WPF MVVM DataGrid绑定后界面更改数据如何在ViewModel中得到更改事件?

[大地主刘发财]

刚刚接触MVVM框架，好多地方都搞不懂，看书看得也是一脸懵逼，在这请教个问题
DataGrid绑定ObservableCollection，增加、删除新行的时候是有事件的，但修改现有行数据时是没有通知的，怎样才能在数据被修改的时候得到通知（事件）？

Model

public class DataModel : INotifyPropertyChanged
    {
        private int _id;
        public int Id
        {
            get { return _id; }
            set
            {
                _id = value;
                OnPropChanged("Id");
            }
        }
        private string _name;
        public string Name
        {
            get { return _name; }
            set
            {
                _name = value;
                OnPropChanged("Name");
            }
        }
        private decimal _qty;
        public decimal Qty
        {
            get { return _qty; }
            set
            {
                _qty = value;
                OnPropChanged("Qty");
            }
        }
        private decimal _price;
        public decimal Price
        {
            get { return _price; }
            set
            {
                _price = value;
                OnPropChanged("Price");
            }
        }
        private decimal _amount;
        public decimal Amount {
            get { return _amount; }
            set
            {
                _amount = value;
                OnPropChanged("Amount");
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

ViewModel

        private ObservableCollection<DataModel> _dataList = new ObservableCollection<DataModel>();
        public ObservableCollection<DataModel> DataList
        {
            get { return _dataList; }
            set { SetProperty(ref _dataList, value); }
        }
public MainWindowViewModel()
        {
            DataList.Add(new DataModel
            {
                Id = 1,
                Name = "杯",
                Qty=5,
                Price=20,
                Amount=100
            });
            DataList.Add(new DataModel
            {
                Id=2,
                Name="纸",
                Qty=100,
                Price=30,
                Amount=3000
            });
}

View

<DataGrid x:Name="dgv" ItemsSource="{Binding DataList}" Margin="57,40,446,247"/>

如图，当在界面上修改“数量”时，要自动计算金额，在MVVM这种模式下要如何实现？

[github_36000833]

DataModel实现了INotifyPropertyChanged接口，你可以通过订阅每一个DataModel的PropertyChanged事件来得到比如金额的变化通知。

public MainWindowViewModel()
{
    DataList.CollectionChanged += (sender, e) =>
    {
        if (e.OldItems != null)
        {
            foreach (var item in e.OldItems?.OfType<DataModel>()) item.PropertyChanged -= DataModel_PropertyChanged;
        }
        if (e.NewItems != null)
        {
            foreach (var item in e.NewItems?.OfType<DataModel>()) item.PropertyChanged += DataModel_PropertyChanged;
        }
        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Remove)
        {
            ShowTotal();
        }
    };

    DataList.Add(new DataModel
    {
        Id = 1,
        Name = "杯",
        Qty = 5,
        Price = 20,
        Amount = 100
    });
    DataList.Add(new DataModel
    {
        Id = 2,
        Name = "纸",
        Qty = 100,
        Price = 30,
        Amount = 3000
    });
}

private void DataModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    switch (e.PropertyName)
    {
        case nameof(DataModel.Amount):
            ShowTotal();
            break;
    }
}

// ShowTotal示例而已，不是MVVM模式。
private void ShowTotal()
{
    System.Windows.Threading.Dispatcher.CurrentDispatcher.BeginInvoke(
        System.Windows.Threading.DispatcherPriority.Normal,
        (Action)delegate 
        {
            var grandTotal = DataList.Sum(x => x.Amount);
            MessageBox.Show("总金额:" + grandTotal);
        });
}

[E次奥]

Amount 这个属性可以直接在定义的时候就用 数量和 单价计算 赋值

[大地主刘发财]

明白了 非常感谢