Windows phone 8 是一个单任务操作系统，任何时候都只有一个应用处于活跃状态，这里的多任务是指对后台任务的支持。本节我们先讲讲应用程序的运行状态，然后看看支持的后台任务，包括：后台代理、后台音频、后台文件传输、后台辅助线程等。

快速导航：
一、应用的状态
二、后台代理
三、后台音频
四、后台文件传输
五、后台辅助线程

一、应用的状态

1）应用的运行状态

我们通过图解来分析应用的运行状态，启动并置于前台界面的应用是唯一处于运行状态的，其他的操作，比如win键，后退导出应用，打开选择器和启动器时都会让当前运行的应用进入休眠状态，如果系统内存不足，处于休眠状态的应用可能会被系统逻辑删除。下面的图示演示了这个过程。

2）如何恢复状态

当应用处于休眠状态时，它的状态信息仍然保留在内存中，用户下次切换进去后不会有任何变化。但是当应用被逻辑删除后，这些状态信息就会丢失，比如表单填写的内容都会消失，为了避免这种情况，我们需要手动保留状态信息。
    首先，我们在mainpage定义一些页面表单控件：

[XAML]

        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <TextBox Text="{Binding TextBox1Text, Mode=TwoWay}" Height="72"                 HorizontalAlignment="Left" Margin="20,20,0,0" Name="textBox1"                 VerticalAlignment="Top" Width="440" />                            <CheckBox IsChecked="{Binding CheckBox1IsChecked, Mode=TwoWay}"                 Content="CheckBox" Height="71" Name="checkBox1" Margin="20,100,0,0"                  VerticalAlignment="Top"/>                            <Slider Value="http://www.mamicode.com/{Binding Slider1Value, Mode=TwoWay}" Height="84" Name="slider1"                 Width="440" Margin="20,180,0,0"  VerticalAlignment="Top"/>                            <RadioButton IsChecked="{Binding RadioButton1IsChecked, Mode=TwoWay}"                 Content="RadioButton 1" Height="71" Name="radioButton1"                 GroupName="RadioButtonGroup"  Margin="20,260,0,0" VerticalAlignment="Top"/>                            <RadioButton IsChecked="{Binding RadioButton1IsChecked, Mode=TwoWay}"                 Content="RadioButton 2" Height="71" Name="radioButton2"                 GroupName="RadioButtonGroup"  Margin="20,340,0,0" VerticalAlignment="Top"/>        </Grid>

我们需要实现在应用逻辑删除后能将其状态保持到页面的State字典中，但是需要我们的数据源支持序列化，所以我们定义与表单关联的ViewModel如下：

[C#]

    [DataContract]    public class ViewModel : INotifyPropertyChanged    {        private string _textBox1Text;        private bool _checkBox1IsChecked;        private bool _radioButton1IsChecked;        private bool _radioButton2IsChecked;        private double _slider1Value;        [DataMember]        public string TextBox1Text        {            get { return _textBox1Text; }            set            {                _textBox1Text = value;                NotifyPropertyChanged("TextBox1Text");            }        }        [DataMember]        public bool CheckBox1IsChecked        {            get { return _checkBox1IsChecked; }            set            {                _checkBox1IsChecked = value;                NotifyPropertyChanged("CheckBox1IsChecked");            }        }        [DataMember]        public double Slider1Value        {            get { return _slider1Value; }            set            {                _slider1Value = http://www.mamicode.com/value;"Slider1Value");            }        }        [DataMember]        public bool RadioButton1IsChecked        {            get { return _radioButton1IsChecked; }            set            {                _radioButton1IsChecked = value;                NotifyPropertyChanged("RadioButton1IsChecked");            }        }        [DataMember]        public bool RadioButton2IsChecked        {            get { return _radioButton2IsChecked; }            set            {                _radioButton2IsChecked = value;                NotifyPropertyChanged("RadioButton2IsChecked");            }        }        public event PropertyChangedEventHandler PropertyChanged;        private void NotifyPropertyChanged(string propertyName)        {            if (null != PropertyChanged)                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));        }    }

我需要对mainpage代码添加页面导航入、导航出的事件。导航出页面的时候，如果不是向后导航，则存储状态。导航入的时候，我们需要判断页面是否为逻辑删除后正在恢复的状态，如果是，则通过状态字典恢复状态。mainpage代码如下：

[C#]

    public partial class MainPage : PhoneApplicationPage    {        // 构造函数        public MainPage()        {            InitializeComponent();            _isNewPageInstance = true;        }        ViewModel _viewModel = null;        /// <summary>        /// 新实例还是现有实例        /// </summary>        bool _isNewPageInstance = false;        private void Button_Click_1(object sender, RoutedEventArgs e)        {            NavigationService.Navigate(new Uri("/Page1.xaml", UriKind.Relative));        }        protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)        {            //如果不是向后导航，则保存状态            if (e.NavigationMode != System.Windows.Navigation.NavigationMode.Back)            {                State["ViewModel"] = _viewModel;            }        }        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)        {            if (_isNewPageInstance)            {                if (_viewModel == null)                {                    if (State.Count > 0)                    {                        _viewModel = (ViewModel)State["ViewModel"];                    }                    else                    {                        _viewModel = new ViewModel();                    }                }                DataContext = _viewModel;            }            _isNewPageInstance = false;        }    }

然后我们添加一page1页面，该页添加一个返回按钮。用于测试。为了达到调试时即时进入逻辑删除的效果，我们需要设置下。右键项目文件，点属性，在调试选项卡勾选“在调试期间取消激活时逻辑删除”。

二、后台代理

后台代理可以在应用退出以后独立在系统后台运行，它包含两种类型的代理，分别是定期代理和资源密集型代理，前者用于频繁执行小任务，后者用于在系统空闲时执行耗时大任务。要使用后台代理，我们需要添加一个名为Windows phone 计划任务代理的项目，并在应用的项目中添加对其的引用，现在我们要实现在后台代理中弹出Toast，我们需要如下修改ScheduledAgent.cs的OnInvoke方法，代码如下

[C#]

        protected override void OnInvoke(ScheduledTask task)        {            string toastMessage = "";            if (task is PeriodicTask)            {                toastMessage = "定期代理正在运行";            }            else            {                toastMessage = "资源密集型代理正在运行";            }            // 用于向用户显示Toast，如果当前任务的前台正在运行，则不显示            ShellToast toast = new ShellToast();            toast.Title = "标题";            toast.Content = toastMessage;            toast.Show();            // 在调试的时候需要及时执行查看效果            #if DEBUG_AGENT                ScheduledActionService.LaunchForTest(task.Name, TimeSpan.FromSeconds(15));            #endif            NotifyComplete();        }

接着，我们在应用项目的mainpage中调用代理，代码如下：

[XAML]

        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <StackPanel>                <StackPanel  Orientation="Vertical" Name="PeriodicStackPanel" Margin="0,0,0,40">                    <TextBlock Text="定期代理" Style="{StaticResource PhoneTextTitle2Style}"/>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="名称: " Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding Name}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="是否可用" VerticalAlignment="Center"  Style="{StaticResource PhoneTextAccentStyle}"/>                        <CheckBox Name="PeriodicCheckBox" IsChecked="{Binding IsEnabled}" Checked="PeriodicCheckBox_Checked" Unchecked="PeriodicCheckBox_Unchecked"/>                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="是否已计划： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding IsScheduled}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="上次计划运行时间： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding LastScheduledTime}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="计划结束时间： " Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding ExpirationTime}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="上一次代理运行退出的原因： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding LastExitReason}" />                    </StackPanel>                </StackPanel>                <StackPanel  Orientation="Vertical" Name="ResourceIntensiveStackPanel" Margin="0,0,0,40">                    <TextBlock Text="资源密集型代理" Style="{StaticResource PhoneTextTitle2Style}"/>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="名称: " Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding Name}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="是否可用" VerticalAlignment="Center"  Style="{StaticResource PhoneTextAccentStyle}"/>                        <CheckBox Name="ResourceIntensiveCheckBox" IsChecked="{Binding IsEnabled}" Checked="ResourceIntensiveCheckBox_Checked" Unchecked="ResourceIntensiveCheckBox_Unchecked"/>                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="是否已计划： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding IsScheduled}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="上次计划运行时间： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding LastScheduledTime}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="计划结束时间： " Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding ExpirationTime}" />                    </StackPanel>                    <StackPanel Orientation="Horizontal">                        <TextBlock Text="上一次代理运行退出的原因： "  Style="{StaticResource PhoneTextAccentStyle}"/>                        <TextBlock Text="{Binding LastExitReason}" />                    </StackPanel>                </StackPanel>            </StackPanel>        </Grid>

[C#]

    public partial class MainPage : PhoneApplicationPage    {        /// <summary>        /// 定期代理        /// </summary>        PeriodicTask periodicTask;        /// <summary>        /// 资源密集型代理        /// </summary>        ResourceIntensiveTask resourceIntensiveTask;        string periodicTaskName = "PeriodicAgent";        string resourceIntensiveTaskName = "ResourceIntensiveAgent";        public bool agentsAreEnabled = true;        // 构造函数        public MainPage()        {            InitializeComponent();        }        //启动定期代理        private void StartPeriodicAgent()        {            agentsAreEnabled = true;            // 获取当前名称的定期代理，如果存在则移除            periodicTask = ScheduledActionService.Find(periodicTaskName) as PeriodicTask;            if (periodicTask != null)            {                RemoveAgent(periodicTaskName);            }            periodicTask = new PeriodicTask(periodicTaskName);            periodicTask.Description = "这是一个定期代理的描述信息。";            try            {                ScheduledActionService.Add(periodicTask);                PeriodicStackPanel.DataContext = periodicTask;                //在调试的时候需要及时执行查看效果                #if(DEBUG_AGENT)                    ScheduledActionService.LaunchForTest(periodicTaskName, TimeSpan.FromSeconds(60));                #endif            }            catch (InvalidOperationException exception)            {                if (exception.Message.Contains("BNS Error: The action is disabled"))                {                    MessageBox.Show("本应用的后台计划被用户禁用。");                    agentsAreEnabled = false;                    PeriodicCheckBox.IsChecked = false;                }                if (exception.Message.Contains("BNS Error: The maximum number of ScheduledActions of this type have already been added."))                {                    MessageBox.Show("定期代理数量达到最大限制。");                }                PeriodicCheckBox.IsChecked = false;            }            catch (SchedulerServiceException)            {                PeriodicCheckBox.IsChecked = false;            }        }        private void StartResourceIntensiveAgent()        {            agentsAreEnabled = true;            // 获取当前名称的资源密集型代理，如果存在则移除            resourceIntensiveTask = ScheduledActionService.Find(resourceIntensiveTaskName) as ResourceIntensiveTask;            if (resourceIntensiveTask != null)            {                RemoveAgent(resourceIntensiveTaskName);            }            resourceIntensiveTask = new ResourceIntensiveTask(resourceIntensiveTaskName);            resourceIntensiveTask.Description = "这是一个资源密集型代理的描述信息。";            try            {                ScheduledActionService.Add(resourceIntensiveTask);                ResourceIntensiveStackPanel.DataContext = resourceIntensiveTask;                //在调试的时候需要及时执行查看效果                #if(DEBUG_AGENT)                    ScheduledActionService.LaunchForTest(resourceIntensiveTaskName, TimeSpan.FromSeconds(15));                #endif            }            catch (InvalidOperationException exception)            {                if (exception.Message.Contains("BNS Error: The action is disabled"))                {                    MessageBox.Show("本应用的后台计划被用户禁用。");                    agentsAreEnabled = false;                }                ResourceIntensiveCheckBox.IsChecked = false;            }            catch (SchedulerServiceException)            {                ResourceIntensiveCheckBox.IsChecked = false;            }        }        bool ignoreCheckBoxEvents = false;        private void PeriodicCheckBox_Checked(object sender, RoutedEventArgs e)        {            if (ignoreCheckBoxEvents)                return;            StartPeriodicAgent();        }        private void PeriodicCheckBox_Unchecked(object sender, RoutedEventArgs e)        {            if (ignoreCheckBoxEvents)                return;            RemoveAgent(periodicTaskName);        }        private void ResourceIntensiveCheckBox_Checked(object sender, RoutedEventArgs e)        {            if (ignoreCheckBoxEvents)                return;            StartResourceIntensiveAgent();        }        private void ResourceIntensiveCheckBox_Unchecked(object sender, RoutedEventArgs e)        {            if (ignoreCheckBoxEvents)                return;            RemoveAgent(resourceIntensiveTaskName);        }        /// <summary>        /// 删除代理        /// </summary>        private void RemoveAgent(string name)        {            try            {                ScheduledActionService.Remove(name);            }            catch (Exception)            {            }        }        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)        {            ignoreCheckBoxEvents = true;            periodicTask = ScheduledActionService.Find(periodicTaskName) as PeriodicTask;            if (periodicTask != null)            {                PeriodicStackPanel.DataContext = periodicTask;            }            resourceIntensiveTask = ScheduledActionService.Find(resourceIntensiveTaskName) as ResourceIntensiveTask;            if (resourceIntensiveTask != null)            {                ResourceIntensiveStackPanel.DataContext = resourceIntensiveTask;            }            ignoreCheckBoxEvents = false;        }    }

三、后台音频

通过后台音频的功能我们可以实现在系统后台播放音乐的功能，由于后台音频代理只能访问本地文件夹，所以我们务必要先把需要播放的音乐文件拷贝到本地文件夹中。本示例是把安装文件夹的音频文件拷贝到本地文件夹，代码如下：

[C#]

        //把安装文件夹下的文件拷贝到本地文件夹        private void CopyToIsolatedStorage()        {            using (IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication())            {                string[] files = new string[] { "Ring01.wma", "Ring02.wma", "Ring03.wma" };                foreach (var _fileName in files)                {                    if (!storage.FileExists(_fileName))                    {                        string _filePath = "Audio/" + _fileName;                        StreamResourceInfo resource = Application.GetResourceStream(new Uri(_filePath, UriKind.Relative));                        using (IsolatedStorageFileStream file = storage.CreateFile(_fileName))                        {                            int chunkSize = 4096;                            byte[] bytes = new byte[chunkSize];                            int byteCount;                            while ((byteCount = resource.Stream.Read(bytes, 0, chunkSize)) > 0)                            {                                file.Write(bytes, 0, byteCount);                            }                        }                    }                }                string[] icons = new string[] { "Ring01.jpg", "Ring02.jpg", "Ring03.jpg" };                foreach (var _fileName in icons)                {                    if (!storage.FileExists(_fileName))                    {                        string _filePath = "Images/" + _fileName;                        StreamResourceInfo iconResource = Application.GetResourceStream(new Uri(_filePath, UriKind.Relative));                        using (IsolatedStorageFileStream file = storage.CreateFile( _fileName))                        {                            int chunkSize = 4096;                            byte[] bytes = new byte[chunkSize];                            int byteCount;                            while ((byteCount = iconResource.Stream.Read(bytes, 0, chunkSize)) > 0)                            {                                file.Write(bytes, 0, byteCount);                            }                        }                    }                }            }        }

我们需要在解决方案中添加Windows phone 音频播放代理项目，并在应用项目中添加对其的引用。修改AudioPlayer.cs代码如下：

[C#]

    public class AudioPlayer : AudioPlayerAgent    {        private static volatile bool _classInitialized;        private static List<AudioTrack> _playList = new List<AudioTrack>        {            new AudioTrack(new Uri("Ring01.wma", UriKind.Relative),"曲目1","艺术家1","专辑1",new Uri("Ring01.jpg", UriKind.Relative)),            new AudioTrack(new Uri("Ring02.wma", UriKind.Relative),"曲目2","艺术家2","专辑2",new Uri("Ring02.jpg", UriKind.Relative)),             new AudioTrack(new Uri("Ring03.wma", UriKind.Relative),"曲目3","艺术家3","专辑3",new Uri("Ring03.jpg", UriKind.Relative))                            };        /// <summary>        /// 当前播放位置        /// </summary>        static int currentTrackNumber = 0;        /// <remarks>        /// AudioPlayer 实例可共享同一进程。        /// 静态字段可用于在 AudioPlayer 实例之间共享状态        /// 或与音频流代理通信。        /// </remarks>        public AudioPlayer()        {            if (!_classInitialized)            {                _classInitialized = true;                // 订阅托管异常处理程序                Deployment.Current.Dispatcher.BeginInvoke(delegate                {                    Application.Current.UnhandledException += AudioPlayer_UnhandledException;                });            }        }        /// 出现未处理的异常时执行的代码        private void AudioPlayer_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)        {            if (System.Diagnostics.Debugger.IsAttached)            {                // 出现未处理的异常；强行进入调试器                System.Diagnostics.Debugger.Break();            }        }        /// <summary>        /// playstate 更改时调用，但 Error 状态除外(参见 one rror)        /// </summary>        /// <param name="player">BackgroundAudioPlayer</param>        /// <param name="track">在 playstate 更改时播放的曲目</param>        /// <param name="playState">播放机的新 playstate </param>        /// <remarks>        /// 无法取消播放状态更改。即使应用程序        /// 导致状态自行更改，假定应用程序已经选择了回调。        ///         /// 值得注意的 playstate 事件        /// (a) TrackEnded: 播放器没有当前曲目时激活。代理可设置下一曲目。        /// (b) TrackReady: 音轨已设置完毕，现在可以播放。        ///         /// 只在代理请求完成之后调用一次 NotifyComplete()，包括异步回调。        /// </remarks>        protected override void OnPlayStateChanged(BackgroundAudioPlayer player, AudioTrack track, PlayState playState)        {            switch (playState)            {                case PlayState.TrackEnded:                    player.Track = GetPreviousTrack();                    break;                case PlayState.TrackReady:                    player.Play();                    break;                case PlayState.Shutdown:                    // TODO: 在此处理关机状态(例如保存状态)                    break;                case PlayState.Unknown:                    break;                case PlayState.Stopped:                    break;                case PlayState.Paused:                    break;                case PlayState.Playing:                    break;                case PlayState.BufferingStarted:                    break;                case PlayState.BufferingStopped:                    break;                case PlayState.Rewinding:                    break;                case PlayState.FastForwarding:                    break;            }            NotifyComplete();        }        /// <summary>        /// 在用户使用应用程序/系统提供的用户界面请求操作时调用        /// </summary>        /// <param name="player">BackgroundAudioPlayer</param>        /// <param name="track">用户操作期间播放的曲目</param>        /// <param name="action">用户请求的操作</param>        /// <param name="param">与请求的操作相关联的数据。        /// 在当前版本中，此参数仅适合与 Seek 操作一起使用，        /// 以指明请求的乐曲的位置</param>        /// <remarks>        /// 用户操作不自动对系统状态进行任何更改；如果用户操作受支持，        /// 执行用户操作(如果这些操作受支持)。        ///         /// 只在代理请求完成之后调用一次 NotifyComplete()，包括异步回调。        /// </remarks>        protected override void OnUserAction(BackgroundAudioPlayer player, AudioTrack track, UserAction action, object param)        {            switch (action)            {                case UserAction.Play:                    if (player.PlayerState != PlayState.Playing)                    {                        player.Play();                    }                    break;                case UserAction.Stop:                    player.Stop();                    break;                case UserAction.Pause:                    player.Pause();                    break;                case UserAction.FastForward:                    player.FastForward();                    break;                case UserAction.Rewind:                    player.Rewind();                    break;                case UserAction.Seek:                    player.Position = (TimeSpan)param;                    break;                case UserAction.SkipNext:                    player.Track = GetNextTrack();                    break;                case UserAction.SkipPrevious:                    AudioTrack previousTrack = GetPreviousTrack();                    if (previousTrack != null)                    {                        player.Track = previousTrack;                    }                    break;            }            NotifyComplete();        }        /// <summary>        /// 实现逻辑以获取下一个 AudioTrack 实例。        /// 在播放列表中，源可以是文件、Web 请求，等等。        /// </summary>        /// <remarks>        /// AudioTrack URI 确定源，它可以是:        /// (a) 独立存储器文件(相对 URI，表示独立存储器中的路径)        /// (b) HTTP URL(绝对 URI)        /// (c) MediaStreamSource (null)        /// </remarks>        /// <returns>AudioTrack 实例，或如果播放完毕，则返回 null</returns>        private AudioTrack GetNextTrack()        {            // TODO: 添加逻辑以获取下一条音轨            if (++currentTrackNumber >= _playList.Count) currentTrackNumber = 0;            AudioTrack track = _playList[currentTrackNumber];            // 指定曲目            return track;        }        /// <summary>        /// 实现逻辑以获取前一个 AudioTrack 实例。        /// </summary>        /// <remarks>        /// AudioTrack URI 确定源，它可以是:        /// (a) 独立存储器文件(相对 URI，表示独立存储器中的路径)        /// (b) HTTP URL(绝对 URI)        /// (c) MediaStreamSource (null)        /// </remarks>        /// <returns>AudioTrack 实例，或如果不允许前一曲目，则返回 null</returns>        private AudioTrack GetPreviousTrack()        {            // TODO: 添加逻辑以获取前一条音轨            if (--currentTrackNumber < 0) currentTrackNumber = _playList.Count - 1;            AudioTrack track = _playList[currentTrackNumber];            // 指定曲目            return track;        }        /// <summary>        /// 每次播放出错(如 AudioTrack 未正确下载)时调用        /// </summary>        /// <param name="player">BackgroundAudioPlayer</param>        /// <param name="track">出现错误的曲目</param>        /// <param name="error">出现的错误</param>        /// <param name="isFatal">如果为 true，则播放不能继续并且曲目播放将停止</param>        /// <remarks>        /// 不保证在所有情况下都调用此方法。例如，如果后台代理程序         /// 本身具有未处理的异常，则不会回调它来处理它自己的错误。        /// </remarks>        protected override void one rror(BackgroundAudioPlayer player, AudioTrack track, Exception error, bool isFatal)        {            if (isFatal)            {                Abort();            }            else            {                NotifyComplete();            }        }        /// <summary>        /// 取消代理请求时调用        /// </summary>        /// <remarks>        /// 取消请求后，代理需要 5 秒钟完成其工作，        /// 通过调用 NotifyComplete()/Abort()。        /// </remarks>        protected override void OnCancel()        {        }    }

最后，我们在mainpage中添加对播放的控制。

[XAML]

    <Grid x:Name="LayoutRoot" Background="Transparent">        <Grid.RowDefinitions>            <RowDefinition Height="Auto"/>            <RowDefinition Height="*"/>        </Grid.RowDefinitions>                <!--TitlePanel 包含应用程序的名称和页标题-->        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">            <TextBlock x:Name="ApplicationTitle" Text="后台音频" Style="{StaticResource PhoneTextNormalStyle}"/>        </StackPanel>        <!--ContentPanel - 在此处放置其他内容-->        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <Button x:Name="button2" Content="〈" HorizontalAlignment="Left" Margin="13,10,0,0" VerticalAlignment="Top" Click="Button_Click_2"/>            <Button x:Name="button1" Content="?" HorizontalAlignment="Left" Margin="75,10,0,0" VerticalAlignment="Top" Click="Button_Click_1"/>            <Button x:Name="button3" Content="〉" HorizontalAlignment="Left" Margin="136,10,0,0" VerticalAlignment="Top" Click="Button_Click_3" />            <TextBlock x:Name="textblock1" HorizontalAlignment="Left" Margin="22,87,0,0" TextWrapping="Wrap" VerticalAlignment="Top"/>            <Image x:Name="imge1" HorizontalAlignment="Left" Height="100" Margin="22,142,0,0" VerticalAlignment="Top" Width="100"/>        </Grid>       </Grid>

[C#]

    public partial class MainPage : PhoneApplicationPage    {        // 构造函数        public MainPage()        {            InitializeComponent();            BackgroundAudioPlayer.Instance.PlayStateChanged += new EventHandler(Instance_PlayStateChanged);        }        //刚加载时确定播放状态        protected override void OnNavigatedTo(NavigationEventArgs e)        {            if (PlayState.Playing == BackgroundAudioPlayer.Instance.PlayerState)            {                button1.Content = "■";                textblock1.Text = "曲目：" + BackgroundAudioPlayer.Instance.Track.Title                    + " 艺术家：" + BackgroundAudioPlayer.Instance.Track.Artist                    + " 专辑：" + BackgroundAudioPlayer.Instance.Track.Album                    + " 曲目长度：" +BackgroundAudioPlayer.Instance.Track.Duration.Minutes + ":" + BackgroundAudioPlayer.Instance.Track.Duration.Seconds;                    using (IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication())                    {                        var stream = storage.OpenFile(BackgroundAudioPlayer.Instance.Track.AlbumArt.OriginalString, System.IO.FileMode.Open);                        var bitmapImage = new BitmapImage();                        bitmapImage.SetSource(stream);                        imge1.Source = bitmapImage;                        stream.Close();                    }            }            else            {                button1.Content = "?";                textblock1.Text = "未播放曲目";            }        }        void Instance_PlayStateChanged(object sender, EventArgs e)        {            switch (BackgroundAudioPlayer.Instance.PlayerState)            {                case PlayState.Playing:                    button1.Content = "■";                    button2.IsEnabled = true;                    button3.IsEnabled = true;                    break;                case PlayState.Paused:                case PlayState.Stopped:                    button1.Content = "?";                    break;            }            if (null != BackgroundAudioPlayer.Instance.Track && BackgroundAudioPlayer.Instance.PlayerState!= PlayState.Stopped)            {                textblock1.Text = "曲目：" + BackgroundAudioPlayer.Instance.Track.Title                    + " 艺术家：" + BackgroundAudioPlayer.Instance.Track.Artist                    + " 专辑：" + BackgroundAudioPlayer.Instance.Track.Album                    + " 曲目长度：" + BackgroundAudioPlayer.Instance.Track.Duration.Minutes + ":" + BackgroundAudioPlayer.Instance.Track.Duration.Seconds;                    using (IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication())                    {                        var stream = storage.OpenFile(BackgroundAudioPlayer.Instance.Track.AlbumArt.OriginalString, System.IO.FileMode.Open);                        var bitmapImage = new BitmapImage();                        bitmapImage.SetSource(stream);                        imge1.Source = bitmapImage;                        stream.Close();                    }            }        }        //播放/暂停        private void Button_Click_1(object sender, RoutedEventArgs e)        {            if (PlayState.Playing == BackgroundAudioPlayer.Instance.PlayerState)                BackgroundAudioPlayer.Instance.Pause();            else                BackgroundAudioPlayer.Instance.Play();        }        //向前        private void Button_Click_2(object sender, RoutedEventArgs e)        {            BackgroundAudioPlayer.Instance.SkipPrevious();            button2.IsEnabled = false;        }        //向后        private void Button_Click_3(object sender, RoutedEventArgs e)        {            BackgroundAudioPlayer.Instance.SkipNext();            button3.IsEnabled = false;        }    }

四、后台文件传输

后台文件传输允许我们实现下载上传文件的功能，他限制系统中同时运行的传输任务不能超过两个，并且下载的文件只能存放在本地文件夹的/shared/transfers目录下。下面我们实现一个后台传输任务，下载博客相册中的一张照片。

[XAML]

    <Grid x:Name="LayoutRoot" Background="Transparent">        <Grid.RowDefinitions>            <RowDefinition Height="Auto"/>            <RowDefinition Height="*"/>        </Grid.RowDefinitions>        <!--TitlePanel 包含应用程序的名称和页标题-->        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">            <TextBlock x:Name="ApplicationTitle" Text="后台传输" Style="{StaticResource PhoneTextNormalStyle}"/>        </StackPanel>        <!--ContentPanel - 在此处放置其他内容-->        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <TextBlock x:Name="textblock1" HorizontalAlignment="Left" Margin="10,198,0,0" TextWrapping="Wrap" VerticalAlignment="Top"/>            <Button Content="清除传输队列中已完成的任务" HorizontalAlignment="Left" Margin="0,85,0,0" VerticalAlignment="Top" Click="Button_Click_2"/>        </Grid>        <Button x:Name="button1" Content="添加一个后台传输" HorizontalAlignment="Left" Margin="12,10,0,0" Grid.Row="1" VerticalAlignment="Top" Click="Button_Click_1"/>    </Grid>

[C#]

    public partial class MainPage : PhoneApplicationPage    {        // 构造函数        public MainPage()        {            InitializeComponent();        }        protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)        {            initTransferRequest();            base.OnNavigatedTo(e);        }        private void initTransferRequest()        {            //获取第一个后台传输任务            var transferRequest = BackgroundTransferService.Requests.FirstOrDefault();            if (transferRequest == null)            {                textblock1.Text = "无后台传输任务";                button1.IsEnabled = true;                return;            }            //当传输状态改变时：            transferRequest.TransferStatusChanged += new EventHandler<BackgroundTransferEventArgs>(transfer_TransferStatusChanged);            //当传输进度改变时：            transferRequest.TransferProgressChanged += new EventHandler<BackgroundTransferEventArgs>(transfer_TransferProgressChanged);            updatesStatus(transferRequest);            button1.IsEnabled = false;        }        void transfer_TransferStatusChanged(object sender, BackgroundTransferEventArgs e)        {            updatesStatus(e.Request);        }        void transfer_TransferProgressChanged(object sender, BackgroundTransferEventArgs e)        {            updatesStatus(e.Request);        }        void updatesStatus(BackgroundTransferRequest transferRequest)        {            textblock1.Text = "传输状态：" + transferRequest.TransferStatus.ToString()                + " 已下载字节：" + transferRequest.BytesReceived                + "总字节：" + transferRequest.TotalBytesToReceive;        }        private void Button_Click_1(object sender, RoutedEventArgs e)        {            string fileurlstring = "http://images.cnblogs.com/cnblogs_com/lipan/319399/o_Large.png";            Uri uri = new Uri(Uri.EscapeUriString(fileurlstring), UriKind.RelativeOrAbsolute);            BackgroundTransferRequest transferRequest = new BackgroundTransferRequest(uri);            transferRequest.Method = "GET";            using (IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication())            {                if (!isoStore.DirectoryExists("/shared/transfers"))                {                    isoStore.CreateDirectory("/shared/transfers");                }            }            //文件下载后存放位置（为本地文件夹相对位置）            transferRequest.DownloadLocation = new Uri("shared/transfers/1.png", UriKind.RelativeOrAbsolute);            //外接电源、WiFi的可用性对传输的影响            transferRequest.TransferPreferences = TransferPreferences.AllowCellularAndBattery;            try            {                //添加到后台传输队列中                BackgroundTransferService.Add(transferRequest);            }            catch (Exception ex)            {                MessageBox.Show("无法添加后台传输请求。" + ex.Message);            }            initTransferRequest();        }        //清除传输队列已完成的任务        private void Button_Click_2(object sender, RoutedEventArgs e)        {            foreach (var transferRequest in BackgroundTransferService.Requests)            {                if (transferRequest.TransferStatus == TransferStatus.Completed)                {                    try                    {                        BackgroundTransferService.Remove(transferRequest);                    }                    catch                    {                    }                }            }            initTransferRequest();        }    }

五、后台辅助线程

后台辅助线程虽然名字这么叫，但是它不能在后台运行，我们可以用它来执行一个任务，并且可以实时获取执行的进度，实现代码如下：

[XAML]

        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">            <StackPanel>                <StackPanel Orientation="Horizontal"                         HorizontalAlignment="Left" VerticalAlignment="Top"                         Margin="10" >                    <Button x:Name="buttonStart" Content="开始" Click="buttonStart_Click"                        Width="200" />                    <Button x:Name="buttonCancel" Content="取消" Click="buttonCancel_Click"                        Width="200" />                </StackPanel>                <StackPanel Margin="10,50,0,0" Orientation="Horizontal">                    <TextBlock Text="进度: " />                    <TextBlock x:Name="tbProgress" />                </StackPanel>            </StackPanel>        </Grid>

[C#]

    public partial class MainPage : PhoneApplicationPage    {                      private BackgroundWorker bw = new BackgroundWorker();          public MainPage()        {            InitializeComponent();            bw.WorkerReportsProgress = true;            bw.WorkerSupportsCancellation = true;            bw.DoWork += new DoWorkEventHandler(bw_DoWork);            bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);        }        private void buttonStart_Click(object sender, RoutedEventArgs e)        {            if (bw.IsBusy != true)            {                bw.RunWorkerAsync();            }        }        private void buttonCancel_Click(object sender, RoutedEventArgs e)        {            if (bw.WorkerSupportsCancellation == true)            {                bw.CancelAsync();            }        }        private void bw_DoWork(object sender, DoWorkEventArgs e)        {            BackgroundWorker worker = sender as BackgroundWorker;            for (int i = 1; i <= 10; i++)            {                if ((worker.CancellationPending == true))                {                    e.Cancel = true;                    break;                }                else                {                    // Perform a time consuming operation and report progress.                    System.Threading.Thread.Sleep(500);                    worker.ReportProgress(i * 10);                }            }        }        private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)        {            if (e.Cancelled == true)            {                this.tbProgress.Text = "Canceled!";            }            else if (!(e.Error == null))            {                this.tbProgress.Text = ("Error: " + e.Error.Message);            }            else            {                this.tbProgress.Text = "Done!";            }        }        private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)        {            this.tbProgress.Text = (e.ProgressPercentage.ToString() + "%");        }    }