今天我们来谈一下ListBoxItem这个控件，ListBoxItem是直接从ContentControl继承而来的，所以可以添加到任何具有Content属性的控件中去，常见的ListBoxItem可以放到ListBox中，也可以放到ItemsControl中去，ListBoxItem可以横向和TreeViewItem进行比较，只不过TreeViewItem是直接从HeaderedItemsControl继承过来的，然后再继承自ItemsControl。两者有很多的共同之处，可以做更多的横向比较，我们今天只是来讲ListBoxItem，首先看看我们使用的样式，这里贴出前端代码：
<Style TargetType="local:FormItem">
<Setter Property="Margin" Value="0 0 0 15"></Setter>
<Setter Property="Background" Value="#fff"></Setter>
<Setter Property="Height" Value="50"></Setter>
<Setter Property="HorizontalAlignment" Value="Stretch"></Setter>
<Setter Property="VerticalAlignment" Value="Stretch"></Setter>
<Setter Property="Padding" Value="6"></Setter>
<Setter Property="Foreground" Value="{StaticResource DarkColor}"></Setter>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:FormItem">
<Grid Background="{TemplateBinding Background}" Height="{TemplateBinding Height}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="{Binding HeaderWidth,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=local:Form}}"></ColumnDefinition>
<ColumnDefinition Width="*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Rectangle Width="3" HorizontalAlignment="Left" Fill="{StaticResource Highlight}"></Rectangle>
<StackPanel VerticalAlignment="Center" HorizontalAlignment="Left" Margin="13 0 0 0" Orientation="Horizontal">
<Image x:Name="Icon" Source="{TemplateBinding Icon}" Width="24" Height="24" Margin="0 0 10 0" VerticalAlignment="Center" HorizontalAlignment="Left"></Image>
<TextBlock Text="{TemplateBinding Title}" Foreground="{TemplateBinding Foreground}" VerticalAlignment="Center" HorizontalAlignment="Left"></TextBlock>
</StackPanel>
<ContentPresenter Margin="{TemplateBinding Padding}" Grid.Column="1" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="{TemplateBinding VerticalAlignment}"></ContentPresenter>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Icon" Value="{x:Null}">
<Setter Property="Visibility" Value="Collapsed" TargetName="Icon"></Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
这里我们重写了ListBoxItem 的ControlTemplate，我们需要注意的一个地方就是我们使用了 <ContentPresenter Margin="{TemplateBinding Padding}" Grid.Column="1" HorizontalAlignment="{TemplateBinding HorizontalAlignment}" VerticalAlignment="{TemplateBinding VerticalAlignment}"></ContentPresenter>来替代ListBoxItem的Content，我们需要始终记住，只有控件拥有Content属性才能使用ContentPresenter ，这个属性是用来呈现控件的content。
另外一个需要重点介绍的就是FormItem这个类中的代码，这个控件在加载的时候所有的效果都是在后台中进行加载的，首先贴出相关的类的实现，然后再做进一步的分析。
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; using System.Windows; using System.Windows.Controls; using System.Windows.Media; using System.Windows.Media.Animation; namespace X.UI { public class FormItem : ListBoxItem { static FormItem() { DefaultStyleKeyProperty.OverrideMetadata(typeof(FormItem), new FrameworkPropertyMetadata(typeof(FormItem))); } public FormItem() { System.Windows.Media.TranslateTransform transform = EnsureRenderTransform<System.Windows.Media.TranslateTransform>(this); transform.X = transform.Y = 100; Opacity = 0; IsVisibleChanged += FormItem_IsVisibleChanged; } void FormItem_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e) { if (this.Parent is Form) { if (!IsVisible) { int index = (this.Parent as Form).Items.IndexOf(this); System.Windows.Media.TranslateTransform transform = EnsureRenderTransform<System.Windows.Media.TranslateTransform>(this); DoubleAnimation da = new DoubleAnimation() { From = 0, To = 100, EasingFunction = new CircleEase { EasingMode = EasingMode.EaseOut } }; transform.BeginAnimation(System.Windows.Media.TranslateTransform.XProperty, da); transform.BeginAnimation(System.Windows.Media.TranslateTransform.YProperty, da); DoubleAnimation daopacity = new DoubleAnimation { From = 1, To = 0, }; this.BeginAnimation(UIElement.OpacityProperty, daopacity); } else { int index = (this.Parent as Form).Items.IndexOf(this); System.Windows.Media.TranslateTransform transform = EnsureRenderTransform<System.Windows.Media.TranslateTransform>(this); DoubleAnimation da = new DoubleAnimation() { From = 100, To = 0, BeginTime = TimeSpan.FromMilliseconds(100 * (index + 1)), Duration = TimeSpan.FromMilliseconds(666), EasingFunction = new CircleEase { EasingMode = EasingMode.EaseOut } }; transform.BeginAnimation(System.Windows.Media.TranslateTransform.XProperty, da); transform.BeginAnimation(System.Windows.Media.TranslateTransform.YProperty, da); DoubleAnimation daopacity = new DoubleAnimation { From = 0, To = 1, BeginTime = TimeSpan.FromMilliseconds(100 * (index + 1)), Duration = TimeSpan.FromMilliseconds(666), EasingFunction = new CircleEase { EasingMode = EasingMode.EaseOut } }; this.BeginAnimation(UIElement.OpacityProperty, daopacity); } } } private T EnsureRenderTransform<T>(UIElement uiTarget) where T : Transform { if (uiTarget.RenderTransform is T) return uiTarget.RenderTransform as T; else { T instance = typeof(T).Assembly.CreateInstance(typeof(T).FullName) as T; uiTarget.RenderTransform = instance; return instance; } } public string Title { get { return (string)GetValue(TitleProperty); } set { SetValue(TitleProperty, value); } } // Using a DependencyProperty as the backing store for Title. This enables animation, styling, binding, etc... public static readonly DependencyProperty TitleProperty = DependencyProperty.Register("Title", typeof(string), typeof(FormItem), new PropertyMetadata("")); public ImageSource Icon { get { return (ImageSource)GetValue(IconProperty); } set { SetValue(IconProperty, value); } } // Using a DependencyProperty as the backing store for Icon. This enables animation, styling, binding, etc... public static readonly DependencyProperty IconProperty = DependencyProperty.Register("Icon", typeof(ImageSource), typeof(FormItem), new PropertyMetadata(null)); } }
这里在FormItem的构造函数中，添加了一个IsVisibleChanged事件，这个事件会在加载当前控件的时候发生，另外当当前控件的属性值发生变化的时候会触发该效果。其实效果就是同时在X和Y方向做一个平移的效果，这个也是一个常用的效果。
我们重点讨论的是下面的这段代码：
private T EnsureRenderTransform<T>(UIElement uiTarget)
where T : Transform
{
if (uiTarget.RenderTransform is T)
return uiTarget.RenderTransform as T;
else
{
T instance = typeof(T).Assembly.CreateInstance(typeof(T).FullName) as T;
uiTarget.RenderTransform = instance;
return instance;
}
}
这里我们创建TranslateTransform的时候是使用的System.Windows.Media.TranslateTransform transform = EnsureRenderTransform<System.Windows.Media.TranslateTransform>(this);这个方法，而不是每次都new一个对象，每次new一个对象的效率是很低的，而且会占据内存，我们如果已经创建过当前对象完全可以重复利用，这里我们使用了带泛型参数的函数来实现当前效果，typeof(T).Assembly.CreateInstance(typeof(T).FullName) as T，核心是通过程序集来创建对象，这种方式我们也是经常会使用的，比如我们可以通过获取应用程序级别的程序集来通过Activator.CreateInstance来创建窗体等一系列的对象，这种通过反射的机制来扩展的方法是我们需要特别留意的，另外写代码的时候必须注重代码的质量和效率，而不仅仅是实现了某一个功能，这个在以后的开发过程中再一点点去积累去吸收。