如何在ViewModel中正確地使用Timer（定時器）

內容摘要：

這是我在某個客戶那邊講課的時候遇到一個小問題，在ViewModel中創建的一個Timer，並不會被自動停止，即便使用該ViewModel的View已經被關閉了。這個問題的原因在於Timer的特殊工作機制，它是運行在一個獨立的工作線程的，除非明確地停止他，或者整個程序關閉了，它才會停止。這一講中，我通過實例重現了這個問題，然后提供了一個可行的解決方法。

 

視頻地址：

http://www.tudou.com/programs/view/uO4b2j0N4L8/

示例代碼：

備注：該范例使用了MvvmLight作為MVVM框架，請自行安裝

Model：

 

using System;
using System.Diagnostics;
using System.Linq;

namespace SilverlightApplicationSample
{
    public class DataService
    {
        public static Customer[] GetCustomers()
        {

            Debug.WriteLine(string.Format("[{0}]正在調用數據服務",DateTime.Now));

            var rnd = new Random();
            return Enumerable.Range(1, rnd.Next(100)).Select(x => new Customer()
            {
                CompanyName = "Company " + x.ToString()
            }).ToArray();

        }
    }

    public class Customer
    {
        public string CompanyName { get; set; }
    }
}

ViewModel：

using System;
using System.Windows.Threading;
using GalaSoft.MvvmLight;

namespace SilverlightApplicationSample
{
    /// <summary>
    /// 使用MVVMLight實現的MVVM ViewModel
    /// </summary>
    public class CustomerWindowViewModel : ViewModelBase
    {
        /// <summary>
        /// 這個方法也不會自動調用
        /// </summary>
        public override void Cleanup()
        {
            base.Cleanup();

            timer.Stop();
        }

 

        DispatcherTimer timer = null;

        public CustomerWindowViewModel()
        {
            //正常情況下的綁定
            //Customers = DataService.GetCustomers();


            //使用定時器調用服務
            timer = new DispatcherTimer();

            timer.Interval = TimeSpan.FromSeconds(1);
            timer.Tick += (o, a) =>
            {
                Customers = DataService.GetCustomers();
            };

            timer.Start();
        }

        private Customer[] _Customers;
        public Customer[] Customers
        {
            get { return _Customers; }
            set
            {
                if (_Customers != value)
                {
                    _Customers = value;
                    RaisePropertyChanged("Customers");
                }
            }
        }
    }
}

 

View：

<controls:ChildWindow x:Class="SilverlightApplicationSample.CustomerWindow"
                      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                      xmlns:controls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls"
                      Width="400"
                      Height="300"
                      Title="CustomerWindow"
                      xmlns:local="clr-namespace:SilverlightApplicationSample">
    <controls:ChildWindow.DataContext>
        <local:CustomerWindowViewModel></local:CustomerWindowViewModel>
    </controls:ChildWindow.DataContext>
    
    <Grid x:Name="LayoutRoot"
          Margin="2">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>



        <ListBox ItemsSource="{Binding Customers}"
                 DisplayMemberPath="CompanyName">
        </ListBox>

        <Button x:Name="CancelButton"
                Content="Cancel"
                Click="CancelButton_Click"
                Width="75"
                Height="23"
                HorizontalAlignment="Right"
                Margin="0,12,0,0"
                Grid.Row="1" />
        <Button x:Name="OKButton"
                Content="OK"
                Click="OKButton_Click"
                Width="75"
                Height="23"
                HorizontalAlignment="Right"
                Margin="0,12,79,0"
                Grid.Row="1" />
    </Grid>
</controls:ChildWindow>

Page：

using System.Windows;
using System.Windows.Controls;

namespace SilverlightApplicationSample
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }

        

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var window = new CustomerWindow();
            window.Closed += (o, a) =>
            {
                var vm = window.DataContext as CustomerWindowViewModel;
                vm.Cleanup();
            };

            window.Show();
        }
    }
}