SwiftUI setting Object Variables within ForEach is not working - swiftui

Question

I have a scrolling view that displays an Object's Name as a Text View within a ForEach, I also have a GeometryReader because I need to know the position of each Object within the ScrollView. When displaying the Text View I have a function showObject() set the object's number to be it's position according to the GeometryReader.
So here's my problem. Above the ScrollView I have 3 Texts that show the name and number of the Objects for debug purposes. When scrolling, object[0]'s number updates like I expect it to, but object[1] and object[2] stay at the initialized 100 value. I have a print set up in my showObject() function and it's receiving the correct information, however it appears that it's failing to set the number on my objects after object[0]. Does anyone know why this is happening? Or perhaps a better way of achieving what I'm trying to do?
struct MyObject {
var name:String
var number:CGFloat = 100
}
struct MyScrollView: View {
#State var objects: [MyObject] = [MyObject(name: "a"), MyObject(name: "b"), MyObject(name: "c")]
var body: some View {
VStack{
Text(objects[0].name + " --- " + (objects[0].number.description))
Text(objects[1].name + " --- " + (objects[1].number.description))
Text(objects[2].name + " --- " + (objects[2].number.description))
ScrollView(showsIndicators: false){
VStack{
ForEach(self.options.indices) { i in
GeometryReader {geo in
self.showObject(index: i, num: geo.frame(in: .global).minY)
} // geo
} // ForEach
} // VStack inside ScrollView
} // ScrollView
} // top vstack
} // body
func showObject(index: Int, num: CGFloat) -> Text
{
objects[index].number = num
print (objects[index].name + " -- should be: " + num.description + " -- actual: " + objects[index].number.description)
return Text(objects[index].name)
}
} // view

Answer 1

Any change in #State var objects result in your MyScrollView redraw, ie recreation. So, rendering engine starts drawing your MyScrollView, comes to objects[index].number = num, which modifies #State var objects that reports that new redraw needed...
a -- should be: 94.0 -- actual: 100.0
2019-11-22 08:14:11.535085+0200 Testing[73866:816126] [SwiftUI] Modifying state during view update, this will cause undefined behavior.
... rendering interrupted and goes from start. This is why you observed only 1st MyObject changes.
Solution... well, you need to place .number out of workflow affecting recursive redraw. The simplest is defer modifications to next event cycle, like
if self.objects[index].number != num { // avoid redraw for same values
DispatchQueue.main.async {
self.objects[index].number = num
}
}
However, in general it is better to put MyModel outside of view and make some ViewModel wrapper, which would have view-effecting published name but not number, but this depends on your real needs.