一个加载时带动画效果的ListBoxItem

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 = ;
            Opacity = ;

            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 = ,
                        To = ,
                        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 = ,
                        To = ,
                    };
                    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 = ,
                        To = ,
                        BeginTime = TimeSpan.FromMilliseconds( * (index + )),
                        Duration = TimeSpan.FromMilliseconds(),
                        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 = ,
                        To = ,
                        BeginTime = TimeSpan.FromMilliseconds( * (index + )),
                        Duration = TimeSpan.FromMilliseconds(),
                        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。