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 here
SystemTrigger 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 name
taskBuilder.TaskEntryPoint = typeof(MyBackgroundTask.TheTask).FullName;

BackgroundTaskRegistration registration = taskBuilder.Register();

// Optionally, handle the progress/completed events of the task
registration.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 instance
if (taskRegistration != null)
{
    taskRegistration.Unregister(true);
}

// Release the progress/completed event subscriptions
registration.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