Windows phone 8 学习笔记(6) 多任务（转）

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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]

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 = value;
NotifyPropertyChanged("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]

[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 OnError)
/// </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 OnError(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]

[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]

[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]

[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() + "%");
}
}