Question

我正在Xamarin.Forms中实现购物车。在我的购物车页面中有ListView个数据。每个单元格都包含一个按钮，用于选择项目和金额的计数。在购物车视图中有一个总标签。

我的问题是，当数字选择器发生变化时，总计不会更新。在项目添加视图单元格时调用计算方法。我知道我需要为此实施INotifyProperty，但我不确定如何做到这一点。

我有一个基本视图模型，它继承了包含事件的INotifyProperty。

 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;
    }

Answer 1

作为一种设计方法，最好将MVVM作为子类实现，并将其实现到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));
    }
}

我还强烈建议将ICommand实现为字典结构，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了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]}"/>

Answer 2

例如尝试此视图模型：

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;
    }
}

并在继承的类中使用它：

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

Answer 3

当我开始Xamarin编码时，MVVM有点令人困惑，直到我发现ViewModel上的PropertyChangedEvent向视图（ContentPage）发出信号，并更新了Label / textbox / etc。

对于那些寻找“最新，最伟大”的人……这是一些修改后的代码：

>>> import sqlite3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/sqlite3/__init__.py", line 23, in <module>
    from sqlite3.dbapi2 import *
  File "/usr/local/lib/python3.7/sqlite3/dbapi2.py", line 27, in <module>
    from _sqlite3 import *
ImportError: libsqlite3.so.0: cannot open shared object file: No such file or directory

并在您的属性设置器上：

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

好吗？没有？无需每次都传递属性名称！

如何在Xamarin.Forms

3 个答案: