MVVM Community Toolkit .NET MAUI中NotifyCanExecuteChangedFor无法触发命令

问题根源 #

你当前仅绑定了TimePicker的Time属性，但未将时间变更事件与TimeSelectedCommand关联，也没有在属性值变化时主动触发命令。

方案一：直接在XAML中绑定命令 #

修改XAML，给每个TimePicker添加Command绑定，让时间变化时自动触发命令：

<Border Style="{StaticResource TimePickerBorderStyle}">
    <TimePicker Style="{StaticResource TimePickerStyle}"
                Time="{Binding StartTime}"
                Command="{Binding TimeSelectedCommand}" />
</Border>

<Border Grid.Column="2"
        Style="{StaticResource TimePickerBorderStyle}">
    <TimePicker Time="{Binding EndTime}"
                Style="{StaticResource TimePickerStyle}"
                Command="{Binding TimeSelectedCommand}" />
</Border>

方案二：利用ObservableProperty的变更回调（推荐） #

借助CommunityToolkit.Mvvm的ObservableProperty特性，重写属性变更后的回调方法，主动调用命令：

[ObservableProperty]
[NotifyCanExecuteChangedFor(nameof(TimeSelectedCommand))]
TimeSpan startTime;

[ObservableProperty]
[NotifyCanExecuteChangedFor(nameof(TimeSelectedCommand))]
TimeSpan endTime;

// StartTime变更时触发命令
partial void OnStartTimeChanged(TimeSpan value)
{
    TimeSelectedCommand.Execute(null);
}

// EndTime变更时触发命令
partial void OnEndTimeChanged(TimeSpan value)
{
    TimeSelectedCommand.Execute(null);
}

[RelayCommand]
private void TimeSelected()
{
    // TimeSpan是值类型，无需判空
    if (StartTime < EndTime)
    {
        Preview.NumberOfSelectedIntervals = (int)(EndTime.Hours - StartTime.Hours);
    }
}

补充：原代码中StartTime != null && EndTime != null的判断多余，因为TimeSpan是值类型永远不会为null。

方案三：使用EventToCommandBehavior（精准控制触发时机） #

如果需要只在用户确认时间选择时触发命令，可借助MAUI Toolkit的行为绑定TimeChanged事件：

<!-- 先添加命名空间 xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit" -->
<Border Style="{StaticResource TimePickerBorderStyle}">
    <TimePicker Style="{StaticResource TimePickerStyle}"
                Time="{Binding StartTime}">
        <TimePicker.Behaviors>
            <toolkit:EventToCommandBehavior
                EventName="TimeChanged"
                Command="{Binding TimeSelectedCommand}" />
        </TimePicker.Behaviors>
    </TimePicker>
</Border>

<Border Grid.Column="2"
        Style="{StaticResource TimePickerBorderStyle}">
    <TimePicker Time="{Binding EndTime}"
                Style="{StaticResource TimePickerStyle}">
        <TimePicker.Behaviors>
            <toolkit:EventToCommandBehavior
                EventName="TimeChanged"
                Command="{Binding TimeSelectedCommand}" />
        </TimePicker.Behaviors>
    </TimePicker>
</Border>

内容的提问来源于stack exchange，提问作者Cristi Sandu