利用行为Behavior和依赖属性实现WPF TextBox的水印

Behavior：行为，是WPF中一种用于扩展控件的一种机制。

DependencyProperty：依赖属性，是WPF的一个核心概念，可用于向控件传递数据，数据绑定，数据校验等。

实现步骤：

1. 在工程中添加引用System.Windows.Interactivity，以支持behavior

2. 创建作用于TextBox上的Behavior类文件TextBoxWatermarkBehavior.cs，在其头部使用命名空间:

using System.Windows.Interactivity;

3. TextBoxWatermarkBehavior类的代码如下：

public class TextBoxWatermarkBehavior : Behavior<TextBox>
    {
        public static readonly DependencyProperty WatermarkProperty;
        static TextBoxWatermarkBehavior()
        {
            WatermarkProperty = DependencyProperty.Register("Watermark", typeof(string), typeof(TextBoxWatermarkBehavior), new PropertyMetadata(default(string)));
        }

        bool _hasContent = false;
        protected override void OnAttached()
        {
            base.OnAttached();
            var textBox = AssociatedObject;
            textBox.Loaded += (o, e) =>
            {
                if (!string.IsNullOrEmpty(textBox.Text))
                {
                    _hasContent = true;
                }
                else
                {
                    _hasContent = false;
                    if (!String.IsNullOrEmpty(Watermark))
                    {
                        textBox.Foreground = Brushes.Gray;
                        textBox.Text = Watermark;
                        textBox.GotFocus += TextBox_GotFocus;
                        textBox.LostFocus += TextBox_LostFocus;
                    }
                }
            };
        }

        private void TextBox_LostFocus(object sender, RoutedEventArgs e)
        {
            var textBox = sender as TextBox;
            if (string.IsNullOrEmpty(textBox.Text))
            {
                _hasContent = false;
                if (!string.IsNullOrEmpty(Watermark))
                {
                    textBox.Text = Watermark;
                    textBox.Foreground = Brushes.Gray;
                }
            }
            else
            {
                _hasContent = true;
            }
            
        }

        private void TextBox_GotFocus(object sender, RoutedEventArgs e)
        {
            var textBox = sender as TextBox;
            if (!string.IsNullOrEmpty(Watermark) && !_hasContent)
            {
                textBox.Text = "";
                textBox.Foreground = Brushes.Black;
            }
        }

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

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

测试Watermark, MainWindow.xaml:

<Window
    x:Class="WpfApp2.MainWindow"
    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:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:local="clr-namespace:WpfApp2"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="MainWindow"
    Width="300"
    Height="200"
    mc:Ignorable="d">
    <Grid Margin="20">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="60" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <TextBlock  Grid.Row="0" Grid.Column="0"  VerticalAlignment="Center">无水印</TextBlock>
        <TextBox  Grid.Row="0"  Grid.Column="1" Height="30" HorizontalAlignment="Stretch" />
        <TextBlock  Grid.Row="1"  Grid.Column="0"  VerticalAlignment="Center">有水印</TextBlock>
        <TextBox  Grid.Row="1" Grid.Column="1" Height="30">
            <i:Interaction.Behaviors>
                <local:TextBoxWatermarkBehavior Watermark="Please input" />
            </i:Interaction.Behaviors>
        </TextBox>
    </Grid>
</Window>

最终效果：

Animation1111.gif