How to call a func in another view/struct in SwiftUI?

In this post I will try to explain some fundamentals about SwiftUI which I used to solve your problem. First, SwiftUI is a little different from UIKit, it is a declarative programming, state driven so UI Components are not referenced manually for manipulation or update instead, each view has different states and will render accordingly when it's state changes.

In SwiftUI, Apple provides us with built-in state management property wrappers @State and @Binding. SwiftUI will store them and update each UI that component is linked to.

You can have a good read about Swift UI : here or here

I tried not to change many things and kept the properties and class names the same.

Code in ChoiceView

import SwiftUI

struct ChoiceView: View {

var ranId_CV : Int // random id pass from ContentView

var delegate : ChoiceRowView? // delegates can't be used in SwiftUI

@State var qandaData : [Question] = [
    Question(id : 1 , question : "Which ones are odd number?", rawChoices : "A. 1\nB. 2\nC. 3\nD. 4" , answer : "AC"),
    Question(id : 2 , question : "This is another question", rawChoices : "A. 1\nB. 2\nC. 3\nD. 4" , answer : "B")
]

// Array of Line objects containing all the Questions and their states
@State var lines : [Line] = []

var body: some View {
    ScrollView {
        Text(self.qandaData[ranId_CV].question)
        HStack{
            VStack(alignment: .leading){
                ForEach (self.lines.indices, id: \.self){ index in
                    ChoiceRowView(line: self.$lines[index])
                }
            }
            .fixedSize(horizontal: false, vertical: true)
            Spacer()
        }
        .padding([.top, .leading, .bottom, .trailing])
        .onAppear(perform: {
            self.qandaData[self.ranId_CV].choices.forEach { (label) in
                self.lines.append(Line(label: label , answer: self.qandaData[self.ranId_CV].answer))
            }
        })
        
        Button(action: {
            self.test_CV()
        }, label: {
            Text("Test_CV")
        })
    }
}

func test_CV() {
    //        delegate?.checkAnswer()
    
    self.lines.indices.forEach { (index) in
        self.lines[index].state = .none // restore correction state
    }
    
    self.lines.indices.forEach { (index) in
        if self.lines[index].selection == .selected {
            self.qandaData[self.ranId_CV].answer.forEach({ (char) in
                if self.lines[index].label.prefix(1).contains(char) && self.lines[index].selection == .selected {
                    self.lines[index].state = .valid
                }else{
                    self.lines[index].state = .invalid
                }
                
            })
        }
    }
}

}

Walkthrough part 1 :

1. When the view appear, the selected by the random Int Question propositions will be converted into a Line object each.
2. ForEach will loop through the array of Lines and render them depend on each of their state.
3. Line's states are stored in an enum which makes them simple to trigger.
4. Using the @State wrapper will tell SwiftUI to update the concerned views whenever changes happens to that array of objects.

Code in ChoiceRowView

import SwiftUI

protocol checkAnswerProtocol {
    func checkAnswer()
}

struct ChoiceRowView: View {

@Binding var line : Line

//    var label: String
//    var answer: String
@State var isChecked : Bool = false
@State var fgColor : Color = .black


func buttonPress() {

    self.line.selection = isChecked ? .selected : .unselected
    
    isChecked = !isChecked
   
    
    if isChecked == false {
        fgColor = .black
    } else {
        fgColor = .blue
    }
    
    //            selectedAnswer = selectedAnswer.filter { $0 != item_BP.prefix(1) }
}

func checkAnswer() {
    
    print("executed checkAnswer")
    
    switch (isChecked, self.line.label.contains(String(self.line.label.prefix(1)))) {
        
    //selected = isChecked = true, qandaData.answer contain prefix(1)  => change to green
    case (true, true) : fgColor = .green
        
    //selected = isChecked = true, incorrect => change to red
    case (true, false) : fgColor = .red
        
    //unselected = isChecked = false, correct => change to red
    case (false, true) : fgColor = .red
    //unselected = isChecked = false, incorrect => remain unchange
    default: print("do nothing")
    }
    
}


var body: some View {
    HStack{
        Button(action: {
            self.buttonPress()
            
        }) {
            
            Image(systemName: isChecked ? "checkmark.square": "square")
            Text(self.line.label)
        }
        .foregroundColor(self.line.state != .none  ? (self.line.state == .valid ? .green : .red) : .blue )
    }
}

}

Walkthrough part 2 :

1. Using the wrapper @Binding will tell the view that the property does come from the caller or the parent and whenever it's value changes, the view will get updated accordingly.

2. Using enum to store a view's state is a good practice and make your code more readable.

3. The entity Question is used, so we can manipulate data correctly and avoid dealing with raw strings.

4. With no views references in code, delegates are not any more part of this programming paradigm.

   import Foundation
   
   struct Question {
   let id: Int
   let question, answer: String
   let choices : [String]
   
   init(id : Int , question : String, rawChoices : String , answer : String) {
       self.id = id
       self.question = question
       self.choices = rawChoices.components(separatedBy: "\n")
       self.answer = answer
   }
   }
   

And the UI object

import Foundation
struct Line {
        let id = UUID()
        let label : String
        let answer : String
        var state : QState = .none
        var selection : Selecction = .unselected
        
    }
    
    enum QState {
        case valid,invalid, none
    }
    
    enum Selecction {
        case selected, unselected
    }

Finally this is the project structure

Or clone the project from my Github

Feel free to ask me any question I will be glad to help you and don't forget to +1 if I answered your question.