Xcode – Work with Binding inside an external function in SwiftUI

I’m currently learning SwiftUI and want to develop my own app. I have designed a LoginView and a LoginHandler that should take care of all the logic behind a login. When the user enters the wrong username/password, an Alert should appear on the screen. I solved this with the state variable loginError. But now comes the tricky part, as i want to pass a binding of this variable to my login function in the LoginHandler. Take a look at the following code:

import SwiftUI

struct LoginView: View
{
    @EnvironmentObject var loginHandler: LoginHandler
    
    @State private var username: String = ""
    @State private var password: String = ""
    @State private var loginError: Bool = false
    
    ...
    
    private func login()
    {
        loginHandler.login(username: username, password: password, error: $loginError)
    }
}

I am now trying to change the value of error inside my login function:

import Foundation
import SwiftUI

class LoginHandler: ObservableObject
{
    public func login(username: String, password: String, error: Binding<Bool>)
    {
        error = true
    }
}

But I’m getting the error

Cannot assign to value: ‘error’ is a ‘let’ constant

which makes sense I think because you can’t edit the parameters in swift. I have also tried _error = true because I once saw the underscore in combination with a binding, but this doesn’t worked either.

But then I came up with a working solution: error.wrappedValue = true. My only problem with that is the following statement from Apples Developer Documentation:

This property provides primary access to the value’s data. However, you don’t access wrappedValue directly. Instead, you use the property variable created with the @Binding attribute.

Although I’m super happy that it works, I wonder if there is any better way to solve this situation?

Update 20.3.21: New edge case

In the comment section I mentioned a case where you don’t know how many times your function will be used. I will now provide a little code example:

Imagine a list of downloadable files (DownloadView) that you will get from your backend:

import SwiftUI

struct DownloadView: View
{
    @EnvironmentObject var downloadHandler: DownloadHandler
    
    var body: some View
    {
        VStack
        {
            ForEach(downloadHandler.getAllDownloadableFiles())
            {
                file in DownloadItemView(file: file)
            }
        }
    }
}

Every downloadable file has a name, a small description and its own download button:

import SwiftUI

struct DownloadItemView: View
{
    @EnvironmentObject var downloadHandler: DownloadHandler
    
    @State private var downloadProgress: Double = -1
    
    var file: File
    
    var body: some View
    {
        HStack
        {
            VStack
            {
                Text(file.name)
                Text(file.description)
            }
            
            Spacer()
            
            if downloadProgress < 0
            {
                // User can start Download
                Button(action: {
                    downloadFile()
                })
                {
                    Text("Download")
                }
            }
            else
            {
                // User sees download progress
                ProgressView(value: $downloadProgress)
            }
        }
    }
    
    func downloadFile()
    {
        downloadHandler.downloadFile(file: file, progress: $downloadProgress)
    }
}

And now finally the ‘DownloadHandler’:

import Foundation
import SwiftUI

class DownloadHandler: ObservableObject
{
    public func downloadFile(file: File, progress: Binding<Double>)
    {
        // Example for changing the value
        progress = 0.5
    }
}