How to do validation using mvvmcross - validation

Question

I have just started learning to write mobile apps using Xamarin and MvvmCross. I have found it quite easy to pick up the basics due to the great support including the N+1 days of MvvmCross videos on YouTube (Huge thanks to Stuart Lodge).
However I am struggling with valudation data. I'm hoping someone on Stackoverflow can point me in the direction of some useful blogs or tutorials on performing validation using MvvmCross. I want to be able validate the data entered and then update the view indicating the issue.
I need something from first principles as I don't know what I don't know (If that makes sense). I need some best practice to follow.

Answer 1

Data validation can be displayed in the UI in different ways.
For example, you can show a message box or show a label.
Suppose you want to have a label with red text somewhere in the UI to show the error.
I assume you have a 'Save' button or similar in your UI.
You can bind the button to a SaveCommand in the view-model.
In the implementation of the SaveCommand, you can check if all the data is valid and set an Error string property.
You can have a label's text bound to the Error property. Moreover, you could also bind the label's visibility to the condition (Error != null).
public class SettingsViewModel : MvxViewModel
{
string firstName;
public string FirstName
{
get { return this.firstName; }
set
{
if(this.firstName != value)
{
this.firstName = value;
this.RaisePropertyChanged(()=> this.FirstName);
this.Error = null; // reset error
}
}
}
public string Error { get; private set; }
public ICommand SaveCommand { get { return new MvxCommand(this.Save); } }
void Save()
{
// reset error
this.Error = null;
if(string.IsNullOrEmpty(this.FirstName))
{
this.Error = "First name is empty";
}
if(string.IsNullOrEmtpy(this.Error))
{
// no error, save settings...
}
else
{
this.RaisePropertyChanged(()=> this.Error);
}
}
}