有没有办法在 SwiftUI 中创建新的手势？

Question

SwiftUI is missing a Pan gesture (that is, both scale and offset), so I was trying to create one. However, it appears that the Gesture struct depends on private classes. For example:

public struct PinchGesture: Gesture {

    public struct PinchGestureValue: Equatable {
        var scale: CGFloat
        var anchor: UnitPoint
        var offset: CGSize
        var isPinching: Bool
    }
    
    public typealias Value = PinchGestureValue
    public typealias Body = Never
    
    var minimumScaleDelta: CGFloat
    var minimumDistance: CGFloat
    var coordinateSpace: CoordinateSpace
    
    
    public init(minimumScaleDelta: CGFloat = 0.01, minimumDistance: CGFloat = 10, coordinateSpace: CoordinateSpace = .local) {
        self.minimumScaleDelta = minimumScaleDelta
        self.minimumDistance = minimumDistance
        self.coordinateSpace = coordinateSpace
    }
    
    public static func _makeGesture(gesture: _GraphValue<PinchGesture>, inputs: _GestureInputs) -> _GestureOutputs<PinchGestureValue> {
      // Unable to complete
    }

}

This code cannot be completed, as the _GraphValue, _GestureInputs, and _GestureOutputs are private. Before I give in completely, I wanted to see if anyone has figured out a workaround.

Answer 1

SwiftUI provides a default implementation of _makeGesture :

extension Gesture where Self.Value == Self.Body.Value {
  public static func _makeGesture(gesture: SwiftUI._GraphValue<Self>, inputs: SwiftUI._GestureInputs) -> SwiftUI._GestureOutputs<Self.Body.Value>
}

The difficulty here is the constraint Self.Value === Self.Body.Value . That means your gesture's body can't be declared to return some Gesture , because some Gesture can't satisfy the constraint (even if its Value would match). So you have to give body a specific type. The easiest solution is to use the AnyGesture type eraser:

public struct PinchGesture: Gesture {

    ...

    public var body: AnyGesture<PinchGestureValue> {
        AnyGesture(
            DragGesture(minimumDistance: 0, coordinateSpace: .global)
               .map { PinchGestureValue($0) }
        )
    }
}

In this code, Swift can infer PinchGesture.Value = PinchGestureValue and PinchGesture.Body = AnyGesture<PinchGestureValue> . Then it can prove AnyGesture<PinchGestureValue>.Value == PinchGesture.Value , so it can use the default implementation of _makeGesture provided by SwiftUI.

Unfortunately, I doubt you can use this to create your PinchGesture . Ultimately, your body is still restricted to combining SwiftUI's primitive gestures, which don't give you access the current UIEvent or UITouch objects.