Windows 8 学习笔记（十八）--.后台任务BackgroundTask(I)

后台任务即是当应用程序处于挂起状态时，不会影响由于程序持起而带来的某些滞后而执行的一些后台操作，如下载文件、最新信息的推送等。当然，也应尽量减少一些没有必要的后台操作，保证电池的寿命。

后台任务处理一般用于以下几种情况：

1、消息推送

2、播放背景音乐

3、文件传输

4、共享文件  

后台任务中一般经常会使用以下一些类和方法：

1、BackgroundTaskBuilder 系统注册后台任务类

2、BackgroundTaskRegistration BackgroundTaskBuilder.Register()注册后台任务

3、SystemTrigger 一个后台任务必须有一个明确的Trigger才执行后台任务，通过SetTrigger()添加，一般Trigger的类型有以下一些类型 

Background task trigger type	Trigger event	When the background task is triggered
ControlChannelTrigger	ControlChannelTrigger	On incoming messages on the control channel.
MaintenanceTrigger	MaintenanceTrigger	It’s time for maintenance background tasks.
PushNotificationTrigger	PushNotificationTrigger	A raw notification arrives on the WNS channel.
SystemEventTrigger	InternetAvailable	The Internet becomes available.
SystemEventTrigger	LockScreenApplicationAdded	An app tile is added to the lock screen.
SystemEventTrigger	LockScreenApplicationRemoved	An app tile is removed from the lock screen.
SystemEventTrigger	ControlChannelReset	A network channel is reset.
SystemEventTrigger	NetworkStateChange	A network change such as a change in cost or connectivity occurs.
SystemEventTrigger	OnlineIdConnectedStateChange	Online ID associated with the account changes.
SystemEventTrigger	ServicingComplete	The system has finished updating an application.
SystemEventTrigger	SessionConnected	The session is connected.
SystemEventTrigger	SessionDisconnected	The session is disconnected.
SystemEventTrigger	SmsReceived	A new SMS message is received by an installed mobile broadband device.
SystemEventTrigger	TimeZoneChange	The time zone changes on the device (for example, when the system adjusts the clock for daylight saving time).
SystemEventTrigger	UserAway	The user becomes absent.
SystemEventTrigger	UserPresent	The user becomes present.
TimeTrigger	TimeTrigger	A time event occurs.

4、SystemCondition

     后台任务可以设置0个或多个条件，其用来描述当达到某种条件时执行后台任务才是有意义的。通过BackgroundTaskBuilder.AddCondition()添加.

5、BackgroundTaskProgressEventHandler

      BackgroundTaskBuilder可以注册ProgressEventHandler事件来报告UI后台执行的进度，如当下载文件事，可以报告UI当前文件下载的百分比。

6、BackgroundTaskCompletedEventArgs

     同样注册CompletedEventArgs事件可以在后台任务执行完后，通知UI。

7、Unregister

    通过Unregister()可以取消后台任务的注册

说了这么多，先来创建一个简单的后台任务，功能非常简单，就是后台任务定时刷新UI：

 第一步：先创建一个空白的Metro项目，项目名为BackgroundProject

前台页面放了两个按钮（注册Task、注销Task）、一个TextBlock控件(显示后台任务的状态)

 第二步：创建一个Class Library，该类库用于实现后台的具体任务，项目名为BackgroundSecniao，需要特别注意的时，这里需要将该项目的输出更改为Winmd文件，不要默认生成DLL

 

第三步：在BackgroundSecniao中新建一个类TimerBackTask.cs类，该类必须继承IBackgroundTask接口

 public sealed class TimerBackTask : IBackgroundTask    

 {
        volatile bool _cancelRequested = false;
        BackgroundTaskDeferral _deferral = null;
        ThreadPoolTimer _periodicTimer = null;
        uint _progress = 0;
        IBackgroundTaskInstance _taskInstance;

        public void Run(IBackgroundTaskInstance taskInstance)
        {
            taskInstance.Canceled += new BackgroundTaskCanceledEventHandler(OnCanceled);
            _deferral = taskInstance.GetDeferral();
            _taskInstance = taskInstance;

            _periodicTimer = ThreadPoolTimer.CreatePeriodicTimer(new TimerElapsedHandler(PeriodicTimerCallback), TimeSpan.FromMilliseconds(1000));
        }

        private void OnCanceled(IBackgroundTaskInstance sender, BackgroundTaskCancellationReason reason)
        {
            _cancelRequested = true;           
        }

        
        private void PeriodicTimerCallback(ThreadPoolTimer timer)
        {
            if ((_cancelRequested == false) && (_progress < 100))
            {
                _progress += 10;
                _taskInstance.Progress = _progress;
            }
            else
            {
                _periodicTimer.Cancel();

                var settings = ApplicationData.Current.LocalSettings;
                var key = _taskInstance.Task.TaskId.ToString();
                if (_cancelRequested)
                {
                    settings.Values[key] = "Canceled";
                }
                else
                {
                    settings.Values[key] = "Completed";
                }
                _deferral.Complete();
            }
}

 第四步：BackgroundProject中的Package.appxmanifest需要设置下，需要在声明中添加相应的后台任务

 

 第五步：在Register按钮中先注册Task

var builder = new BackgroundTaskBuilder();
builder.Name = SampleBackgroundTaskName;
builder.TaskEntryPoint = SampleBackgroundTaskEntryPoint;
SystemTrigger trigger = new SystemTrigger(SystemTriggerType.TimeZoneChange, false);
builder.SetTrigger(trigger);
SystemCondition condition = new SystemCondition(SystemConditionType.UserPresent);

if (condition != null)
{
    builder.AddCondition(condition);
}                
 BackgroundTaskRegistration task = builder.Register();
 task.Completed += task_Completed;

 task.Progress += task_Progress;    

 实现BackgroundTaskProgressEventHandler和BackgroundTaskCompletedEventArgs事件 

     void task_Progress(BackgroundTaskRegistration sender, BackgroundTaskProgressEventArgs args)

        {
            Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
                () =>
                    {
                         var taskRegistration = sender as IBackgroundTaskRegistration;
                         var progressArgs = args as BackgroundTaskProgressEventArgs;

                          if ((taskRegistration != null) && (progressArgs != null))
                           {                                                 
                                 tbProgress.Text = "Progress: " + progressArgs.Progress + "%";
                           }
                      });
        }

        void task_Completed(BackgroundTaskRegistration sender, BackgroundTaskCompletedEventArgs args)
        {
            Dispatcher.RunAsync(CoreDispatcherPriority.Normal,
                   () =>
                    {
                       try
                       {
                          var taskCompletion = sender as IBackgroundTaskRegistration;
                          var completionArgs = args as BackgroundTaskCompletedEventArgs;

                           if ((taskCompletion != null) && (completionArgs != null))
                           {
                               var key = taskCompletion.TaskId.ToString();
                               var settings = ApplicationData.Current.LocalSettings;                                                    
                               tbProgress.Text = settings.Values[key].ToString();                                                   
                           }
                        }
                       catch (Exception ex)
                       {
                        }
              });
        }

Ungister按钮事件：

 foreach (var cur in BackgroundTaskRegistration.AllTasks)           

  {
        if (cur.Value.Name == SampleBackgroundTaskName)
        {
            cur.Value.Unregister(true);
        } 
 }              

 在本地运行调试时，需要将TimerBackTask挂起，才能看到效果，挂起设断点也可以Debug后台程序

 

 后台任务还有好多东西要学习，如锁屏时后台任务的执行、推送、后台任务与程序的生命周期等，内容相当之多，上面的是最基本最简单的内容了！