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:

    /// <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：

<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：