Progress view updated by threads in SwiftUI

contentView.swift

import SwiftUI

struct ContentView: View {
    
    @State var total = 0
    @State var progress = 0
    @State private var workItem: DispatchWorkItem?
    
    var body: some View {
        
        if(self.total = 0){
            VStack {
                //Some view to select items from the array here
                
                
                //Button to begin the process
                Button(action: {
                    self.doStuff()
                }) {
                    Image(systemName: "square.and.arrow.down")
                }
            }.padding()
         } else {
            
          VStack{
                //Loading bar
                ProgressView("Saving...", value: Double(self.progress), total: Double(self.total))
                
                //Add a cancel button
                Button(action: {
                    self.workItem?.cancel()
                    self.total = 0
                    self.progress = 0
                }) {
                    Image(systemName: "xmark.octagon")
                }
            }.padding()
        }
    }
    
    func doStuff() {
        let items = [] //For my case I was looping through an array
        self.total = items.count  //but you can replace this with the total number of items that need to be processed
        self.progress = 0
        
        self.workItem = DispatchWorkItem {            
            for item in items {
                //Do your complicated work here
                
              
                //Update progress using the main thread each loop iteration
                DispatchQueue.main.async { self.progress += 1 }
            }
            
            //Reset variables on the main thread once finished
            DispatchQueue.main.async{
                self.progress = 0
                self.total = 0
            }
        }
        
        //Start the work item
        if(workItem != nil) {
            DispatchQueue.global().async(execute: workItem!)
        }
    }
}