Silverlight：双向绑定综合应用-自动更新集合汇总字段

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:

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/// <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:

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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：

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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：

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<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" 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="5,5,0" 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:

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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：

http://www.cnblogs.com/yjmyzz/archive/2011/06/26/2090538.html