wpf/mvvm:命令禁用的wpf按钮的奇怪行为

只是一些代码…: 问题在底部。

XAML :

 <StackPanel Orientation="Horizontal">
            <Button Content="Start" Command="{Binding FirstDateCommand}" />
            <Button Content="Back" Command="{Binding PreviousDateCommand}" />
            <DatePicker SelectedDate="{Binding SelectedDate}" DisplayDateStart="{Binding MinDate}" DisplayDateEnd="{Binding MaxDate}" />
            <Button Content="Forward" Command="{Binding NextDateCommand}"  />
            <Button Content="End" Command="{Binding LastDateCommand}" />
        </StackPanel>

视图模型 :

public class LessonPlannerViewModel : ViewModelBase
    {    
        private ILessonPlannerRepository _lessonplannerRepo;    

        private ObservableCollection<LessonDay> _lessons;

        private RelayCommand _firstDateCommand;
        private RelayCommand _lastDateCommand;
        private RelayCommand _nextDateCommand;
        private RelayCommand _previousDateCommand;

        public LessonPlannerViewModel()
        {
            _lessonplannerRepo = new LessonPlannerRepository();

            MinDate = DateTime.Now.AddDays(-2);
            MaxDate = DateTime.Now.AddDays(2);

            SelectedDate = DateTime.Now;                   
        }

        public RelayCommand FirstDateCommand
        {
            get { return _firstDateCommand ?? (_firstDateCommand = new RelayCommand(() => MoveFirstDate(), () => CanMoveFirstDate())); }
        }

        public RelayCommand LastDateCommand
        {
            get { return _lastDateCommand ?? (_lastDateCommand = new RelayCommand(() => MoveLastDate(), () => CanMoveLastDate())); }
        }

        public RelayCommand PreviousDateCommand
        {
            get { return _previousDateCommand ?? (_previousDateCommand = new RelayCommand(() => MovePreviousDate(), () => CanMovePreviousDate())); }
        }

        public RelayCommand NextDateCommand
        {
            get { return _nextDateCommand ?? (_nextDateCommand = new RelayCommand(() => MoveNextDate(), () => CanMoveNextDate())); }
        }

        private void MoveFirstDate()
        {
            SelectedDate = MinDate;
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MoveLastDate()
        {
            SelectedDate = MaxDate;
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MoveNextDate()
        {
            SelectedDate = SelectedDate.AddDays(1);
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private void MovePreviousDate()
        {
            SelectedDate = SelectedDate.AddDays(-1);
            Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
        }

        private bool CanMoveFirstDate()
        {
            return SelectedDate != MinDate;
        }

        private bool CanMoveLastDate()
        {
            return SelectedDate != MaxDate;
        }

        private bool CanMoveNextDate()
        {
            return SelectedDate < MaxDate;
        }

        private bool CanMovePreviousDate()
        {
            return SelectedDate > MinDate;
        }   

        private DateTime _selectedDate;
        public DateTime SelectedDate
        {
            get { return _selectedDate; }
            set
            {
                if (_selectedDate == value)
                    return;

                _selectedDate = value;
                this.RaisePropertyChanged("SelectedDate");
                //Lessons = _lessonplannerRepo.GetLessonDayByDate(SelectedDate);
            }
        }

        public DateTime MinDate { get; set; }

        public DateTime MaxDate { get; set; }        

        public ObservableCollection<LessonDay> Lessons
        {
            get { return _lessons; }
            set
            {
                _lessons = value;
                this.RaisePropertyChanged("Lessons");
            }
        }
...

当我在datepicker中选择一个等于mindate的日期时,previouusdate命令返回canexecute=false;t 帽子可以,按预期工作 .

但是为什么lastdatecommand 不返回canexecute=false吗?

当我按下 “上一个日期”按钮 相反 通过选择日期 数据记录器 .

我做错什么了?

更新 :

我没有怀疑我的逻辑是错误的,但是…我试过一些东西,用这个密码

这真的很奇怪。我现在更改了lastdate和previousdate canexecute方法的逻辑,两个按钮现在都可以更改日期选择器。

private bool CanMoveFirstDate()
{
    Debug.WriteLine("SelectedDate FirstDate: " + SelectedDate);
    return SelectedDate > MinDate;
}

private bool CanMovePreviousDate()
{
    Debug.WriteLine("SelectedDate PreviousDate: " + SelectedDate);
    return SelectedDate > MinDate;
}

有人知道如何使nextdate+lastdate按钮工作得到解决方案吗?P

更新2:

绑定功能强大,但可能难以控制…

我又犯了一些疯狂的逻辑错误,现在看来它起作用了:

        private bool CanMoveNextDate()
        {
            Debug.WriteLine("SelectedDate NextDate: " + SelectedDate);
            return SelectedDate.AddDays(1) < MaxDate;
        }

        private bool CanMoveLastDate()
        {
            Debug.WriteLine("SelectedDate LastDate: " + SelectedDate);
            return SelectedDate.AddDays(1) < MaxDate;
        }  

如果有人能 解释 这种奇怪的逻辑,那将是很好的,我认为原因在于日期选择器和命令的绑定,以及哪个绑定首先被更新或被调用等等…