c# - 使用 MVVM 对 ComboBox 选择的异步方法调用

Question

我在显示通过 ComboBox 选择过滤的图表时遇到了问题，而没有锁定 UI。统计过滤非常繁重，需要运行 async。在我尝试从 Property setter 调用 FilterStatisticsAsync 和 MonthSelectionChanged 之前，一切正常。有没有人对如何解决或解决这个问题有好的建议？

XAML 如下所示:

        <ComboBox x:Name="cmbMonth"
                  ItemsSource="{Binding Months}" 
                  SelectedItem="{Binding SelectedMonth }"
                  IsEditable="True"
                  IsReadOnly="True"

像这样的 ViewModel 属性 setter :

    public string SelectedMonth
    {
        get { return _selectedMonth; }
        set { SetProperty(ref _selectedMonth, value); LoadStatisticsAsync(); MonthSelectionChanged(); }
    }

SetProperty 派生自封装 INPC 的基类，如下所示:

        public event PropertyChangedEventHandler PropertyChanged = delegate { };

        protected virtual void SetProperty<T>(ref T member, T value, [CallerMemberName] string propertyName = null)
        {
            if (Equals(member, value))
                return;

            member = value;
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

Answer 1

我会用这个来做:

    public class AsyncProperty<T> : INotifyPropertyChanged
    {
        public async Task UpdateAsync(Task<T> updateAction)
        {
            LastException = null;
            IsUpdating = true;

            try
            {
                Value = await updateAction.ConfigureAwait(false);
            }
            catch (Exception e)
            {
                LastException = e;
                Value = default(T);
            }

            IsUpdating = false;
        }

        private T _value;

        public T Value
        {
            get { return _value; }
            set
            {
                if (Equals(value, _value)) return;
                _value = value;
                OnPropertyChanged();
            }
        }

        private bool _isUpdating;

        public bool IsUpdating
        {
            get { return _isUpdating; }
            set
            {
                if (value == _isUpdating) return;
                _isUpdating = value;
                OnPropertyChanged();
            }
        }

        private Exception _lastException;

        public Exception LastException
        {
            get { return _lastException; }
            set
            {
                if (Equals(value, _lastException)) return;
                _lastException = value;
                OnPropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

属性的定义

 public AsyncProperty<string> SelectedMonth { get; } = new AsyncProperty<string>();

代码中的其他地方:

SelectedMonth.UpdateAsync(Task.Run(() => whateveryourbackground work is));

在 xaml 中绑定(bind):

SelectedItem="{Binding SelectedMonth.Value }"

请注意，属性应该反射(reflect)当前状态，而不是触发可能需要无限长时间的过程。因此需要以不同于仅仅分配它的方式更新属性。