Commands' canExecute isn't evaluated

Question

In my MainPage.xaml I've these:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:VM="clr-namespace:Emon"
             x:Class="Emon.MainPage">
    <ContentPage.BindingContext>
        <VM:MainVM/>
    </ContentPage.BindingContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackLayout Margin="20,20,20,0">
            <Entry Placeholder="First Name" Text="{Binding FirstName}"/>
            <Entry Placeholder="Last Name" Text="{Binding LastName}" />
            <Button Text="Change Full Name" Command="{Binding MyCommand}"/>
        </StackLayout>
        <ListView Grid.Row="1" ItemsSource="{Binding Names}"/>
    </Grid>
</ContentPage> 

and in the MainVM.cs these:

public class MainVM : INotifyPropertyChanged
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public ObservableCollection<string> Names { get; set; }
    public Command MyCommand { get; set; }
    public event PropertyChangedEventHandler PropertyChanged;
    public MainVM()
    {
        MyCommand = new Command(setName, canSetName);
        Names = new ObservableCollection<string>();
    }
    void setName()
    {
        Names.Add(FirstName + " " + LastName);
        FirstName = string.Empty; LastName = null;
        OnPropertyChanged(nameof(FirstName));
        OnPropertyChanged(nameof(LastName));
    }
    bool canSetName() => !string.IsNullOrWhiteSpace(FirstName) && !string.IsNullOrWhiteSpace(LastName);
    void OnPropertyChanged(string name) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}

In WPF I've used UpdateSourceTrigger=PropertyChanged in TextBox but there's no such thing in Entry so even if I type in those Entries canSetName isnt called and as a result the Button remains disabled.

Answer 1

Hello,​

Welcome to our Microsoft Q&A platform!

The return value of the canSetName Func cannot by updated automatically. Try adding a bool property in the MainVM class to receive the result of !string.IsNullOrWhiteSpace(FirstName) && !string.IsNullOrWhiteSpace(LastName) code. Detect the property changed event of the FirstName and LastName property to update the value of the bool property.

Check the code:

   public class MainVM : INotifyPropertyChanged  
   {  
       public ObservableCollection<string> Names { get; set; }  
       public Command MyCommand { get; set; }  
       public MainVM()  
       {  
           MyCommand = new Command(setName, canSetName);  
           Names = new ObservableCollection<string>();  
       }  
       void setName()  
       {  
           Names.Add(FirstName + " " + LastName);  
           FirstName = string.Empty; LastName = null;  
       }  
       bool canSetName()  
       {  
           return Result;  
       }  
     
       public string firstName;  
       public string FirstName  
       {  
           get  
           {  
               return firstName;  
           }  
           set  
           {  
               if (firstName != value)  
               {  
                   firstName = value;  
                   Result = !string.IsNullOrWhiteSpace(value) && !string.IsNullOrWhiteSpace(LastName) ? true : false;  
                   OnPropertyChanged(nameof(FirstName));  
               }  
           }  
       }  
     
       public string lastName;  
       public string LastName  
       {  
           get  
           {  
               return lastName;  
           }  
           set  
           {  
               if (lastName != value)  
               {  
                   lastName = value;  
                   Result = !string.IsNullOrWhiteSpace(value) && !string.IsNullOrWhiteSpace(FirstName) ? true : false;  
                   OnPropertyChanged(nameof(LastName));  
               }  
           }  
       }  
     
       public bool result;  
       public bool Result  
       {  
           get  
           {  
               return result;  
           }  
           set  
           {  
               if (result != value)  
               {  
                   result = value;  
                   MyCommand?.ChangeCanExecute();  
                   OnPropertyChanged(nameof(Result));  
               }  
           }  
       }  
     
       void OnPropertyChanged(string name) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));  
       public event PropertyChangedEventHandler PropertyChanged;  
   }  

Best Regards,

Jarvan Zhang

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