Silverlight渐进学习系列(2)——关于依赖属性

原理

1. Silverlight和WPF一样都包含依赖属性，它是通过 DependencyProperty类来实现的。

CLR属性和依赖属性还是有本质的差别，什么是CLR属性？很简单，大家都用过，比如：

public class CLRButton
{
    public string Name { get; set; }

    public double FontSize { get; set; }

    public string Background { get; set; }

    //其他属性...
}

传统的属性成员写法。那依赖属性会是如何书写，截取一段Control（实际上也是Button的基类）的代码：

代码

public abstract class Control : FrameworkElement
{
    public static readonly DependencyProperty FontWeightProperty = DependencyProperty.RegisterCoreProperty(0x80003717, typeof(FontWeight));
    public static readonly DependencyProperty FontSizeProperty = DependencyProperty.RegisterCoreProperty(0x80003714, typeof(double));
    public double FontSize
    {
        get
        {
            return (double)base.GetValue(FontSizeProperty);
        }
        set
        {
            base.SetValue(FontSizeProperty, value);
        }
    }
    public FontWeight FontWeight
    {
        get
        {
            return (FontWeight)base.GetValue(FontWeightProperty);
        }
        set
        {
            base.SetValue(FontWeightProperty, value);
        }
    }
}

其中GetValue和SetValue方法调用DependencyObject基类的方法。

比起CLR属性，这样有什么好处？如果CLR属性类，包含相当多的属性成员，势必将造成每个实例占用大量的内存，因为每个属性成员都要分配一块内存空间。这依赖属性就没有这个问题，因为它通过GetValue和SetValue来使用，因为DependencyObject维护着一套高效的存储系统。

2. Control对于DependencyProperty的使用，针对内置依赖属性的注册，使用了RegisterCoreProperty方法；而如果需要实现自己的依赖属性，那么就需要使用Register方法，那么接下来看下如何使用Register方法：

这个实例实现一个焦点高亮显示的文本框，那么直接继承 TextBox：

声明两个DependencyProperty属性：

代码

private static readonly DependencyProperty FocusBackgroundProperty;
private static readonly DependencyProperty BlurBackgroundProperty;

/// <summary>
/// 获得焦点时的文本框背景色
/// </summary>
public Brush FocusBackground
{
     set
     {
         SetValue (FocusBackgroundProperty, value);
     }
     get
     {
         return (Brush)GetValue(FocusBackgroundProperty);
     }
}

/// <summary>
/// 失去焦点时的文本框背景色
/// </summary>
public Brush BlurBackground
{
     set
     {
         SetValue (BlurBackgroundProperty, value);
     }
     get
     {
         return (Brush)GetValue(BlurBackgroundProperty);
     }
}

那么需要在静态构造函数来注册这两个依赖属性：

代码

//注册FocusBackgroundProperty依赖属性
FocusBackgroundProperty = DependencyProperty.Register(
    "FocusBackground", typeof(Brush), typeof(HightLightTextBox),
    new PropertyMetadata(new SolidColorBrush(Colors.Yellow), (obj, args) =>
    {
        TextBox txt = obj as TextBox;
        txt.GotFocus += new RoutedEventHandler((sender, e) =>
        {
            txt.Background = (Brush)args.NewValue;
        });
    }));

//注册BlurBackgroundProperty依赖属性
BlurBackgroundProperty = DependencyProperty.Register(
    "BlurBackground", typeof(Brush), typeof(HightLightTextBox),
    new PropertyMetadata(new SolidColorBrush(Colors.White), (obj, args) =>
    {
        TextBox txt = obj as TextBox;
        txt.LostFocus += new RoutedEventHandler((sender, e) =>
        {
            txt.Background = (Brush)args.NewValue;
        });
    }));

可以发现两个都执行了Register方法，它的声明为：

public static DependencyProperty Register(string name, Type propertyType, Type ownerType, PropertyMetadata typeMetadata);

第一个对应属性的名称，第二个对应属性的类型，第三个就是执行该依赖属性的载体，第四个为对属性赋值时，将执行的动作；

那么两个方法分别调用了TextBox中的获取焦点以及失去焦点时执行的事件，并且设置了背景色。

该高亮文本框完整代码如下：

HighLightTextBox代码

public class HightLightTextBox : TextBox
{
    private static readonly DependencyProperty FocusBackgroundProperty;
    private static readonly DependencyProperty BlurBackgroundProperty;

    static HightLightTextBox()
    {
        //注册FocusBackgroundProperty依赖属性
        FocusBackgroundProperty = DependencyProperty.Register(
            "FocusBackground", typeof(Brush), typeof(HightLightTextBox),
            new PropertyMetadata(new SolidColorBrush(Colors.Yellow), (obj, args) =>
            {
                TextBox txt = obj as TextBox;
                txt.GotFocus += new RoutedEventHandler((sender, e) =>
                {
                    txt.Background = (Brush)args.NewValue;
                });
            }));

        //注册BlurBackgroundProperty依赖属性
        BlurBackgroundProperty = DependencyProperty.Register(
            "BlurBackground", typeof(Brush), typeof(HightLightTextBox),
            new PropertyMetadata(new SolidColorBrush(Colors.White), (obj, args) =>
            {
                TextBox txt = obj as TextBox;
                txt.LostFocus += new RoutedEventHandler((sender, e) =>
                {
                    txt.Background = (Brush)args.NewValue;
                });
            }));
    }

    /// <summary>
    /// 获得焦点时的文本框背景色
    /// </summary>
    public Brush FocusBackground
    {
        set
        {
            SetValue(FocusBackgroundProperty, value);
        }
        get
        {
            return (Brush)GetValue(FocusBackgroundProperty);
        }
    }

    /// <summary>
    /// 失去焦点时的文本框背景色
    /// </summary>
    public Brush BlurBackground
    {
        set
        {
            SetValue(BlurBackgroundProperty, value);
        }
        get
        {
            return (Brush)GetValue(BlurBackgroundProperty);
        }
    }
}

接着，编写xaml用户控件的代码：

代码