Writing to Bool property not agreeing with SwiftUI - swiftui

Question

Continuing on my learning path of SwiftUI, I stumble on what seems like a simple problem, but not to me !
I am trying to display a Text in a colored border. The color depends on a condition that I set inside a func.
I declared a var Bool in order to keep track of that condition, but when I try to write to it, I get the dreaded Cannot assign to property: 'self' is immutable error.
If I declare that Bool using the #State wrapper, I then get the other dreaded Modifying state during view update, this will cause undefined behavior message.
If anyone can help, would be [again] much appreciated.
Here is a summary of my code, simplified for clarity :
import SwiftUI
import CoreData
struct ContentView: View {
var isCurrent = false
var body: some View {
VStack {
Spacer()
Text(minimumEventsCheck().0) // Currency string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(isCurrent ? Color.green : Color.red, lineWidth: 1))
Text(minimumEventsCheck().1) // details string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(isCurrent ? Color.green : Color.red, lineWidth: 1))
Spacer()
} // END of main VStack
} // END of body
private func minimumEventsCheck() -> (String, String) {
var aString = ""
var bString = ""
let refDate = Date ()
let date90Prior = refDate.addingTimeInterval(-7776000)
if date90Prior < refDate { // This is always true, but it's for demo only
aString = "True"
bString = "Also true"
isCurrent = true // TRIGGERS Xcode error : Cannot assign to property: 'self' is immutable
} else {
aString = "False"
bString = "Also false"
isCurrent = false // TRIGGERS Xcode error : Cannot assign to property: 'self' is immutable
}
return (aString, bString)
} // End of func currencyText() -> String
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
return ContentView().environment(\.managedObjectContext, context)
}
}

Answer 1

use
#State var isCurrent = false
The view is a struct and structs are by default immutable. Therefore SwiftUI has propertywrapper #State to change the struct/variables.

Answer 2

I see two possible purpose of trying to do that.
First is to check some date once one start. You can do that in init and store the result in a struct property like this:
struct ContentView: View {
var isCurrent = false
var aString: String
var bString: String
init(refDate: Date){
if refDate < Date().addingTimeInterval(-7776000) {
self.aString = "True"
self.bString = "Also true"
self.isCurrent = true
} else {
self.aString = "False"
self.bString = "Also false"
self.isCurrent = false
}
}
var body: some View {
VStack {
Spacer()
Text(self.aString) // Currency string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(isCurrent ? Color.green : Color.red, lineWidth: 1))
Text(self.bString) // details string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(isCurrent ? Color.green : Color.red, lineWidth: 1))
Spacer()
} // END of main VStack
}
}
In a parent View (or in sceneDelegate file if it is the root view) you will have to pass this date like this:
ContentView(refDate: Date())
Then, when you change the date outside of the View, this view will be triggered to initiate with new values, and the dates will be checked again. This is SwiftUI way of coding.
If you trying to make some kind of function that will updates this view from the outside, you will have to use a passthroughSubject when changing something, and catch notification in .onReceive modifier. This is the only way of updating values inside the View without it being initialize again.

Answer 3

OK, Got this to work the following way :
Got rid of the #State var Bool in my View struct.
Modified the func to return the tuple (String, String, Bool) iso ((String, String) and called minimumEventsCheck().2 to access it in code :
import SwiftUI
import CoreData
struct ContentView: View {
var body: some View {
VStack {
Spacer()
Text(minimumEventsCheck().0) // Currency string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(minimumEventsCheck().2 ? Color.green : Color.red, lineWidth: 1))
Text(minimumEventsCheck().1) // details string
.lineLimit(nil) // allows unlimited lines
.padding(.all)
.overlay(RoundedRectangle(cornerRadius: 10).stroke(minimumEventsCheck().2 ? Color.green : Color.red, lineWidth: 1))
Spacer()
} // END of main VStack
} // END of body
private func minimumEventsCheck() -> (String, String, Bool) {
var isCurrent = false
var aString = ""
var bString = ""
let refDate = Date ()
let date90Prior = refDate.addingTimeInterval(-7776000)
if date90Prior < refDate { // This is always true, but it's for demo only
aString = "True"
bString = "Also true"
isCurrent = true
} else {
aString = "False"
bString = "Also false"
isCurrent = false
}
return (aString, bString, isCurrent)
} // End of func currencyText() -> String
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
return ContentView().environment(\.managedObjectContext, context)
}
}