback = tappedCallback
        super.init(target: target, action: nil)
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
        for touch in touches {
            let location = touch.location(in: touch.view)
            // print("Start: (\(location.x)/\(location.y))")
            touchViews[touch] = location
            tappedCallback(touch, location)
        }
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        for touch in touches {
            let newLocation = touch.location(in: touch.view)
            // let oldLocation = touchViews[touch]!
            // print("Move: (\(oldLocation.x)/\(oldLocation.y)) -> (\(newLocation.x)/\(newLocation.y))")
            touchViews[touch] = newLocation
            tappedCallback(touch, newLocation)
        }
    }

    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
        for touch in touches {
            // let oldLocation = touchViews[touch]!
            // print("End: (\(oldLocation.x)/\(oldLocation.y))")
            touchViews.removeValue(forKey: touch)
            tappedCallback(touch, nil)
        }
    }

}

Update:

You need to wrap this into an UIViewRepresentable to use it as a SwiftUI component:

import Foundation
import SwiftUI

struct TapView: UIViewRepresentable {

    var tappedCallback: (UITouch, CGPoint?) -> Void

    func makeUIView(context: UIViewRepresentableContext<TapView>) -> TapView.UIViewType {
        let v = UIView(frame: .zero)
        let gesture = NFingerGestureRecognizer(target: context.coordinator, tappedCallback: tappedCallback)
        v.addGestureRecognizer(gesture)
        return v
    }
    
    func updateUIView(_ uiView: UIView, context: UIViewRepresentableContext<TapView>) {
        // empty
    }

}

Update 2:

A little example of how to use TapView in SwiftUI:

var body: some View {
    guard let img = UIImage(named: "background.png") else {
        fatalError("Unable to load image")
    }
    return GeometryReader { geometry in
        ZStack {
            Image(uiImage: img)
                    .resizable()
                    .aspectRatio(geometry.size, contentMode: .fill)
                    .edgesIgnoringSafeArea(.all)


            TapView { touch, optLocation in
                // touch can be used as a dictionary key.
                // optLocation is nil when the touch ends.
            }
        }
    }
}