在前两篇文章中，我们分别使用了templatepart和visualstate的方法实现了相同的功能，其中visualstate显然更具灵活性。在这种情况下，我通常更倾向于使用visualstate。然而，在实际应用中，这两种实现方式并不是互斥的，许多模板化控件会同时使用这两种方法。
使用VisualState的好处包括：
- 代码和UI分离，使得控件扩展更灵活。
- 可以使用Blend轻松实现动画。
尽管VisualState有诸多优势，但并不是说所有功能都必须使用VisualState实现。以下情况我会选择使用TemplatePart：
- 需要快速实现一个控件。
- 某个行为是固定的，不需要扩展。
- 需要在代码中操作UI，例如Slider或ComboBox。
- 为了强调某个部件是控件必需的。
- 为了隐藏实现细节，限制派生类或ControlTemplate修改重要的逻辑。
其中，使用TemplatePart产生的扩展性问题是我谨慎使用这种方案的最大因素。
除了VisualState，TemplatePart的功能也常常会被TemplateBinding所替代。前面的例子展示了使用VisualState在UI上的优势，这次我们用另一个控件DateTimeSelector来讨论使用TemplatePart在扩展性上的其他问题。
使用TemplatePart
DateTimeSelector组合了CalendarDatePicker和TimePicker，用于选择日期和时间（SelectedDateTime）。其XAML如下：
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:DateTimeSelector">
<StackPanel Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<CalendarDatePicker Margin="0,0,0,5" x:Name="DateElement"></CalendarDatePicker>
<TimePicker x:Name="TimeElement"></TimePicker>
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>相应的代码如下：
[TemplatePart(Name = DateElementPartName, Type = typeof(CalendarDatePicker))]
[TemplatePart(Name = TimeElementPartName, Type = typeof(TimePicker))]
public class DateTimeSelector : Control
{
public const string DateElementPartName = "DateElement";
public const string TimeElementPartName = "TimeElement";
public static readonly DependencyProperty SelectedDateTimeProperty =
DependencyProperty.Register("SelectedDateTime", typeof(DateTime), typeof(DateTimeSelector), new PropertyMetadata(DateTime.Now, OnSelectedDateTimeChanged));
private static void OnSelectedDateTimeChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
DateTimeSelector target = obj as DateTimeSelector;
DateTime oldValue = (DateTime)args.OldValue;
DateTime newValue = (DateTime)args.NewValue;
if (oldValue != newValue)
target.OnSelectedDateTimeChanged(oldValue, newValue);
}
public DateTimeSelector()
{
this.DefaultStyleKey = typeof(DateTimeSelector);
}
public DateTime SelectedDateTime
{
get { return (DateTime)GetValue(SelectedDateTimeProperty); }
set { SetValue(SelectedDateTimeProperty, value); }
}
private CalendarDatePicker _dateElement;
private TimePicker _timeElement;
private bool _isUpdatingDateTime;
protected override void OnApplyTemplate()
{
base.OnApplyTemplate();
if (_dateElement != null)
_dateElement.DateChanged -= OnDateElementDateChanged;
_dateElement = GetTemplateChild(DateElementPartName) as CalendarDatePicker;
if (_dateElement != null)
_dateElement.DateChanged += OnDateElementDateChanged;
if (_timeElement != null)
_timeElement.TimeChanged -= OnTimeElementTimeChanged;
_timeElement = GetTemplateChild(TimeElementPartName) as TimePicker;
if (_timeElement != null)
_timeElement.TimeChanged += OnTimeElementTimeChanged;
UpdateElement();
}
protected virtual void OnSelectedDateTimeChanged(DateTime oldValue, DateTime newValue)
{
UpdateElement();
}
private void OnDateElementDateChanged(CalendarDatePicker sender, CalendarDatePickerDateChangedEventArgs args)
{
UpdateSelectDateTime();
}
private void OnTimeElementTimeChanged(object sender, TimePickerValueChangedEventArgs e)
{
UpdateSelectDateTime();
}
private void UpdateElement()
{
_isUpdatingDateTime = true;
try
{
if (_dateElement != null)
_dateElement.Date = SelectedDateTime.Date;
if (_timeElement != null)
_timeElement.Time = SelectedDateTime.TimeOfDay;
}
finally
{
_isUpdatingDateTime = false;
}
}
private void UpdateSelectDateTime()
{
if (_isUpdatingDateTime)
return;
DateTime dateTime = DateTime.Now;
if (_dateElement != null && _dateElement.Date.HasValue)
dateTime = _dateElement.Date.Value.Date;
if (_timeElement != null)
dateTime = dateTime.Add(_timeElement.Time);
SelectedDateTime = dateTime;
}
}可以看出，DateTimeSelector通过监视CalendarDatePicker的DateChanged和TimePicker的TimeChanged来改变SelectedDateTime的值。
DateTimeSelector的代码非常简单，控件也运行良好，但如果某天需要将CalendarDatePicker替换为DatePicker或某个第三方的日期选择控件，DateTimeSelector就无能为力了，既不能通过修改ControlTemplate，也不能通过继承来达到目的。
使用TemplateBinding
通常在构建这类控件时，应先考虑其数据和行为，而不关心其UI。DateTimeSelector最核心的功能是通过选择Date和Time得出组合起来的DateTime，那么就可以先写出如下的类：
public class DateTimeSelector2 : Control
{
public static readonly DependencyProperty DateProperty =
DependencyProperty.Register("Date", typeof(DateTime), typeof(DateTimeSelector2), new PropertyMetadata(DateTime.Now, OnDateChanged));
private static void OnDateChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
DateTimeSelector2 target = obj as DateTimeSelector2;
DateTime oldValue = (DateTime)args.OldValue;
DateTime newValue = (DateTime)args.NewValue;
if (oldValue != newValue)
target.OnDateChanged(oldValue, newValue);
}
public static readonly DependencyProperty TimeProperty =
DependencyProperty.Register("Time", typeof(TimeSpan), typeof(DateTimeSelector2), new PropertyMetadata(TimeSpan.Zero, OnTimeChanged));
private static void OnTimeChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
DateTimeSelector2 target = obj as DateTimeSelector2;
TimeSpan oldValue = (TimeSpan)args.OldValue;
TimeSpan newValue = (TimeSpan)args.NewValue;
if (oldValue != newValue)
target.OnTimeChanged(oldValue, newValue);
}
public static readonly DependencyProperty DateTimeProperty =
DependencyProperty.Register("DateTime", typeof(DateTime), typeof(DateTimeSelector2), new PropertyMetadata(DateTime.Now, OnDateTimeChanged));
private static void OnDateTimeChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
DateTimeSelector2 target = obj as DateTimeSelector2;
DateTime oldValue = (DateTime)args.OldValue;
DateTime newValue = (DateTime)args.NewValue;
if (oldValue != newValue)
target.OnDateTimeChanged(oldValue, newValue);
}
public DateTimeSelector2()
{
this.DefaultStyleKey = typeof(DateTimeSelector2);
}
public DateTime Date
{
get { return (DateTime)GetValue(DateProperty); }
set { SetValue(DateProperty, value); }
}
public TimeSpan Time
{
get { return (TimeSpan)GetValue(TimeProperty); }
set { SetValue(TimeProperty, value); }
}
public DateTime DateTime
{
get { return (DateTime)GetValue(DateTimeProperty); }
set { SetValue(DateTimeProperty, value); }
}
private bool _isUpdatingDateTime;
protected virtual void OnDateChanged(DateTime oldValue, DateTime newValue)
{
UpdateDateTime();
}
protected virtual void OnTimeChanged(TimeSpan oldValue, TimeSpan newValue)
{
UpdateDateTime();
}
protected virtual void OnDateTimeChanged(DateTime oldValue, DateTime newValue)
{
_isUpdatingDateTime = true;
try
{
Date = newValue.Date;
Time = newValue.TimeOfDay;
}
finally
{
_isUpdatingDateTime = false;
}
}
private void UpdateDateTime()
{
if (_isUpdatingDateTime)
return;
DateTime = Date.Date.Add(Time);
}
}控件的代码并不清楚ControlTemplate中包含什么控件，它只关心自己的数据。
在XAML中通过绑定使用这些数据。
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:DateTimeSelector2">
<StackPanel Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<CalendarDatePicker Date="{Binding Date, RelativeSource={RelativeSource Mode=TemplatedParent}, Mode=TwoWay, Converter={StaticResource DateTimeOffsetConverter}}" Margin="0,0,0,5"></CalendarDatePicker>
<TimePicker Time="{Binding Time, RelativeSource={RelativeSource Mode=TemplatedParent}, Mode=TwoWay}"></TimePicker>
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:DateTimeSelector2">
<StackPanel Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<DatePicker Date="{Binding Date, RelativeSource={RelativeSource Mode=TemplatedParent}, Mode=TwoWay, Converter={StaticResource DateTimeOffsetConverter}}" Margin="0,0,0,5"></DatePicker>
<TimePicker Time="{Binding Time, RelativeSource={RelativeSource Mode=TemplatedParent}, Mode=TwoWay}"></TimePicker>
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>![[UWP 自定义控件]了解模板化控件(5.1)：TemplatePart vs. VisualState](https://img.php.cn/upload/article/001/503/042/175746492436940.jpg)
这里给出了两个Style，分别使用了CalendarDatePicker和DatePicker，通过TwoWay Binding访问DateTimeSelector2中的Date属性。如果你的TemplatedControl需要有良好的扩展能力，可以尝试使用这种方式。
