Return value from ViewModel

Question

Hello all,

I'm trying to program a login-flow (API with Bearer-Token) and getting the
result back from a ViewModel to the caller.

Here's what i try to achieve:

1) On startup check for username/password in SecureStorage
2) Not found? PushModalAsync a login-page (with a ViewModel behind it)
3) The login-button is bound to a Command (or AsyncCommand in MVVM Helpers)
When the button is clicked, the command shall return a Model-Class
<LoginDataModel> with username and password as string.
4) Login-Page disappears, and the returned <LoginDataModel> is further processed
and sent to the API with httpclient

Everything works - except step 3. How is it done "correctly"?

• On Disappearing?
• Can a command return a value/object?
• Messaging System?

Thank you for your tips and help!
Heiko

Answer 1

Hello,

Welcome to our Microsoft Q&A platform!

3) The login-button is bound to a Command (or AsyncCommand in MVVM Helpers)
When the button is clicked, the command shall return a Model-Class
<LoginDataModel> with username and password as string.
4) Login-Page disappears, and the returned <LoginDataModel> is further processed
and sent to the API with httpclient

When the Login-Page disappears(popup from the stack), the ViewModel behind it will been released .

But if you want to keep the returned model (LoginDataModel) for later usage, there are several methods to achieve this.

For example:

1.define the LoginDataModel as a global class and a prefix for field UserName and field Password as ,for example:

   public  class LoginDataModel  
{  
    public static string UserName { get; set; }  
    public static string Password { get; set; }  

    public LoginDataModel()  
    {  
        UserName = "";  
        Password = "";  
    }  
}  

2.When the command shall return a Model-Class, then the value of LoginDataModel will been changed.And the value will not lost even if the Login-Page be pop up from the page stack.
Then you can get the value of LoginDataModel in any other class you want.

    System.Diagnostics.Debug.WriteLine("Name = " + LoginDataModel.UserName +"<----->  pwd = " + LoginDataModel.Password);  

Note:
Of course, you can also use MessagingCenter to achieve this.
You need to Publish a message ,Subscribe to a message as follows:

Publish a message

  string[] values = { name, password};  
  MessagingCenter.Send<ViewModelBase, string[]> (this, "userInfo", values);  

Subscribe to a message

MessagingCenter.Subscribe<ViewModelBase, string[]> (this, "userInfo", (sender, values) => {  
       DisplayAlert(values[0], values[1], "Ok");  
 });  

Best Regards,

Jessie Zhang

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Answer 2

Hello Jessie,

thank you for your helpful answer!

I thought about your suggestion 2 and realized that my ViewModel still exists, if I pass it as a paramater to my LoginView.
So for now i did:

1) put my LoginDataModel in the ViewModel

public class LoginViewModel
{
    private LoginDataModel _loginDataModel;
    public LoginDataModel loginDataModel
    {
        get { return _loginDataModel; }
        set { _loginDataModel = value; }
     }
    [...]

2) Bind my entry fields directly to my LoginDataModel

<Entry Placeholder="E-mail" Text="{Binding loginDataModel.email}" />

3) create all login related things

 LoginViewModel loginViewModel = new LoginViewModel();
 await MainPage.Navigation.PushModalAsync(new LoginPage(loginViewModel));

4) do the login-work in the ViewModel, and close the Page

OnLoginClicked = new Xamarin.Forms.Command(doLogin);

private async void doLogin(object obj)
{
    await Application.Current.MainPage.Navigation.PopModalAsync();
}

After that the login-Data is still accessable in the ViewModel:

Debug.WriteLine(loginViewModel.loginDataModel.email);

And it works :-)

Best regards
Heiko