How to implement INotifyPropertyChanged in Xamarin.Forms

Question

I am implementing a cart in Xamarin.Forms. On my page there are baskets ListViewwith data. Each cell contains a button for selecting the number of elements and the number. There is a total amount in the cart.

My problem is that the total amount is not updated when changing the choice of number. The calculation method is called the element of adding the position of the element. I know what I need to implement INotifyPropertyfor this, but I'm not sure how to do it.

I have a base view model that inherits INotifyPropertythat contains an event.

 public class BaseViewModel : INotifyPropertyChanged
{
    private double  _price;
    public double Price
    {
        get 
        { 
            return _price; 
        }
        set 
        { 
            _price = value;
            OnPropertyChanged("Price");}
        } 

 protected virtual void OnPropertyChanged(string propertyName)
 {
     if (PropertyChanged != null)
     {
         PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
     }
 }

View Model

    public BaseViewModel()
    {
        App.Instance.ViewModel = this;
        TempList = TempList ?? new ObservableCollection<cm_items>();
        this.Title = AppResources.AppResource.Cart_menu_title;
        this.Price = CartCell.price;
    }

Answer 1

MVVM ViewModel.

:

public class ObservableProperty : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

ICommand , :

public abstract class ViewModelBase : ObservableProperty
{
    public Dictionary<string,ICommand> Commands { get; protected set; }

    public ViewModelBase()
    {
        Commands = new Dictionary<string,ICommand>();
    }
}

, todo ViewModel ViewModelBase

class LoginViewModel : ViewModelBase
{
    #region fields
    string userName;
    string password;
    #endregion

    #region properties
    public string UserName 
    {
         get {return userName;}
        set 
        {
            userName = value;
            OnPropertyChanged("UserName");
        }
     }

    public string Password 
    {
        get{return password;}
        set
        {
            password = value;
            OnPropertyChanged("Password");
        }
    }
    #endregion

    #region ctor
    public LoginViewModel()
    {
        //Add Commands
        Commands.Add("Login", new Command(CmdLogin));
    }
    #endregion


    #region UI methods

    private void CmdLogin()
    {
        // do your login jobs here
    }
    #endregion
}

: Xaml:

<Entry Placeholder="Username"  Text="{Binding UserName}"/>
<Entry Placeholder="Password" Text="{Binding Password}" IsPassword="True"/>
<Button Text="Login" Command="{Binding Commands[Login]}"/>

Answer 2

, :

public abstract class BaseViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected bool SetPropertyValue<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
    {
        if (value == null ? field != null : !value.Equals(field))
        {
            field = value;

            var handler = this.PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
            return true;
        }
        return false;
    }
}

:

    private int myProperty;
    public int MyProperty
    {
        get { return this.myProperty; }
        set { this.SetPropertyValue(ref this.myProperty, value); }
    }

Answer 3

Xamarin, MVVM , , PropertyChangedEvent ViewModel View (ContentPage), Label/textbox/etc.

, " "... :

private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
    var handler = PropertyChanged;
    if (handler != null)
    {
        handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

Setter:

public string SomeProperty
{
    get { return _somProperty; }
    set
    {
       _someProperty= value;
            OnPropertyChanged();
        }
    }
}

? ? !

Answer 4

INPC . Fody..... https://github.com/Fody/PropertyChanged

INPC , .

(, ) ([DoNotNotify]).

....

NuGet. XML. , !