WPF---依赖属性（二）

一、概要

我们将会通过一个简单的综合例子来阐述下依赖属性的变化。

场景：我们在一个文本框中输入一个数字，然后对应的panel中会出现对应的椭圆，椭圆的个数与输入的文本相同。

我们在MainWindow中定义一个名字叫CountProperty的依赖属性，该属性关联到一个回调方法OnCountChanged，当依赖属性发生变化的时候，会触发该方法。

在文本框的事件处理程序来动态修改依赖属性的值，使回调方法OnCountChanged触发，然后在这个函数中画出椭圆。

参考代码如下：

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Shapes;

namespace DependencyDemo
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        public int Count
        {
            get { return (int)GetValue(CountProperty); }
            set { SetValue(CountProperty, value); }
        }

        // Using a DependencyProperty as the backing store for Count.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty CountProperty =
            DependencyProperty.Register("Count", typeof(int), typeof(MainWindow), new FrameworkPropertyMetadata(new PropertyChangedCallback(OnCountChanged)));

       public static void OnCountChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            MainWindow win = d as MainWindow;
            win.wrapPanel?.Children.Clear();
            int myCount = (int)e.NewValue;
            for (int i = 0; i < myCount; i++)
            {
                win.wrapPanel?.Children.Add(new Ellipse() { Height = 20, Width = 20, Stroke = Brushes.Red, Margin = new Thickness(1) });
            }
        }

        private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            int myCount = 0;
            bool isSuccess = int.TryParse(this.tbInput.Text, out myCount);
            if(isSuccess && myCount > 0)
            {
                Count = myCount;
            }
        }
    }
}

<Window x:Class="DependencyDemo.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:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:DependencyDemo"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="315">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="21*"/>
            <RowDefinition Height="139*"/>
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal">
            <Label Content="椭圆个数：" FontSize="16"></Label>
            <TextBox Width="200" Margin="5" TextChanged="TextBox_TextChanged" Name="tbInput"></TextBox>
        </StackPanel>
        <ScrollViewer  Grid.Row="1">
            <WrapPanel Name="wrapPanel" >
            </WrapPanel>
        </ScrollViewer>
    </Grid>
</Window>

运行结果如下：