数据绑定（十）Binding的数据转换

当Source端Path所关联的数据与Target端目标属性数据类型不一致时，需要添加数据转换器，数据转换器是一个自定义的类，这个类需要实现IValueConverter接口，这个接口有两个方法需要实现：Convert和ConvertBack，当数据从Source流向Target时，将调用Convert方法，反之，将调用ConvertBack方法

例子，首先定义飞机类型

    public enum Category
    {
        Bomber,
        Fighter
    }

    public enum State
    {
        Available,
        Locked,
        Unknown
    }

    public class Plane
    {
        public Category Category { get; set; }
        public string Name { get; set; }
        public State State { get; set; }
    }

Plane类型的Category将在界面上转换为图片，而State类型将会转换成界面上的CheckBox显示，由于存在两个转换，因此需要提供两个Converter，第一个转换是做Category类型与字符类型的转换，字符串是图片的路径

    class CategoryToSourceConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            Category c = (Category)value;
            switch (c)
            {
                case Category.Bomber:
                    {
                        return @"\Icons\close.png";
                    }
                case Category.Fighter:
                    {
                        return @"\Icons\closeing.png";
                    }
                default:
                    {
                        return null;
                    }
            }
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return new NotImplementedException();
        }
    }

由于UI上不能修改图片，所以只实现了从Source到Target的转换

另一个转换用于将State数据转换为bool?

    public class StateToNullableBoolConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            State s = (State)value;
            switch (s)
            {
                case State.Locked:
                    {
                        return false;
                    }
                case State.Available:
                    {
                        return true;
                    }
                case State.Unknown:
                default:
                    {
                        return null;
                    }
            }
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            bool? nb = (bool?)value;
            switch (nb)
            {
                case true:
                    {
                        return State.Available;
                    }
                case false:
                    {
                        return State.Locked;
                    }
                case null:
                default:
                    {
                        return State.Unknown;
                    }
            }
        }
    }

界面代码：

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="275" Width="275">
    <Window.Resources>
        <local:CategoryToSourceConverter x:Key="cts" />
        <local:StateToNullableBoolConverter x:Key="stnb" />
    </Window.Resources>
    <StackPanel Background="LightBlue">
        <ListBox x:Name="listBoxPlane" Height="160" Margin="5">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <Image Width="20" Height="20" Source="{Binding Path=Category, Converter={StaticResource cts}}" />
                        <TextBlock Text="{Binding Path=Name}" Width="60" Margin="80,0" />
                        <CheckBox IsThreeState="True" IsChecked="{Binding Path=State, Converter={StaticResource stnb}}" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Button x:Name="buttonLoad" Content="Load" Height="25" Margin="5,0" Click="buttonLoad_Click" />
        <Button x:Name="buttonSave" Content="Save" Height="25" Margin="5,0" Click="buttonSave_Click" />
    </StackPanel>
</Window>

加粗的部分是XAML中对转换器的使用，后台代码中实现了Load和Save两个按钮的点击事件

        private void buttonLoad_Click(object sender, RoutedEventArgs e)
        {
            List<Plane> planeList = new List<Plane>()
            {
                new Plane(){Category=Category.Bomber, Name="B-1", State=State.Unknown},
                new Plane(){Category=Category.Bomber, Name="B-2", State=State.Unknown},
                new Plane(){Category=Category.Fighter, Name="F-22", State=State.Unknown},
                new Plane(){Category=Category.Fighter, Name="Su-47", State=State.Unknown},
                new Plane(){Category=Category.Bomber, Name="B-52", State=State.Unknown},
                new Plane(){Category=Category.Fighter, Name="J-10", State=State.Unknown}
            };

            listBoxPlane.ItemsSource = planeList;
        }

        private void buttonSave_Click(object sender, RoutedEventArgs e)
        {
            StringBuilder sb = new StringBuilder();
            foreach (Plane p in listBoxPlane.Items)
            {
                sb.AppendLine(string.Format("Category={0}, Name={1}, State={2}", p.Category, p.Name, p.State));
            }
            File.WriteAllText(@"d:\PlaneList.txt", sb.ToString());
        }

界面效果如图：

当改变checkbox的选中状态时，Plane对象中的值会发生变化