场景：有一家公司(类名：Company)，它有N多员工(类名:Employee)。要在界面上用网格显示所有员工的姓名、工资，并且当操作用户在网格里对员工进行增减或修改其工资时，能自动汇总出员工工资的总和并显示出来。
员工类 Employee代码如下:
/// <summary>
/// 员工类
/// </summary>
public class Employee:INotifyPropertyChanged
{
private string _name = "";
public string Name { set { _name = value; OnPropertyChanged("Name"); } get { return _name; } }
private int _salary = 0;
public int Salary
{
get { return _salary; }
set { _salary = value; OnPropertyChanged("Salary"); }
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null) {
PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
}
}
}
公司类 Company代码原型如下:
public class Company {
private ObservableCollection<Employee> _employeeCollection = new ObservableCollection<Employee>();
/// <summary>
/// 公司的"员工集合"
/// </summary>
public ObservableCollection<Employee> EmployeeCollection
{
get { return _employeeCollection; }
}
private void computeSalaryTotal()
{
_salaryTotal = _employeeCollection.Sum(c => c.Salary);
}
private int _salaryTotal = 0;
/// <summary>
/// 工资汇总
/// </summary>
public int SalaryTotal
{
get
{
computeSalaryTotal();
return _salaryTotal;
}
}
}
常规解决办法：
可以在Grid每行“工资”字段对应的TextBox上，注册TextChanged或LostFocus事件，在输入值变化或失去焦点时，去更新总和。
这是很容易想到的办法，但是并不优雅，原因：
1、每行的TextBox上都要去绑定事件，并在xaml.cs上写代码处理类似 TextBoxTotal.text = company.SalaryTotal 的逻辑。这样界面逻辑代码与UI绑得太紧，应对变化的能力有限。比如以后将TextBox换成其它形式的控件，一旦并不支持TextChanged事件，原来的代码就得修改。
2、代码重用率低，如果其它界面上也需要类似的需求，只能把本页面Xaml、Xaml.cs的代码复制一遍，如果以后需求有变化，更增加了维护成本。
所以，理想的解决方法，应该是Company类自身能“智能感知”员工的变化，并自动更新工资汇总字段。（即：员工Employee的工资有变化时，应该主动通知Company类。这跟实际公司的运营管理也比较接近，人事给员工调整了工资，肯定会主动通知财务，所以财务肯定也就知道了最新的工资汇总数据。）
这时，双向绑定就再一次体现了这种威力，我们把Company类改造一下：
public class Company:INotifyPropertyChanged {
private ObservableCollection<Employee> _employeeCollection = new ObservableCollection<Employee>();
/// <summary>
/// 公司的"员工集合"
/// </summary>
public ObservableCollection<Employee> EmployeeCollection
{
get { return _employeeCollection; }
}
/// <summary>
/// 构造函数
/// </summary>
public Company()
{
_employeeCollection.CollectionChanged += new NotifyCollectionChangedEventHandler(_employeeCollection_CollectionChanged);
}
/// <summary>
/// 员工有“增减”时自动触发
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void _employeeCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
//重新计算工资总和
computeSalaryTotal();
//每个员工的“工资”属性变化时，自动触发指定事件
foreach (var item in _employeeCollection)
{
item.PropertyChanged -= new PropertyChangedEventHandler(item_PropertyChanged);
item.PropertyChanged += new PropertyChangedEventHandler(item_PropertyChanged);
}
}
/// <summary>
/// 员工属性变化时自动调用本方法
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
//如果是"工资"属性变化，则自动重新计算工资汇总
if (e.PropertyName == "Salary")
{
computeSalaryTotal();
}
}
private void computeSalaryTotal()
{
_salaryTotal = _employeeCollection.Sum(c => c.Salary);
OnPropertyChanged("SalaryTotal");//工资总合重新计算后，向外广播事件，以便UI能自动更新
}
private int _salaryTotal = 0;
/// <summary>
/// 工资汇总
/// </summary>
public int SalaryTotal
{
get
{
return _salaryTotal;
}
}
private void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
这里，我们充分利用了INotifyPropertyChanged接口的PropertyChanged事件，以及INotifyCollectionChanged接口的CollectionChanged事件，实现了自动通知。
这样一来，界面UI部分就轻松多了，只需要简单的绑定即可。
Xaml部分：
Xaml部分：
<UserControl x:Class="XmlClassSerelizer.MainPage"
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"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk">
<StackPanel x:Name="LayoutRoot" Background="White">
<sdk:DataGrid AutoGenerateColumns="False" HorizontalAlignment="Center" Margin="0,10,0,0" Name="dataGrid1" VerticalAlignment="Center"
ItemsSource="{Binding EmployeeCollection}">
<sdk:DataGrid.Columns>
<sdk:DataGridTemplateColumn Header="姓名">
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBox Text="{Binding Name,Mode=TwoWay}" VerticalAlignment="Center" Margin="1"></TextBox>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
</sdk:DataGridTemplateColumn>
<sdk:DataGridTemplateColumn Header="工资">
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBox Text="{Binding Salary,Mode=TwoWay}" VerticalAlignment="Center" Margin="1"></TextBox>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
</sdk:DataGridTemplateColumn>
<sdk:DataGridTemplateColumn Header="操作">
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<Button Click="RemoveEmployee" HorizontalAlignment="Center" VerticalAlignment="Center" Padding="10,1">-</Button>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
</sdk:DataGridTemplateColumn>
</sdk:DataGrid.Columns>
</sdk:DataGrid>
<StackPanel HorizontalAlignment="Center" Orientation="Horizontal" Margin="5">
<TextBlock VerticalAlignment="Center">工资总合：</TextBlock>
<TextBlock Text="{Binding SalaryTotal, Mode=TwoWay}" Margin="5,0,5,0" Width="60"></TextBlock>
</StackPanel>
<StackPanel HorizontalAlignment="Center" Orientation="Horizontal" Margin="5">
<Button Click="AddEmployee" Padding="10,1">+</Button>
</StackPanel>
</StackPanel>
</UserControl>
Xaml.cs部分:
using System.Windows;
using System.Windows.Controls;
namespace XmlClassSerelizer
{
public partial class MainPage : UserControl
{
Company c = new Company();
public MainPage()
{
InitializeComponent();
this.Loaded += new RoutedEventHandler(MainPage_Loaded);
}
void MainPage_Loaded(object sender, RoutedEventArgs e)
{
Employee e1 = new Employee(){ Name="张三",Salary=3000};
Employee e2 = new Employee(){ Name="李四",Salary=4000};
c.EmployeeCollection.Add(e1);
c.EmployeeCollection.Add(e2);
this.DataContext = c;
}
/// <summary>
/// 删除员工
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void RemoveEmployee(object sender, RoutedEventArgs e)
{
var emp = (sender as Button).DataContext as Employee;
if (emp != null) { c.EmployeeCollection.Remove(emp); }
}
/// <summary>
/// 添加员工
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void AddEmployee(object sender, RoutedEventArgs e)
{
c.EmployeeCollection.Add(new Employee() { Name = "新人", Salary = 1000 });
}
}
}
运行效果：
