原文来自：wp教程网
原理：在失去焦点和获取焦点的时候，判断Text值是否为空或者是否与水印值相同，然后修改TextBox中的Text和Foreground。
代码如下：
/* ==============================================================================
2 * 类名称：WatermarkTextBox
3 * 类描述：
4 * 创建人：neoyee
5 * 创建时间：2014/2/25 17:24:11
6 * 修改人：
7 * 修改时间：
8 * 修改备注：
9 * @version 1.0
10 * ==============================================================================*/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using Windows.UI;
namespace WP8.Controls
{
public sealed class WatermarkTextBox : TextBox
{
private static readonly DependencyProperty WatermarkTextProperty =
DependencyProperty.Register("WatermarkText", typeof(string), typeof(WatermarkTextBox), new PropertyMetadata(string.Empty, new PropertyChangedCallback(WatermarkTextChanged)));
private static readonly DependencyProperty WatermarkForegroundProperty =
DependencyProperty.Register("WatermarkForeground", typeof(SolidColorBrush), typeof(WatermarkTextBox), new PropertyMetadata(new SolidColorBrush(Colors.Black)));
private static readonly DependencyProperty WatermarkBackgroundProperty =
DependencyProperty.Register("WatermarkBackground", typeof(SolidColorBrush), typeof(WatermarkTextBox), new PropertyMetadata(new SolidColorBrush(Colors.White)));
private static readonly DependencyProperty NormalForegroundProperty =
DependencyProperty.Register("NormalForeground", typeof(SolidColorBrush), typeof(WatermarkTextBox), new PropertyMetadata(new SolidColorBrush(Colors.Black), NormalForegroundPropertyChanged));
private static readonly DependencyProperty NormalBackgroundProperty =
DependencyProperty.Register("NormalBackground", typeof(SolidColorBrush), typeof(WatermarkTextBox), new PropertyMetadata(new SolidColorBrush(Colors.White)));
private static void NormalForegroundPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
var watermarkTextBox = obj as WatermarkTextBox;
if (watermarkTextBox != null)
watermarkTextBox.NormalForegroundChanged((SolidColorBrush)args.NewValue);
}
private void NormalForegroundChanged(SolidColorBrush value)
{
Foreground = value;
}
public SolidColorBrush NormalBackground
{
get { return (SolidColorBrush)GetValue(NormalBackgroundProperty); }
set { SetValue(NormalBackgroundProperty, value); }
}
public SolidColorBrush NormalForeground
{
get { return (SolidColorBrush)GetValue(NormalForegroundProperty); }
set { SetValue(NormalForegroundProperty, value); }
}
public SolidColorBrush WatermarkBackground
{
get { return (SolidColorBrush)GetValue(WatermarkBackgroundProperty); }
set { SetValue(WatermarkBackgroundProperty, value); }
}
public SolidColorBrush WatermarkForeground
{
get { return (SolidColorBrush)GetValue(WatermarkForegroundProperty); }
set { SetValue(WatermarkForegroundProperty, value); }
}
public string WatermarkText
{
get { return (string)GetValue(WatermarkTextProperty); }
set { SetValue(WatermarkTextProperty, value); }
}
public WatermarkTextBox()
{
this.LostFocus += WatermarkTextBox_LostFocus;
this.GotFocus += WatermarkTextBox_GotFocus;
this.TextChanged += WatermarkTextBox_TextChanged;
if (string.IsNullOrEmpty(this.Text))
{
this.Text = WatermarkText;
Foreground = WatermarkForeground;
}
}
void WatermarkTextBox_TextChanged(object sender, TextChangedEventArgs e)
{
if (Text == WatermarkText)
{
this.Text = WatermarkText;
Foreground = WatermarkForeground;
}
else if (Text != WatermarkText)
{
Foreground = NormalForeground;
}
}
private static void WatermarkTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
((WatermarkTextBox)obj).WatermarkTextChanged(args.OldValue, args.NewValue);
}
private void WatermarkTextChanged(object OldValue, object NewValue)
{
}
void WatermarkTextBox_GotFocus(object sender, RoutedEventArgs e)
{
if (this.Text == WatermarkText && Foreground == WatermarkForeground)
{
this.Text = string.Empty;
Foreground = NormalForeground;
}
}
void WatermarkTextBox_LostFocus(object sender, RoutedEventArgs e)
{
if (string.IsNullOrEmpty(this.Text) || Text == WatermarkText)
{
this.Text = WatermarkText;
Foreground = WatermarkForeground;
}
}
}
}
详细说明：http://wp.662p.com/thread-8105-1-1.html
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