How to implement a left or right DragGesture() that trigger a switch case in SwiftUI?

Question

I have created a DragGesture in a View that should select a @State(Bool) whether the user swipe left or right.

The thing is that only swiping right is detected.

How to use .gesture() to capture whether a user is swiping left or right on his screen?

import SwiftUI

struct SwiftUIView: View {

  //MARK: Value to change on swipe gesture
  @State var swipeRight: Bool

  var body: some View {
    VStack {
      //MARK: Value displayed after user swiped
      Text($swipeRight ? "Right" : "Left")
    }
    .gesture(
      DragGesture()
        .onChanged { (value) in
          //MARK: What is it missing here?
          switch value.location.x {
          case ...(-0.5):
            self.swipeRight = false
            print("Swipe Left return false")
          case 0.5...:
            self.swipeRight = true
            print("Swipe Right return true")
          default: ()
          }
    })
}

Answer 1

You should compare old and new locations instead:

if value.startLocation.x > value.location.x {
    print("Swipe Left")
} else {
    print("Swipe Right")
}

So the refactored version of your code would be:

struct ContentView: View {
    enum SwipeHorizontalDirection: String {
        case left, right, none
    }

    @State var swipeHorizontalDirection: SwipeHorizontalDirection = .none { didSet { print(swipeHorizontalDirection) } }

    var body: some View {
        VStack {
            Text(swipeHorizontalDirection.rawValue)
        }
        .gesture(
            DragGesture()
                .onChanged {
                    if $0.startLocation.x > $0.location.x {
                        self.swipeHorizontalDirection = .left
                    } else if $0.startLocation.x == $0.location.x {
                        self.swipeHorizontalDirection = .none
                    } else {
                        self.swipeHorizontalDirection = .right
                    }
        })
    }
}

Answer 2

Swift 5, iOS 13

An improved version presented by [Mojtaba Hosseini ][1] here.

[1]: https://stackoverflow.com/users/5623035/mojtaba-hosseini. Place the enum and function before the body of ContentView.

enum SwipeHVDirection: String {
    case left, right, up, down, none
}

func detectDirection(value: DragGesture.Value) -> SwipeHVDirection {
if value.startLocation.x < value.location.x - 24 {
            return .left
          }
          if value.startLocation.x > value.location.x + 24 {
            return .right
          }
          if value.startLocation.y < value.location.y - 24 {
            return .down
          }
          if value.startLocation.y > value.location.y + 24 {
            return .up
          }
  return .none
  }

...

With it called inside a DragGesture. Calling it on onEnded to stop it firing multiple times.

.gesture(DragGesture()
        .onEnded { value in
        print("value ",value.translation.width)
          let direction = self.detectDirection(value: value)
          if direction == .left {
            // your code here
          }  
        }
      )

Obviously you need/can add other directions as needed...

Answer 3

I think the directions above were inverted? Or I'm misunderstanding swipe left and right. Anyway, here's a further refinement using view modifiers. You can just at .onSwipe { direction in ... } to your views.

struct SwipeModifier: ViewModifier {
    let action: ((UISwipeGestureRecognizer.Direction) -> Void)?

    init(perform action: ((UISwipeGestureRecognizer.Direction) -> Void)? = nil) {
        self.action = action
    }
        
    func body(content: Content) -> some View {
        content
            .gesture(DragGesture(minimumDistance: 24.0, coordinateSpace: .local)
                        .onEnded { value in
                            guard let action = action else {
                                return
                            }
                            if value.startLocation.x > value.location.x {
                                action(.left)
                            } else if value.startLocation.x < value.location.x {
                                action(.right)
                            } else if value.startLocation.y > value.location.y {
                                action(.down)
                            } else if value.startLocation.y < value.location.y {
                                action(.up)
                            }
                        })
    }
}

extension View {
    public func onSwipe(perform action: ((UISwipeGestureRecognizer.Direction) -> Void)? = nil) -> some View {
        return self.modifier(SwipeModifier(perform: action))
    }
}

Answer 4

Extend DragGesture.Value:

import SwiftUI

extension DragGesture.Value {
    func detectDirection(_ tolerance: Double = 24) -> Direction? {
        if startLocation.x < location.x - tolerance { return .left }
        if startLocation.x > location.x + tolerance { return .right }
        if startLocation.y > location.y + tolerance { return .up }
        if startLocation.y < location.y - tolerance { return .down }
        return nil
    }
    
    enum Direction {
        case left
        case right
        case up
        case down
    }
}

Add a dismissingGesture method to View:

extension View {
    public func dismissingGesture(tolerance: Double = 24, action: @escaping () -> ()) -> some View {
        gesture(DragGesture()
            .onEnded { value in
                let direction = value.detectDirection(tolerance)
                if direction == .left {
                    action()
                }
            }
        )
    }
}

Example usage:

struct DismissingGesture_Previews: PreviewProvider {
    static var previews: some View {
        Preview()
    }
    
    struct Preview: View {
        var body: some View {
            NavigationView {
                NavigationLink {
                    Destination()
                } label: {
                    Text("Go")
                }
            }
        }
    }
    
    struct Destination: View {
        @Environment(\.dismiss) private var dismiss
        
        var body: some View {
            ZStack {
                Color.yellow.edgesIgnoringSafeArea(.all)
                Text("Nothing")
            }
            .dismissingGesture {
                dismiss()
            }
            .navigationBarHidden(true)
        }
    }
}