如何在Silverlight 4中使用TextBox.Watermark？

Question

当浏览MSDN文档时，您可能会遇到这个宝石：TextBox.Watermark。

“太棒了！我一直想要一个内置的方法来在文本框上加水印！这很好，让我去设置XAML！”

<TextBox Watermark="This is my watermark" Margin="20"></TextBox>

很不幸，如果你运行这个代码，你将得到一个出乎意料的结果：

具体细节如下：

这是什么？好的，仔细看一下MSDN文档： [EditorBrowsable(EditorBrowsableState.Never)] public object Watermark { get { StubHelper.ThrowIfNotInDesignMode(); return base.GetValue(WatermarkProperty); } set { StubHelper.ThrowIfNotInDesignMode(); base.SetValue(WatermarkProperty, value); } }

这里有一个问题 - 只要不在设计模式下，它就会抛出异常。这没有意义，对吧？为什么微软会这样做呢？

很遗憾，我还没有找到明确的答案，但如果我猜的话，可能是因为微软计划在未来版本（可能是v5）中在TextBox控件上实现水印行为，并希望有效地保留此属性，以便第三方控件创建者不会子类化TextBox并创建自己的Watermark属性。 我知道至少有一个控件供应商ComponentOne，他们的控件继承自TextBox并提供Watermark属性。 对我来说，这似乎是微软阻止人们在自己的TextBox子类上使用此属性名称的方法。

- Unknown

Answer 1

创建一个类库项目。添加类文件，使用以下代码......之后在您的项目中添加此dll。

public class WatermarkTextBox : TextBox 
{ 
    private bool displayWatermark = true; 
    private bool hasFocus = false; 
     public WatermarkTextBox() 
    { 
        this.GotFocus += new RoutedEventHandler(WatermarkTextBox_GotFocus); 
        this.LostFocus += new RoutedEventHandler(WatermarkTextBox_LostFocus); 
        this.TextChanged += new TextChangedEventHandler(WatermarkTextBox_TextChanged); 
        this.Unloaded += new RoutedEventHandler(WatermarkTextBox_Unloaded); 
    } 

    private void WatermarkTextBox_TextChanged(object sender, TextChangedEventArgs e) 
    { 
        if (!hasFocus && Text == "") 
        { 
            setMode(true); 
            displayWatermark = true; 
            this.Text = Watermark; 
        } 
    } 

    private void WatermarkTextBox_Unloaded(object sender, RoutedEventArgs e) 
    { 
        this.GotFocus -= WatermarkTextBox_GotFocus; 
        this.LostFocus -= WatermarkTextBox_LostFocus; 
        this.Unloaded -= WatermarkTextBox_Unloaded; 
        this.TextChanged -= WatermarkTextBox_TextChanged; 
    } 

    private void WatermarkTextBox_GotFocus(object sender, RoutedEventArgs e) 
    { 
        hasFocus = true; 
        if (displayWatermark) 
        { 
            setMode(false); 
            this.Text = ""; 
        } 
    } 
    private void WatermarkTextBox_LostFocus(object sender, RoutedEventArgs e) 
    { 
        hasFocus = false; 
        if (this.Text == "") 
        { 
            displayWatermark = true; 
            setMode(true); 
            this.Text = Watermark; 
        } 
        else 
        { 
            displayWatermark = false; 
        } 
    } 
    private void setMode(bool watermarkStyle) 
    { 
        if (watermarkStyle) 
        { 
            this.FontStyle = FontStyles.Italic; 
            this.Foreground = new SolidColorBrush(Colors.Gray); 
        } 
        else 
        { 
            this.FontStyle = FontStyles.Normal; 
            this.Foreground = new SolidColorBrush(Colors.Black); 
        } 
    } 
    public new string Watermark 
    { 
        get { return GetValue(WatermarkProperty) as string; } 
        set { SetValue(WatermarkProperty, value); } 
    } 
    public static new readonly DependencyProperty WatermarkProperty = 
        DependencyProperty.Register("Watermark", typeof(string), typeof(WatermarkTextBox), new PropertyMetadata(watermarkPropertyChanged)); 
    private static void watermarkPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e) 
    { 
        WatermarkTextBox textBox = obj as WatermarkTextBox; 
        if (textBox.displayWatermark) 
        { 
            textBox.Text = e.NewValue.ToString(); 
            textBox.setMode(true); 
        } 
    } 

XAML：

  xmlns:watertext="clr-namespace:SilverlightClassLibrary1;assembly=SilverlightClassLibrary1"


    <watertext:WatermarkTextBox Watermark="WElcome" Margin="150,115,120,166"></watertext:WatermarkTextBox>

- mani kandan

---

非常好的解决方案，点赞！顺便说一句，在Silverlight 5中运行良好。有趣的是，SL5 MSDN文档仍然存在关于Watermark属性的相互矛盾的信息。 - Dave Marley

请注意，我稍微调整了代码，主要是为了解决在未设置水印属性时的设计时错误。更新后的代码已作为新答案添加如下。 - Dave Marley

你已经在构造函数中注册了事件，但在卸载事件中又取消了注册（这样做是可以的），但这会导致选项卡控件出现问题。选项卡控件将在适当的标签被点击时卸载其内容并重新加载，因此也许你应该在Loaded事件中注册事件，而不是在构造函数中。 - Sonosar

Answer 2

看看这个基于行为的案例

namespace MyNamespace
{
    public class WatermarkBehavior : Behavior<TextBox>
    {
        public String Watermark
        {
            get { return this.GetValue(WatermarkProperty) as String; }
            set { this.SetValue(WatermarkProperty, value); }
        }

        public static readonly DependencyProperty WatermarkProperty = DependencyProperty.Register("Watermark", typeof(String), typeof(WatermarkBehavior), new PropertyMetadata("", new PropertyChangedCallback(OnWatermarkChanged)));

        private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var behavior = d as WatermarkBehavior;
            if (!String.IsNullOrWhiteSpace(e.NewValue as String))
            {
                behavior.SetWatermarkIfNeeded();
            }
        }

        protected override void OnAttached()
        {
            base.OnAttached();
            this.AssociatedObject.GotFocus += GotFocus;
            this.AssociatedObject.LostFocus += LostFocus;
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();
            this.AssociatedObject.GotFocus -= GotFocus;
            this.AssociatedObject.LostFocus -= LostFocus;
        }

        private void GotFocus(object sender, RoutedEventArgs e)
        {
            if (this.AssociatedObject.Text == this.Watermark)
            {
                this.AssociatedObject.Text = String.Empty;
                this.AssociatedObject.Foreground = new SolidColorBrush(Colors.Black);
            }
        }

        private void LostFocus(object sender, RoutedEventArgs e)
        {
            this.SetWatermarkIfNeeded();
        }

        private void SetWatermarkIfNeeded()
        {
            if (String.IsNullOrWhiteSpace(this.AssociatedObject.Text))
            {
                this.SetWatermark();
            }
        }

        private void SetWatermark()
        {
            this.AssociatedObject.Text = this.Watermark;
            this.AssociatedObject.Foreground = new SolidColorBrush(Colors.Gray);
        }
    }
}

XAML

<UserControl x:Class="GreenField.Targeting.Controls.BasketTargets.EditorView"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
  xmlns:local="clr-namespace:MyNamespace"
  xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
  mc:Ignorable="d" d:DesignHeight="300" d:DesignWidth="400">
  <TextBox Text="{Binding Text}">
    <i:Interaction.Behaviors>
      <local:WatermarkBehavior Watermark="{Binding Watermark}" />
    </i:Interaction.Behaviors>
  </TextBox>
</UserControl>

- Trident D'Gao

Answer 3

我稍微修改了 @mani kandan 的解决方案，以修复在未设置水印属性时出现的设计时错误。还添加了一个 HasValue 布尔属性，以便轻松检查用户是否在 TextBox 中输入了文本，并最后更改为将所有空白条目视为非条目（即继续显示水印）。

修改后的代码：

public class WatermarkTextBox : TextBox
{

    private bool displayWatermark = true;
    private bool hasFocus = false;

    public WatermarkTextBox()
    {
        this.GotFocus += new RoutedEventHandler(WatermarkTextBox_GotFocus);
        this.LostFocus += new RoutedEventHandler(WatermarkTextBox_LostFocus);
        this.TextChanged += new TextChangedEventHandler(WatermarkTextBox_TextChanged);
        this.Unloaded += new RoutedEventHandler(WatermarkTextBox_Unloaded);
    }

    private void WatermarkTextBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        if (!hasFocus && string.IsNullOrWhiteSpace(this.Text))
        {
            setMode(true);
            displayWatermark = true;
            // avoid design-time error if Watermark not specified
            this.Text = (Watermark == null ? string.Empty : Watermark);
        }
    }

    private void WatermarkTextBox_Unloaded(object sender, RoutedEventArgs e)
    {
        this.GotFocus -= WatermarkTextBox_GotFocus;
        this.LostFocus -= WatermarkTextBox_LostFocus;
        this.Unloaded -= WatermarkTextBox_Unloaded;
        this.TextChanged -= WatermarkTextBox_TextChanged;
    }

    private void WatermarkTextBox_GotFocus(object sender, RoutedEventArgs e)
    {
        hasFocus = true;
        if (displayWatermark)
        {
            setMode(false);
            this.Text = "";
        }
    }

    private void WatermarkTextBox_LostFocus(object sender, RoutedEventArgs e)
    {
        hasFocus = false;
        if (string.IsNullOrWhiteSpace(this.Text))
        {
            displayWatermark = true;
            setMode(true);
            this.Text = (Watermark == null ? string.Empty : Watermark);
        }
        else
        {
            displayWatermark = false;
        }
    }

    private void setMode(bool watermarkStyle)
    {
        if (watermarkStyle)
        {
            this.FontStyle = FontStyles.Italic;
            this.Foreground = new SolidColorBrush(Colors.Gray);
        }
        else
        {
            this.FontStyle = FontStyles.Normal;
            this.Foreground = new SolidColorBrush(Colors.Black);
        }
    }

    public new string Watermark
    {
        get { return GetValue(WatermarkProperty) as string; }
        set { SetValue(WatermarkProperty, value); }
    }

    public static new readonly DependencyProperty WatermarkProperty =
        DependencyProperty.Register("Watermark", typeof(string), typeof(WatermarkTextBox), new PropertyMetadata(watermarkPropertyChanged));
    private static void watermarkPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        WatermarkTextBox textBox = obj as WatermarkTextBox;
        if (textBox.displayWatermark)
        {
            textBox.Text =  e.NewValue.ToString();
            textBox.setMode(true);
        }
    }

    public bool HasValue
    {
        get 
        {
            // if watermark has been specified, then compare to text value to determine if text set by user,
            // otherwise check to see if empty or whitespace.
            if (this.Watermark != null)
                return this.Watermark != this.Text;
            else
                return !string.IsNullOrWhiteSpace(this.Text);
        }
    }

}

- Dave Marley

Answer 4

在Silverlight 5中，您可以成功使用它。

请尝试此链接： TextBox.Watermark

我正在一个Silverlight 5 MVVM应用程序中成功使用WatermarkTextBox。

- Unknown

Answer 5

创建一个类库项目。添加一个类文件并使用以下代码。

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace Project.Controls
{
    public class WatermarkEditBox : TextBox
    {

        TextBlock lbl = new TextBlock()
        {
            IsHitTestVisible = false,
            Foreground = new SolidColorBrush(Colors.LightGray),
            VerticalAlignment = VerticalAlignment.Center,
            HorizontalAlignment = HorizontalAlignment.Left,
            Margin = new Thickness(3,0,0,0)
        };
        public string WatermarkText { get { return lbl.Text; } set { lbl.Text = value; } }

        public WatermarkEditBox()
        {
            this.Loaded += WatermarkEditBox_Loaded;
        }

        void WatermarkEditBox_Loaded(object sender, RoutedEventArgs e)
        {
            this.UpdateLayout();
            Grid g = GetObjectOfType<Grid>(this, "RootElement");
            if (g != null)
            {
                g.Children.Add(lbl);
            }
            this.TextChanged += WatermarkEditBox_TextChanged;
        }

        void WatermarkEditBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (this.Text.Length == 0)
                lbl.Visibility = System.Windows.Visibility.Visible;
            else
                lbl.Visibility = System.Windows.Visibility.Collapsed;
        }

        public TObject GetObjectOfType<TObject>(DependencyObject parent, string name) where TObject : DependencyObject
        {
            int count = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < count; ++i)
            {
                DependencyObject child = VisualTreeHelper.GetChild(parent, i);
                if (child is TObject && child.GetValue(NameProperty).ToString() == name)
                {
                    return child as TObject;
                }
                else
                {
                    TObject obj = GetObjectOfType<TObject>(child, name);
                    if (obj != null)
                    {
                        return obj;
                    }
                }
            }

            return null;
        }

    }
}

XAML:

xmlns:Controls="clr-namespace:Project.Controls"

<Controls:WatermarkEditBox WatermarkText="фильтр" Width="100"/>

- user1737877