如何在Xamarin.Forms中实现INotifyPropertyChanged [英] How to implement INotifyPropertyChanged in Xamarin.Forms

问题描述

我正在Xamarin.Forms中实现购物车.在我的购物车页面上，有一个ListView及其数据.每个单元格都包含一个按钮，用于选择 item 和 amount 的计数.在购物车视图中，有一个总计标签.

I am implementing a cart in Xamarin.Forms. In my cart page there is a ListView with data. Each of the cell contains a button to select the count of item and amount. In the cart view there is a grand total label.

我的问题是，数字选择器更改时，总计没有更新.在项目添加视图单元格时调用该计算方法.我知道我需要为此实现INotifyProperty，但是我不确定如何做到这一点.

My problem is the grand total is not updating while the number picker changes. The calculation method is called upon item adding view cell. I know that i need to implement INotifyProperty for this, but I'm unsure of how to do it.

我有一个基本视图模型，该模型继承了包含事件的INotifyProperty.

I have a base view model which inherits INotifyProperty that 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));
     }
 }

查看模型

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

推荐答案

作为一种设计方法，最好将MVVM作为子类实现并将其实现到您的ViewModel.

As a design methodology, its better to implement MVVM as a subclass and implement it to your ViewModel.

示例实施:

public class ObservableProperty : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

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

我也强烈建议将ICommand实施为Dictionary结构，例如:

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

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

因此，ViewModel中的所有待办事项都只是继承ViewModelBase类并使用它

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
}

最后:Xaml用法:

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

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