In an app I'm working on, there is a part that has, mostly, a "forward" navigation – tapping on buttons would display the next slide. However, a secondary "backward" navigation is also necessary. Here's the approach I've used:
import SwiftUI
struct Sample: View {
@State private var dragOffset: CGFloat = -100
var body: some View {
VStack {
Text("Perhaps a title")
ScrollView {
VStack {
Text("Some scrollable content is going to be here")
// ...
Button(action: {
// Go to the next slide
}) { Text("Next") }
}
}
Text("and, maybe, something else")
}
.overlay(
Image(systemName: "arrow.left").offset(x: dragOffset / 2),
alignment: .leading
)
.gesture(
DragGesture()
.onChanged{
self.dragOffset = $0.translation.width
}
.onEnded {
self.dragOffset = -100 // Hide the arrow
if $0.translation.width > 100 {
// Go to the previous slide
}
}
)
}
}
There is a small indicator (left arrow) that is, initially, hidden (dragOffset = -100). When the drag gesture begins, offset is fed into the dragOffset state variable and that, effectively, shows the arrow. When drag gesture ends, the arrow is hidden again and, if a certain offset is reached, the previous slide is displayed.
Works well enough, except, when the user scrolls the content in the ScrollView, this gesture is also triggered and updated for a while but then is, I assume, cancelled by the ScrollView and the "onEnded" is not called. As a result, the arrow indicator stays on the screen.
Hence the question: what is the correct way to do a gesture like that, that would work together with a ScrollView? Is it even possible with the current state of SwiftUI?
For such temporary states it is better to use GestureState
as it is automatically reset to initial state after gesture cancels/finished.
So here is possible approach
Demo:
Code:
struct Sample: View {
@GestureState private var dragOffset: CGFloat = -100
var body: some View {
VStack {
Text("Perhaps a title")
ScrollView {
VStack {
Text("Some scrollable content is going to be here")
// ...
Button(action: {
// Go to the next slide
}) { Text("Next") }
}
}
Text("and, maybe, something else")
}
.overlay(
Image(systemName: "arrow.left").offset(x: dragOffset / 2),
alignment: .leading
)
.gesture(
DragGesture()
.updating($dragOffset) { (value, gestureState, transaction) in
let delta = value.location.x - value.startLocation.x
if delta > 10 { // << some appropriate horizontal threshold here
gestureState = delta
}
}
.onEnded {
if $0.translation.width > 100 {
// Go to the previous slide
}
}
)
}
}
Note: dragOffset: CGFloat = -100
this might have different effect on different real devices, so probably it is better to calculate it explicitly.