INotifyPropertyChanged（监听数据），当数据改变时调用

public class BaseViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public void OnPropertyChanged(string propertyName) {
            if (PropertyChanged!=null)
            {
                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }