前言
业务开发过程中遇到一个日期范围选择的需求，和Element UI的DateTimePicker组件比较类似，由两个日历控件组成，联动选择起始时间和结束时间。
问题
WPF中提供了一个DatePicker的控件，主要由DatePickerTextBox、Button和一个Calendar组成，其中Calendar是后台代码动态添加的，因此不能直接通过自定义DatePicker的控件模板实现需求。这里通过实现自定义DateTimePicker控件来满足需求。
技术要点与实现
由于Calendar结构比较复杂，本文通过控件组合的方式简单实现自定义DateTimePicker。
先来看下实现效果。
首先创建一个名为DateTimePicker的UserControl,添加依赖属性HoverStart和HoverEnd用于控制日历中的开始日期和结束日期，添加依赖属性DateTimeRangeStart和DateTimeRangeEnd用于设置外部设置/获取起始时间和结束时间。
然后在XAML中添加两个WatermarkTextBox用于输入起始时间和结束时间（增加校验规则验证时间的合法性，这里不再详细说明如何写校验规则，具体可参考ValidationRule实现参数绑定）。
接着添加一个Popup(默认关闭)，并在其中添加两个Calendar用于筛选日期，以及四个ComboBox用于筛选小时和分钟。当WatermarkTextBox捕获到鼠标时触发Popup打开。
<Grid>
<Border Height="30" Width="320" BorderBrush="#c4c4c4" BorderThickness="1" CornerRadius="2">
<StackPanel x:Name="datetimeSelected" Orientation="Horizontal" Height="30">
<local:WatermarkTextBox x:Name="DateStartWTextBox" Style="{StaticResource DateWatermarkTextBoxStyle}" GotMouseCapture="WatermarkTextBox_GotMouseCapture">
<local:WatermarkTextBox.Resources>
<helper:BindingProxy x:Key="dateRangeCeiling" Data="{Binding Text,ElementName=DateEndWTextBox}"/>
</local:WatermarkTextBox.Resources>
<local:WatermarkTextBox.Text>
<Binding Path="DateTimeRangeStart" ElementName="self" StringFormat="{}{0:yyyy-MM-dd HH:mm}" UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<helper:DateTimeValidationRule Type="Range">
<helper:ValidationParams Param1="{x:Static System:DateTime.Today}" Param2="{Binding Data,Source={StaticResource dateRangeCeiling}}"/>
</helper:DateTimeValidationRule>
</Binding.ValidationRules>
</Binding>
</local:WatermarkTextBox.Text>
</local:WatermarkTextBox>
<TextBlock Text="~"/>
<local:WatermarkTextBox x:Name="DateEndWTextBox" Style="{StaticResource DateWatermarkTextBoxStyle}" GotMouseCapture="WatermarkTextBox_GotMouseCapture">
<local:WatermarkTextBox.Resources>
<helper:BindingProxy x:Key="dateRangeFloor" Data="{Binding Text,ElementName=DateStartWTextBox}"/>
</local:WatermarkTextBox.Resources>
<local:WatermarkTextBox.Text>
<Binding Path="DateTimeRangeEnd" ElementName="self" StringFormat="{}{0:yyyy-MM-dd HH:mm}" UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<helper:DateTimeValidationRule Type="Floor">
<helper:ValidationParams Param1="{Binding Data,Source={StaticResource dateRangeFloor}}"/>
</helper:DateTimeValidationRule>
</Binding.ValidationRules>
</Binding>
</local:WatermarkTextBox.Text>
</local:WatermarkTextBox>
<local:ImageButton Width="18" Height="18" Click="ImageButton_Click"
HoverImage="/Platanus;component/Images/calendar_hover.png"
NormalImage="/Platanus;component/Images/calendar.png" />
</StackPanel>
</Border>
<Popup x:Name="DatetimePopup" AllowsTransparency="True" StaysOpen="False" Placement="Top" VerticalOffset="-10" HorizontalOffset="-300" PlacementTarget="{Binding ElementName=datetimeSelected}" PopupAnimation="Slide">
<Grid Background="White" Margin="3">
<Grid.Effect>
<DropShadowEffect Color="Gray" BlurRadius="16" ShadowDepth="3" Opacity="0.2" Direction="0" />
</Grid.Effect>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="42"/>
<RowDefinition Height="42"/>
</Grid.RowDefinitions>
<StackPanel Orientation="Horizontal">
<Calendar x:Name="startCalendar" DockPanel.Dock="Left"
Style="{DynamicResource CalendarStyle}" SelectionMode="SingleRange" SelectedDatesChanged="Calendar_SelectedDatesChanged"/>
<Line Y1="0" Y2="{Binding ActualHeight ,ElementName=startCalendar}" Stroke="#e4e4e4"/>
<Calendar x:Name="endCalendar" DockPanel.Dock="Right"
Style="{DynamicResource CalendarStyle}" SelectionMode="SingleRange" SelectedDatesChanged="Calendar_SelectedDatesChanged" DisplayDate="{Binding DisplayDate,ElementName=startCalendar,Converter={StaticResource DateTimeAddtionConverter},ConverterParameter=1}"/>
</StackPanel>
<Border Grid.Row="1" BorderThickness="0 0 0 1" BorderBrush="#e4e4e4">
<StackPanel Orientation="Horizontal" TextElement.Foreground="#999999" TextElement.FontSize="14">
<TextBlock Text="开始时间:" Margin="15 0 7 0"/>
<ComboBox x:Name="startHours" Width="64" ItemStringFormat="{}{0:D2}" SelectionChanged="startHours_SelectionChanged"/>
<TextBlock Text=":" Margin="5 0 5 0"/>
<ComboBox x:Name="startMins" ItemStringFormat="{}{0:D2}" Width="64"/>
<TextBlock Text="截止时间:" Margin="40 0 7 0"/>
<ComboBox x:Name="endHours" ItemStringFormat="{}{0:D2}" Width="64"/>
<TextBlock Text=":" Margin="5 0 5 0"/>
<ComboBox x:Name="endMins" ItemStringFormat="{}{0:D2}" Width="64"/>
</StackPanel>
</Border>
<StackPanel Grid.Row="2" Orientation="Horizontal" HorizontalAlignment="Right" Margin="0 0 11 0">
<local:ImageButton x:Name="clearBtn" Style="{StaticResource ImageLinkButton}" Content="清空" FontSize="14" Foreground="#0099ff"
Click="clearBtn_Click"
NormalImage="{x:Null}"
HoverImage="{x:Null}"
DownImage="{x:Null}"
/>
<Button x:Name="yesBtn" Content="确定" Width="56" Height="28" Margin="12 0 0 0" Click="yesBtn_Click">
<Button.Style>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource BaseButtonStyle}">
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="BorderBrush" Value="#dcdfe6"/>
<Setter Property="Foreground" Value="#333333"/>
<Setter Property="OverridesDefaultStyle" Value="True"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border x:Name="border" Background="Transparent" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="3" ClipToBounds="True">
<ContentPresenter
RecognizesAccessKey="True"
Margin="{TemplateBinding Padding}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<ControlTemplate.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsPressed" Value="false"/>
<Condition Property="IsMouseOver" Value="true"/>
</MultiTrigger.Conditions>
<Setter Property="BorderBrush" Value="#409eff"/>
<Setter Property="Foreground" Value="#409eff"/>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Button.Style>
</Button>
</StackPanel>
</Grid>
</Popup>
</Grid>
紧接着就是修改Calendar的样式了。通常情况下，自定义控件模板只需要在Visual Studio的设计窗口或者Blend中选中控件，然后右键菜单中编辑模板即可。可能由于Calendar中的部分元素（CalendarButton和CalendarDayButton）是后台代码生成，这个方法编辑Calendar模板副本生成的CalendarStyle不包含完整的可视化树结构，无法对样式进一步修改。
幸运的是微软官方文档公开了控件的默认样式和模板，在此基础上进行修改即可。
通过官方文档可以发现Calendar完整的可视化树中包含了四个类型控件Calendar、CalendarItem、CalendarButton、CalendarDayButton。其中CalendarDayButton对应的就是日历中具体的“天”，管理着具体的“天”的状态，比如选中状态、不可选状态等，这也是我们主要修改的地方，接下来看下CalendarDayButton的样式。
（其他几个元素的样式和模板参照官方文档修改即可）
<Style x:Key="CalendarDayButtonStyle" TargetType="{x:Type CalendarDayButton}">
<Setter Property="MinWidth" Value="5" />
<Setter Property="MinHeight" Value="5" />
<Setter Property="Width" Value="42"/>
<Setter Property="Height" Value="42"/>
<Setter Property="FontSize" Value="12" />
<Setter Property="HorizontalContentAlignment" Value="Center" />
<Setter Property="VerticalContentAlignment" Value="Center" />
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CalendarDayButton}">
<Grid Height="26" MouseUp="Grid_MouseUp">
<Border x:Name="SelectedBackground" Background="#f2f6fc" Visibility="Collapsed">
<Border.CornerRadius>
<MultiBinding Converter="{StaticResource SelectedDatesConverter}">
<Binding/>
<Binding Path="HoverStart" RelativeSource="{RelativeSource AncestorType={x:Type local:DateTimePicker}}"/>
<Binding Path="HoverEnd" RelativeSource="{RelativeSource AncestorType={x:Type local:DateTimePicker}}"/>
</MultiBinding>
</Border.CornerRadius>
</Border>
<Grid Width="22" Height="22">
<Rectangle x:Name="StartStopBackground" Fill="#409eff" RadiusX="11" RadiusY="11" >
<Rectangle.Visibility>
<MultiBinding Converter="{StaticResource SelectedDatesConverter}">
<Binding/>
<Binding Path="HoverStart" RelativeSource="{RelativeSource AncestorType={x:Type local:DateTimePicker}}"/>
<Binding Path="HoverEnd" RelativeSource="{RelativeSource AncestorType={x:Type local:DateTimePicker}}"/>
<Binding Path="IsInactive" RelativeSource="{RelativeSource AncestorType={x:Type CalendarDayButton}}"/>
</MultiBinding>
</Rectangle.Visibility>
</Rectangle>
<Border
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}" />
<Rectangle
x:Name="HighlightBackground"
Grid.ColumnSpan="2"
Fill="#FFBADDE9"
Opacity="0"
RadiusX="11"
RadiusY="11" />
<ContentPresenter
x:Name="NormalText"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
TextElement.Foreground="#FF333333" />
<Path
x:Name="Blackout"
Grid.ColumnSpan="2"
Margin="3"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
Data="M8.1772461,11.029181 L10.433105,11.029181 L11.700684,12.801641 L12.973633,11.029181 L15.191895,11.029181 L12.844727,13.999395 L15.21875,17.060919 L12.962891,17.060919 L11.673828,15.256231 L10.352539,17.060919 L8.1396484,17.060919 L10.519043,14.042364 z"
Fill="#FF000000"
Opacity="0"
RenderTransformOrigin="0.5,0.5"
Stretch="Fill" />
<Rectangle
x:Name="DayButtonFocusVisual"
Grid.ColumnSpan="2"
IsHitTestVisible="false"
RadiusX="11"
RadiusY="1"
Stroke="#FF45D6FA"
Visibility="Collapsed" />
</Grid>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsInactive" Value="True">
<Setter Property="Visibility" Value="Collapsed" TargetName="SelectedBackground"/>
<Setter Property="TextElement.Foreground" Value="#c0c4cc" TargetName="NormalText"/>
</Trigger>
<Trigger Property="IsBlackedOut" Value="true">
<Setter Property="Visibility" Value="Collapsed" TargetName="SelectedBackground"/>
<Setter Property="TextElement.Foreground" Value="#c0c4cc" TargetName="NormalText"/>
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactive" Value="false"/>
<Condition Property="IsSelected" Value="true"/>
</MultiTrigger.Conditions>
<MultiTrigger.Setters>
<Setter Property="Visibility" Value="Visible" TargetName="SelectedBackground"/>
</MultiTrigger.Setters>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactive" Value="false"/>
<Condition Property="IsBlackedOut" Value="false"/>
<Condition Property="IsMouseOver" Value="true"/>
</MultiTrigger.Conditions>
<MultiTrigger.Setters>
<Setter Property="Opacity" Value="0.5" TargetName="HighlightBackground"/>
</MultiTrigger.Setters>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactive" Value="false"/>
<Condition Property="IsToday" Value="true"/>
</MultiTrigger.Conditions>
<MultiTrigger.Setters>
<Setter Property="TextElement.Foreground" Value="#409eff" TargetName="NormalText"/>
</MultiTrigger.Setters>
</MultiTrigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactive" Value="false"/>
<Condition Property="Visibility" Value="Visible" SourceName="StartStopBackground"/>
</MultiTrigger.Conditions>
<MultiTrigger.Setters>
<Setter Property="TextElement.Foreground" Value="#ffffff" TargetName="NormalText"/>
</MultiTrigger.Setters>
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
样式中用到一个MultiBinding绑定CalendarDayButton以及前边提到的两个依赖属性：HoverStart和HoverEnd，然后通过MultiValueConverter转换器比较CalendarDayButton是否处于选中的日期范围，根据不同的状态设置其背景样式和字体颜色。SelectedDatesConverter的实现如下：
public class SelectedDatesConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (targetType.Name == "CornerRadius")
{
if (values.Length < 3) return new CornerRadius(0);
if (values[0].Equals(values[1])) return new CornerRadius(13, 0, 0, 13);
else if (values[0].Equals(values[2])) return new CornerRadius(0, 13, 13, 0);
else return new CornerRadius(0);
}
else
{
if (values.Length < 3) return Visibility.Collapsed;
if ((values[0].Equals(values[1]) || values[0].Equals(values[2])) && System.Convert.ToBoolean(values[3]) == false) return Visibility.Visible;
else return Visibility.Collapsed;
}
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
最后就是在后台代码中根据日历的SelectedDatesChanged事件设置HoverStart和HoverEnd的值，以此来控制DateTimePicker中选中日期的样式。
总结
本文分享了一种简单实现自定义DateTimePicker控件的方式，同时也介绍了另外一种查看原生控件默认样式和模板的方法：查看微软官方文档。
这种方法虽然不如在Visual Studio的设计窗口或者Blend中编辑模板副本方便，但提供了完整的结构、每个元素的组成部分以及可视化状态，方便开发人员清晰的了解控件全貌，可以应对修改复杂的原生控件样式和模板的需求。
