SwiftUI ScrollView with Tap and Drag gesture - ios

Question

I'm trying to implement a ScrollView with elements which can be tapped and dragged. It should work the following way:
The ScrollView should work normally, so swiping up/down should not interfere with the gestures.
Tapping an entry should run some code. It would be great if there would be a "tap indicator", so the user knows that the tap has been registered (What makes this difficult is that the tap indicator should be triggered on touch down, not on touch up, and should be active until the finger gets released).
Long pressing an entry should activate a drag gesture, so items can be moved around.
The code below covers all of those requirements (except the tap indicator). However, I'm not sure why it works, to be specific, why I need to use .highPriorityGesture and for example can't sequence the Tap Gesture and the DragGesture with .sequenced(before: ...) (that will block the scrolling).
Also, I'd like to be notified on a touch down event (not touch up, see 2.). I tried to use LongPressGesture() instead of TapGesture(), but that blocks the ScrollView scrolling as well and doesn't even trigger the DragGesture afterwards.
Does somebody know how this can be achieved? Or is this the limit of SwiftUI? And if so, would it be possible to port UIKit stuff over to achieve this (I already tried that, too, but was unsuccessful, the content of the ScrollView should also be dynamic so porting over the whole ScrollView might be difficult)?
Thanks for helping me out!
struct ContentView: View {
var body: some View {
ScrollView() {
ForEach(0..<5, id: \.self) { i in
ListElem()
.highPriorityGesture(TapGesture().onEnded({print("tapped!")}))
.frame(maxWidth: .infinity)
}
}
}
}
struct ListElem: View {
#GestureState var dragging = CGSize.zero
var body: some View {
Circle()
.frame(width: 100, height: 100)
.gesture(DragGesture(minimumDistance: 0, coordinateSpace: .global)
.updating($dragging, body: {t, state, _ in
state = t.translation
}))
.offset(dragging)
}
}

Answer 1

I tried a few option and I think a combination of sequenced and simultaneously allows two gestures to run the same time. To achieve a onTouchDown I used a DragGesture with minimum distance of 0.
struct ContentView: View {
var body: some View {
ScrollView() {
ForEach(0..<5, id: \.self) { i in
ListElem()
.frame(maxWidth: .infinity)
}
}
}
}
struct ListElem: View {
#State private var offset = CGSize.zero
#State private var isDragging = false
#GestureState var isTapping = false
var body: some View {
// Gets triggered immediately because a drag of 0 distance starts already when touching down.
let tapGesture = DragGesture(minimumDistance: 0)
.updating($isTapping) {_, isTapping, _ in
isTapping = true
}
// minimumDistance here is mainly relevant to change to red before the drag
let dragGesture = DragGesture(minimumDistance: 0)
.onChanged { offset = $0.translation }
.onEnded { _ in
withAnimation {
offset = .zero
isDragging = false
}
}
let pressGesture = LongPressGesture(minimumDuration: 1.0)
.onEnded { value in
withAnimation {
isDragging = true
}
}
// The dragGesture will wait until the pressGesture has triggered after minimumDuration 1.0 seconds.
let combined = pressGesture.sequenced(before: dragGesture)
// The new combined gesture is set to run together with the tapGesture.
let simultaneously = tapGesture.simultaneously(with: combined)
return Circle()
.overlay(isTapping ? Circle().stroke(Color.red, lineWidth: 5) : nil) //listening to the isTapping state
.frame(width: 100, height: 100)
.foregroundColor(isDragging ? Color.red : Color.black) // listening to the isDragging state.
.offset(offset)
.gesture(simultaneously)
}
}
For anyone interested here is a custom scroll view that will not be blocked by other gestures as mentioned in one of the comments. As this was not possible to be solved with the standard ScrollView.
OpenScrollView for SwiftUI on Github
Credit to
https://stackoverflow.com/a/59897987/12764795
http://developer.apple.com/documentation/swiftui/composing-swiftui-gestures
https://www.hackingwithswift.com/books/ios-swiftui/how-to-use-gestures-in-swiftui