How to update a tableview's cell based on values from other cells using MVVM/RxSwift?

Question

I'm new to RxSwift and trying to learn by creating a simple signup form. I want to implement it using a UITableView (as an exercise, plus it will become more complicated in the future) so I'm currently using two types of cells:

• A TextInputTableViewCell with just a UITextField
• A ButtonTableViewCell with just a UIButton

In order to represent each cell, I created an enum which looks like that:

enum FormElement {
    case textInput(placeholder: String, text: String?)
    case button(title: String, enabled: Bool)
}

and use it in a Variable to feed the tableview:

    formElementsVariable = Variable<[FormElement]>([
        .textInput(placeholder: "username", text: nil),
        .textInput(placeholder: "password", text: nil),
        .textInput(placeholder: "password, again", text: nil),
        .button(title: "create account", enabled: false)
        ])

by binding like that:

    formElementsVariable.asObservable()
        .bind(to: tableView.rx.items) {
            (tableView: UITableView, index: Int, element: FormElement) in
            let indexPath = IndexPath(row: index, section: 0)
            switch element {
            case .textInput(let placeholder, let defaultText):
                let cell = tableView.dequeueReusableCell(withIdentifier: "TextInputTableViewCell", for: indexPath) as! TextInputTableViewCell
                cell.textField.placeholder = placeholder
                cell.textField.text = defaultText
                return cell
            case .button(let title, let enabled):
                let cell = tableView.dequeueReusableCell(withIdentifier: "ButtonTableViewCell", for: indexPath) as! ButtonTableViewCell
                cell.button.setTitle(title, for: .normal)
                cell.button.isEnabled = enabled
                return cell
            }
        }.disposed(by: disposeBag)

So far, so good - this is how my form looks like:

Now, the actual problem I'm facing here, is how am I supposed to enable the create account button when all the 3 text inputs are not empty and the password is the same in both password textfields? In other words, what is the right way to apply changes to a cell, based on events happening on one or more of the other cells?

Should my goal be to change this formElementsVariable through the ViewModel or is there any better way to achieve what I want?

Answer 1

I suggest that you change your ViewModel a bit such that you can have more control over the changes in the textfields. If you create streams from your input fields such as username, password and confirmation, you can subscribe for the changes and react to it in any way you want.

Here is how I restructured your code a bit for working with changes in text fields.

internal enum FormElement {
    case textInput(placeholder: String, variable: Variable<String>)
    case button(title: String)
}

ViewModel.

internal class ViewModel {

    let username = Variable("")
    let password = Variable("")
    let confirmation = Variable("")

    lazy var formElementsVariable: Driver<[FormElement]> = {
        return Observable<[FormElement]>.of([.textInput(placeholder: "username",
                                                          variable: username),
                                               .textInput(placeholder: "password",
                                                          variable: password),
                                               .textInput(placeholder: "password, again",
                                                          variable: confirmation),
                                               .button(title: "create account")])
            .asDriver(onErrorJustReturn: [])
    }()

    lazy var isFormValid: Driver<Bool> = {
        let usernameObservable = username.asObservable()
        let passwordObservable = password.asObservable()
        let confirmationObservable = confirmation.asObservable()

        return Observable.combineLatest(usernameObservable,
                                        passwordObservable,
                                        confirmationObservable) { [unowned self] username, password, confirmation in
                                            return self.validateFields(username: username,
                                                                       password: password,
                                                                       confirmation: confirmation)
            }.asDriver(onErrorJustReturn: false)
    }()

    fileprivate func validateFields(username: String,
                                    password: String,
                                    confirmation: String) -> Bool {

        guard username.count > 0,
            password.count > 0,
            password == confirmation else {
                return false
        }

        // do other validations here

        return true
    }
}

ViewController,

internal class ViewController: UIViewController {
    @IBOutlet var tableView: UITableView!

    fileprivate var viewModel = ViewModel()

    fileprivate let disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        viewModel.formElementsVariable.drive(tableView.rx.items) { [unowned self] (tableView: UITableView, index: Int, element: FormElement) in

                let indexPath = IndexPath(row: index, section: 0)

                switch element {

                case .textInput(let placeholder, let variable):

                    let cell = self.createTextInputCell(at: indexPath,
                                                        placeholder: placeholder)

                    cell.textField.text = variable.value
                    cell.textField.rx.text.orEmpty
                        .bind(to: variable)
                        .disposed(by: cell.disposeBag)
                    return cell

                case .button(let title):
                    let cell = self.createButtonCell(at: indexPath,
                                                     title: title)
                    self.viewModel.isFormValid.drive(cell.button.rx.isEnabled)
                        .disposed(by: cell.disposeBag)
                    return cell
                }
            }.disposed(by: disposeBag)
    }

    fileprivate func createTextInputCell(at indexPath:IndexPath,
                                         placeholder: String) -> TextInputTableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "TextInputTableViewCell",
                                                 for: indexPath) as! TextInputTableViewCell
        cell.textField.placeholder = placeholder
        return cell
    }

    fileprivate func createButtonCell(at indexPath:IndexPath,
                                      title: String) -> ButtonInputTableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "ButtonInputTableViewCell",
                                                 for: indexPath) as! ButtonInputTableViewCell
        cell.button.setTitle(title, for: .normal)
        return cell
    }
}

We have three different variables based on which we enable disable button, you can see the power of stream and rx operators here.

I think it is always good to convert plain properties to Rx when they change a lot like username, password and passwordField in our case. You can see that formElementsVariable do not change much, it has no real added value of Rx except the magical tableview binding for creating cell.