附加属性和Behavior

在WPF中，使用附加属性和Behavior都可以给对象附加一些功能。“附加”的好处是不会影响原对象，二者完全隔离，可以独自修改。

下面的示例是“令文本框只能输入数字”，如果要令输入框文本框只能输入数字，容易想到在TextChanged事件中处理。下面两种方式中都有方法ValidateChanged，作用就是限定位数字。

注意

使用Behavior，需要引用System.Windows.Interactivity命名空间。在XAML中使用时，必须另起行，用<i:Interaction.Behaviors>标签包括起来。

附加属性的Snippet是propa，附加属性就像 Grid.Row="1"这样可以直接写在对象标签内。

Behavior的实现方式

public class TextBoxValidateBehavior : Behavior<TextBox>
{
    public static readonly DependencyProperty NumberOnlyProperty =
        DependencyProperty.Register("NumberOnly", typeof(bool), typeof(TextBoxValidateBehavior),
            new PropertyMetadata(false));

    /// <summary>
    /// 是否只允许数字
    /// </summary>
    public bool NumberOnly
    {
        get { return (bool)GetValue(NumberOnlyProperty); }
        set { SetValue(NumberOnlyProperty, value); }
    }

    /// <summary>
    /// 允许的最大数值（NumberOnyl设为False时无效）
    /// </summary>
    public int MaxNumberValue
    {
        get { return (int)GetValue(MaxNumberValueProperty); }
        set { SetValue(MaxNumberValueProperty, value); }
    }

    public static readonly DependencyProperty MaxNumberValueProperty =
        DependencyProperty.Register("MaxNumberValue", typeof(int), typeof(TextBoxValidateBehavior), new PropertyMetadata(Int32.MaxValue));

    protected override void OnAttached()
    {
        base.OnAttached();

        if (AssociatedObject != null)
        {
            AssociatedObject.TextChanged += ValidateChanged;
        }
    }

    private void ValidateChanged(object sender, TextChangedEventArgs e)
    {
        if (NumberOnly)
        {
            //屏蔽中文输入和非法字符粘贴输入
            var textBox = sender as TextBox;
            if (textBox == null) return;
            var change = new TextChange[e.Changes.Count];
            e.Changes.CopyTo(change, 0);

            var offset = change[0].Offset;
            if (change[0].AddedLength <= 0) return;
            int num;
            if (Int32.TryParse(textBox.Text, out num) && num < MaxNumberValue) return;
            textBox.Text = textBox.Text.Remove(offset, change[0].AddedLength);
            textBox.Select(offset, 0);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (AssociatedObject != null)
        {
            AssociatedObject.TextChanged -= ValidateChanged;
        }
    }
}

使用Behavior，需要继承自Behavior<T>对象，此处因为只对TextBox处理，所以T为TextBox。关键的两个重写方法OnAttached和OnDetaching，分别添加、移除事件处理函数。

<TextBox Width="200" Height="25">
    <i:Interaction.Behaviors>
        <td:TextBoxValidateBehavior NumberOnly="True" MaxNumberValue="1000"/>
    </i:Interaction.Behaviors>
</TextBox>

附加属性的实现方式

public class TextBoxValidateAction
{
    public static bool GetNumberOnly(DependencyObject obj)
    {
        return (bool)obj.GetValue(NumberOnlyProperty);
    }

    public static void SetNumberOnly(DependencyObject obj, bool value)
    {
        obj.SetValue(NumberOnlyProperty, value);
    }

    public static readonly DependencyProperty NumberOnlyProperty =
        DependencyProperty.RegisterAttached("NumberOnly", typeof(bool), typeof(TextBoxValidateAction), new PropertyMetadata(false, OnNumberOnlyChanged));

    private static void OnNumberOnlyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        var textbox = obj as TextBox;
        if (textbox == null) return;

        textbox.TextChanged += ValidateChanged;
    }

    private static void ValidateChanged(object sender, TextChangedEventArgs e)
    {
        var numberOnly = GetNumberOnly((DependencyObject)sender);
        if (numberOnly)
        {
            //屏蔽中文输入和非法字符粘贴输入
            var textBox = sender as TextBox;
            if (textBox == null) return;
            var change = new TextChange[e.Changes.Count];
            e.Changes.CopyTo(change, 0);

            var offset = change[0].Offset;
            if (change[0].AddedLength <= 0) return;
            int num;
            if (Int32.TryParse(textBox.Text, out num) && num < 1000) return;
            textBox.Text = textBox.Text.Remove(offset, change[0].AddedLength);
            textBox.Select(offset, 0);
        }
    }

}

使用附加属性，必须针对所有属性设置更新回调函数，附加属性的原理就是在更新回调函数中附加事件处理方法。

<TextBox Width="200" Height="25" td:TextBoxValidateAction.NumberOnly="True"/>

附加属性和Behavior