WPF实现树形下拉列表框(TreeComboBox)

前言

　　树形下拉菜单是许多WPF应用程序中常见的用户界面元素，它能够以分层的方式展示数据，提供更好的用户体验。本文将深入探讨如何基于WPF创建一个可定制的树形下拉菜单控件，涵盖从原理到实际实现的关键步骤。

一、需求分析

树形下拉菜单控件的核心是将ComboBox与TreeView结合起来，以实现下拉时的树状数据展示。在WPF中，可以通过自定义控件模板、样式和数据绑定来实现这一目标。

我们首先来分析一下ComboBox控件的模板。

<ControlTemplate x:Key="ComboBoxTemplate" TargetType="{x:Type ComboBox}">
    <Grid x:Name="templateRoot" SnapsToDevicePixels="true">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
        </Grid.ColumnDefinitions>
        <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource Mode=TemplatedParent}}" Margin="1" Placement="Bottom" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}">
            <theme:SystemDropShadowChrome x:Name="shadow" Color="Transparent" MinWidth="{Binding ActualWidth, ElementName=templateRoot}" MaxHeight="{TemplateBinding MaxDropDownHeight}">
                <Border x:Name="dropDownBorder" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1">
                    <ScrollViewer x:Name="DropDownScrollViewer">
                        <Grid x:Name="grid" RenderOptions.ClearTypeHint="Enabled">
                            <Canvas x:Name="canvas" HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                                <Rectangle x:Name="opaqueRect" Fill="{Binding Background, ElementName=dropDownBorder}" Height="{Binding ActualHeight, ElementName=dropDownBorder}" Width="{Binding ActualWidth, ElementName=dropDownBorder}"/>
                            </Canvas>
                            <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                        </Grid>
                    </ScrollViewer>
                </Border>
            </theme:SystemDropShadowChrome>
        </Popup>
        <ToggleButton x:Name="toggleButton" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource Mode=TemplatedParent}}" Style="{StaticResource ComboBoxToggleButton}"/>
        <ContentPresenter x:Name="contentPresenter" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" Content="{TemplateBinding SelectionBoxItem}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" IsHitTestVisible="false" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
    </Grid>
</ControlTemplate>

　　从以上代码可以看出，其中的Popup控件就是下拉部分，那么按照常理，我们在Popup控件中放入一个TreeView控件即可实现该需求，但是现实情况远没有这么简单。我们开发一个控件，不仅要从外观上实现功能，还需要考虑数据绑定、事件触发、自定义模板等方面的问题，显然，直接放置一个TreeView控件虽然也能实现功能，但是从封装的角度看，它并不优雅，使用也不方便。那么有没有更好的方法满足以上需求呢？下面提供另一种思路，其核心思想就是融合ComboBox控件与TreeView控件模板，让控件既保留TreeView的特性，又拥有ComboBox的外观。

二、代码实现

2.1 编辑TreeView模板；

2.2 提取ComboBox的模板代码；

2.3 将ComboBox的模板代码移植到TreeView模板中；

2.4 将TreeView模板包含ItemsPresenter部分的关键代码放入ComboBox模板中的Popup控件内；

以下为融合后的xaml代码

<ControlTemplate TargetType="{x:Type local:TreeComboBox}">
    <Grid x:Name="templateRoot" SnapsToDevicePixels="true">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="0" MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" />
        </Grid.ColumnDefinitions>
        <Popup
                            x:Name="PART_Popup"
                            Grid.ColumnSpan="2"
                            MaxHeight="{TemplateBinding MaxDropDownHeight}"
                            Margin="1"
                            AllowsTransparency="true"
                            IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                            Placement="Bottom"
                            PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}">
            <Border
                                x:Name="PART_Border"
                                Width="{Binding RelativeSource={RelativeSource AncestorType=local:TreeComboBox}, Path=ActualWidth}"
                                Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"
                                BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"
                                BorderThickness="1"
                                SnapsToDevicePixels="true">
                <ScrollViewer
                                    x:Name="_tv_scrollviewer_"
                                    Padding="{TemplateBinding Padding}"
                                    Background="{TemplateBinding Background}"
                                    CanContentScroll="false"
                                    Focusable="false"
                                    HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
                                    SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                    VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">
                    <ItemsPresenter />
                </ScrollViewer>
            </Border>
        </Popup>
        <ToggleButton
                            x:Name="toggleButton"
                            Grid.ColumnSpan="2"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                            Style="{StaticResource ComboBoxToggleButton}" />
        <ContentPresenter
                            x:Name="contentPresenter"
                            Margin="{TemplateBinding Padding}"
                            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                            Content="{TemplateBinding SelectionBoxItem}"
                            ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                            IsHitTestVisible="False" />
    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsEnabled" Value="false">
            <Setter TargetName="PART_Border" Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" />
        </Trigger>
        <Trigger Property="VirtualizingPanel.IsVirtualizing" Value="true">
            <Setter TargetName="_tv_scrollviewer_" Property="CanContentScroll" Value="true" />
        </Trigger>
        <MultiTrigger>
            <MultiTrigger.Conditions>
                <Condition Property="IsGrouping" Value="true" />
                <Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false" />
            </MultiTrigger.Conditions>
            <Setter Property="ScrollViewer.CanContentScroll" Value="false" />
        </MultiTrigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

以下为使用控件的代码。

<TreeComboBox
                Width="315"
                MinHeight="30"
                Padding="5"
                HorizontalAlignment="Center"
                VerticalAlignment="Top"
                VerticalContentAlignment="Stretch"
                IsAutoCollapse="True"
                ItemsSource="{Binding Collection}">
    <TreeComboBox.SelectionBoxItemTemplate>
        <ItemContainerTemplate>
            <Border>
                <TextBlock VerticalAlignment="Center" Text="{Binding Property1}" />
            </Border>
        </ItemContainerTemplate>
    </TreeComboBox.SelectionBoxItemTemplate>
    <TreeComboBox.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Collection}">
            <TextBlock
                            Margin="5,0,0,0"
                            VerticalAlignment="Center"
                            Text="{Binding Property1}" />
        </HierarchicalDataTemplate>
    </TreeComboBox.ItemTemplate>
</TreeComboBox>