wp8.1 Study15：后台任务

一、介绍

1、多任务处理

　　什么是多任务处理？它意味着当App被挂起时，它仍然可以完成一些开发者设定的任务，比如更新tiles和toasts、预定toast和提醒、后台任务等。

2、后台任务

　　App可以注册后台任务，它被系统运行和管理，但它依旧使用和前台程序一样的数据储存等，同时它使用的CPU资源是由系统限制的。它可以使用toast, tile, badge UI等。一个App可以注册使用多个后台任务。

 

二、添加后台任务

1、添加一个新的项目到解决方案中作为后台任务，如图

然后，添加以下类似代码（重点是添加IBackgroundTask）

public sealed class TheTask : IBackgroundTask  {    public void Run(IBackgroundTaskInstance taskInstance)    {      var deferral = taskInstance.GetDeferral();       taskInstance.canceled+=(s,e)=>{};//当后台任务被限制执行时触发事件        taskInstance.Progress = 0;//后台任务的进度      deferral.Complete();    }

2、后台任务触发设置（在前台程序，不是在TheTask）

　　后台任务将根据触发器是否被触发而运行，以下是多种触发器：

　　同时也可以添加条件，让后台任务根据条件运行，可以设置一下几种条件等

添加条件代码很简单，代码如下：

3、在manifest声明 中，添加后台任务声明

4、请求及注册后台任务（在前台程序中）

async void RegisterBackgroundTasks(){    // On Windows, RequestAccessAsync presents the user with a confirmation     // dialog that requests that an app be allowed on the lock screen.    // On Windows Phone, RequestAccessAsync does not show any user confirmation UI     // but *must* be called before registering any tasks    var access = await BackgroundExecutionManager.RequestAccessAsync();    // A 'good' status return on Phone is BackgroundAccess.AllowedMayUseActiveRealTimeConnectivity    if (access == BackgroundAccessStatus.Denied)    {        // Either the user has explicitly denied background execution for this app        // or the maximum number of background apps across the system has been reached        // Display some informative message to the user...    }    BackgroundTaskBuilder taskBuilder = new BackgroundTaskBuilder();taskBuilder.Name = "MyBackgroundTask";// Many different trigger types could be used hereSystemTrigger trigger = new SystemTrigger(SystemTriggerType.TimeZoneChange, false);taskBuilder.SetTrigger(trigger);taskBuilder.AddCondition(new SystemCondition(SystemConditionType.InternetAvailable));// Entry point is the full name of our IBackgroundTask implementation// Good practice to use reflection as here to ensure correct nametaskBuilder.TaskEntryPoint = typeof(MyBackgroundTask.TheTask).FullName;BackgroundTaskRegistration registration = taskBuilder.Register();// Optionally, handle the progress/completed events of the taskregistration.Progress += registration_Progress;registration.Completed += registration_Completed;}

5、获取后台任务及取消注册后台任务，代码如下：

// AllTasks is a dictionary <Guid, IBackgroundTaskRegistration> so you can get back// to your registration by id or by posiiton, or select First if you only have one registration.var taskRegistration = BackgroundTaskRegistration.AllTasks.Values.FirstOrDefault();// We could then unregister the task, optionally cancelling any running instanceif (taskRegistration != null){    taskRegistration.Unregister(true);}// Release the progress/completed event subscriptionsregistration.Progress -= registration_Progress;registration.Completed -= registration_Completed;

 

tips：在Debugger 中我们可以这样触发后台任务，如图