Focus on the next TextField/SecureField in SwiftUI

Question

I've built a login screen in SwiftUI. I want to focus on the password SecureField when the user is finished entering their email. How can I do this?

struct LoginView: View {
    @State var username: String = ""
    @State var password: String = ""

    var body: some View {
        ScrollView {
            VStack {
                TextField("Email", text: $username)
                    .padding()
                    .frame(width: 300)
                    .background(Color(UIColor.systemGray5))
                    .cornerRadius(5.0)
                    .padding(.bottom, 20)
                    .keyboardType(.emailAddress)

                SecureField("Password", text: $password)
                    .padding()
                    .frame(width: 300)
                    .background(Color(UIColor.systemGray5))
                    .cornerRadius(5.0)
                    .padding(.bottom, 20)

                Button(action: {

                }, label: {
                    Text("Login")
                        .padding()
                        .frame(width: 300)
                        .background((username.isEmpty || password.isEmpty) ? Color.gray : Color(UIColor.cricHQOrangeColor()))
                        .foregroundColor(.white)
                        .cornerRadius(5.0)
                        .padding(.bottom, 20)
                }).disabled(username.isEmpty || password.isEmpty)

Answer 1

I've improved on the answer from Gene Z. Ragan and Razib Mollick. Fixes a crash, this allows for any amount of textfields, supports passwords and made it into its own class.

struct UITextFieldView: UIViewRepresentable {
    let contentType: UITextContentType
    let returnVal: UIReturnKeyType
    let placeholder: String
    let tag: Int
    @Binding var text: String
    @Binding var isfocusAble: [Bool]

    func makeUIView(context: Context) -> UITextField {
        let textField = UITextField(frame: .zero)
        textField.textContentType = contentType
        textField.returnKeyType = returnVal
        textField.tag = tag
        textField.delegate = context.coordinator
        textField.placeholder = placeholder
        textField.clearButtonMode = UITextField.ViewMode.whileEditing

        if textField.textContentType == .password || textField.textContentType == .newPassword {
            textField.isSecureTextEntry = true
        }

        return textField
    }

    func updateUIView(_ uiView: UITextField, context: Context) {
        uiView.text = text

        if uiView.window != nil {
            if isfocusAble[tag] {
                if !uiView.isFirstResponder {
                    uiView.becomeFirstResponder()
                }
            } else {
                uiView.resignFirstResponder()
            }
        }
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    class Coordinator: NSObject, UITextFieldDelegate {
        var parent: UITextFieldView

        init(_ textField: UITextFieldView) {
            self.parent = textField
        }

        func textFieldDidChangeSelection(_ textField: UITextField) {
            // Without async this will modify the state during view update.
            DispatchQueue.main.async {
                self.parent.text = textField.text ?? ""
            }
        }

        func textFieldShouldBeginEditing(_ textField: UITextField) -> Bool {
            setFocus(tag: parent.tag)
            return true
        }

        func setFocus(tag: Int) {
            let reset = tag >= parent.isfocusAble.count || tag < 0

            if reset || !parent.isfocusAble[tag] {
                var newFocus = [Bool](repeatElement(false, count: parent.isfocusAble.count))
                if !reset {
                    newFocus[tag] = true
                }
                // Without async this will modify the state during view update.
                DispatchQueue.main.async {
                    self.parent.isfocusAble = newFocus
                }
            }
        }

        func textFieldShouldReturn(_ textField: UITextField) -> Bool {
            setFocus(tag: parent.tag + 1)
            return true
        }
    }
}

struct UITextFieldView_Previews: PreviewProvider {
    static var previews: some View {
        UITextFieldView(contentType: .emailAddress,
                       returnVal: .next,
                       placeholder: "Email",
                       tag: 0,
                       text: .constant(""),
                       isfocusAble: .constant([false]))
    }
}

Answer 2

iOS 15

In iOS 15 we can now use @FocusState to control which field should be focused.

Here is an example how to add buttons above the keyboard to focus the previous/next field:

struct ContentView: View {
    @State private var email: String = ""
    @State private var username: String = ""
    @State private var password: String = ""

    @FocusState private var focusedField: Field?

    var body: some View {
        NavigationView {
            VStack {
                TextField("Email", text: $email)
                    .focused($focusedField, equals: .email)
                TextField("Username", text: $username)
                    .focused($focusedField, equals: .username)
                SecureField("Password", text: $password)
                    .focused($focusedField, equals: .password)
            }
            .toolbar {
                ToolbarItem(placement: .keyboard) {
                    Button(action: focusPreviousField) {
                        Image(systemName: "chevron.up")
                    }
                    .disabled(!canFocusPreviousField()) // remove this to loop through fields
                }
                ToolbarItem(placement: .keyboard) {
                    Button(action: focusNextField) {
                        Image(systemName: "chevron.down")
                    }
                    .disabled(!canFocusNextField()) // remove this to loop through fields
                }
            }
        }
    }
}

extension ContentView {
    private enum Field: Int, CaseIterable {
        case email, username, password
    }
    
    private func focusPreviousField() {
        focusedField = focusedField.map {
            Field(rawValue: $0.rawValue - 1) ?? .password
        }
    }

    private func focusNextField() {
        focusedField = focusedField.map {
            Field(rawValue: $0.rawValue + 1) ?? .email
        }
    }
    
    private func canFocusPreviousField() -> Bool {
        guard let currentFocusedField = focusedField else {
            return false
        }
        return currentFocusedField.rawValue > 0
    }

    private func canFocusNextField() -> Bool {
        guard let currentFocusedField = focusedField else {
            return false
        }
        return currentFocusedField.rawValue < Field.allCases.count - 1
    }
}

Note: As of Xcode 13 beta 1 @FocusState doesn't work in Form/List. This should be fixed in the next releases.