如何在 Silverlight 4 中使用 TextBox.Watermark？答案

【问题标题】：How to use TextBox.Watermark in Silverlight 4?如何在 Silverlight 4 中使用 TextBox.Watermark？
【发布时间】：2012-04-03 12:11:26
【问题描述】：

在浏览 MSDN 文档时，您可能会遇到这个 gem：TextBox.Watermark。

“太棒了！我一直想要一种在我的文本框上添加水印的内置方法！这太棒了，让我继续在 XAML 中设置它！”

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

不幸的是，如果你运行它，你将不会得到你所期望的：

还有细节：

这是什么？好吧，仔细看看 MSDN 文档：

没错。 Silverlight 4 支持它，但它也说“不要在 Silverlight 4 应用程序中使用”。如果你使用它，你会收到一个 System.NotImplemented 异常。为了验证，下面是通过 Reflector 反编译的属性代码：

[EditorBrowsable(EditorBrowsableState.Never)]
public object Watermark
{
get
{
StubHelper.ThrowIfNotInDesignMode();
return base.GetValue(WatermarkProperty);
}
set
{
StubHelper.ThrowIfNotInDesignMode();
base.SetValue(WatermarkProperty, value);
}
}

就是这样——只要不在设计模式下，它就会抛出异常。这没有意义吧？微软为什么要这样做？

不幸的是，我还没有找到任何明确的答案，但是如果我不得不猜测这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，所以第三方控件创建者不会继承 TextBox 并创建自己的 Watermark 属性。我知道至少有一个控件供应商 ComponentOne，它有一个从 TextBox 继承并提供 Watermark 属性的控件。在我看来，这似乎是 Microsoft 阻止人们在自己的 TextBox 子类中使用此属性名称的方式。

【问题讨论】：

标签： .net silverlight-4.0

【解决方案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这个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>

【讨论】：

+1 非常好的解决方案。顺便说一句，在 Silverlight 5 中运行良好。有趣的是，SL5 MSDN 文档仍然有关于 Watermark 属性的相同冲突信息。
还请注意，我稍微调整了代码，主要是为了解决未设置 Watermark 属性时的设计时错误。更新的代码添加为下面的新答案。
您已在构造函数中注册了事件，但在卸载时您已从它们中注销（这很好），但这会给选项卡控件带来问题，选项卡控件将卸载其内容并在适当的选项卡时加载被按下，所以也许你应该在 Loaded 事件而不是构造函数中注册事件。

【解决方案2】：

如果未设置 Watermark 属性，我稍微修改了@mani kandan 的解决方案以修复设计时错误。还添加了一个 HasValue 布尔属性，以便能够轻松检查用户是否在 TextBox 中输入了文本，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);
        }
    }

}

【讨论】：

【解决方案3】：

根据行为检查这个

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>

【讨论】：

【解决方案4】：

你可以在 Silverlight 5 中成功使用它

试试这个链接： TextBox.Watermark

我在 Silverlight 5 MVVM 应用程序中成功使用了 WatermarkTextBox。

【讨论】：

【解决方案5】：

创建一个类库项目。添加类文件使用以下代码

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"/>

【讨论】：