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Run a background task on a timer

  • Bài viết

Learn how to use the TimeTrigger to schedule a one-time background task, or run a periodic background task.

See Scenario4 in the Background activation sample to see an example of how to implement the time triggered background task described in this topic.

This topic assumes that you have a background task that needs to run periodically, or at a specific time. If you don't already have a background task, there is a sample background task at BackgroundActivity.cs. Or, follow the steps in Create and register an in-process background task or Create and register an out-of-process background task to create one.

Create a time trigger

Create a new TimeTrigger. The second parameter, OneShot, specifies whether the background task will run only once or keep running periodically. If OneShot is set to true, the first parameter (FreshnessTime) specifies the number of minutes to wait before scheduling the background task. If OneShot is set to false, FreshnessTime specifies the frequency at which the background task will run.

The built-in timer for Universal Windows Platform (UWP) apps that target the desktop or mobile device family runs background tasks in 15-minute intervals. (The timer runs in 15-minute intervals so that the system only needs to wake once every 15 minutes to wake up the apps that have requested TimerTriggers--which saves power.)

  • If FreshnessTime is set to 15 minutes and OneShot is true, the task will be scheduled to run once starting between 15 and 30 minutes from the time it is registered. If it is set to 25 minutes and OneShot is true, the task will be scheduled to run once starting between 25 and 40 minutes from the time it is registered.

  • If FreshnessTime is set to 15 minutes and OneShot is false, the task will be scheduled to run every 15 minutes starting between 15 and 30 minutes from the time it is registered. If it is set to n minutes and OneShot is false, the task will be scheduled to run every n minutes starting between n and n + 15 minutes after it is registered.

Note

If FreshnessTime is set to less than 15 minutes, an exception is thrown when attempting to register the background task.

For example, this trigger will cause a background task to run once an hour.

TimeTrigger hourlyTrigger = new TimeTrigger(60, false);

Windows::ApplicationModel::Background::TimeTrigger hourlyTrigger{ 60, false };

TimeTrigger ^ hourlyTrigger = ref new TimeTrigger(60, false);

(Optional) Add a condition

You can create a background task condition to control when the task runs. A condition prevents the background task from running until the condition is met. For more information, see Set conditions for running a background task.

In this example the condition is set to UserPresent so that, once triggered, the task only runs once the user is active. For a list of possible conditions, see SystemConditionType.

SystemCondition userCondition = new SystemCondition(SystemConditionType.UserPresent);

Windows::ApplicationModel::Background::SystemCondition userCondition{
    Windows::ApplicationModel::Background::SystemConditionType::UserPresent };

SystemCondition ^ userCondition = ref new SystemCondition(SystemConditionType::UserPresent);

For more in-depth information on conditions and types of background triggers, see Support your app with background tasks.

Call RequestAccessAsync()

Before registering the ApplicationTrigger background task, call RequestAccessAsync to determine the level of background activity the user allows because the user may have disabled background activity for your app. See Optimize background activity for more information about the ways users can control the settings for background activity.

var requestStatus = await Windows.ApplicationModel.Background.BackgroundExecutionManager.RequestAccessAsync();
if (requestStatus != BackgroundAccessStatus.AlwaysAllowed)
{
    // Depending on the value of requestStatus, provide an appropriate response
    // such as notifying the user which functionality won't work as expected
}

Register the background task

Register the background task by calling your background task registration function. For more information on registering background tasks, and to see the definition of the RegisterBackgroundTask() method in the sample code below, see Register a background task.

Important

For background tasks that run in the same process as your app, do not set entryPoint. For background tasks that run in a separate process from your app, set entryPoint to be the namespace, '.', and the name of the class that contains your background task implementation.

string entryPoint = "Tasks.ExampleBackgroundTaskClass";
string taskName   = "Example hourly background task";

BackgroundTaskRegistration task = RegisterBackgroundTask(entryPoint, taskName, hourlyTrigger, userCondition);

std::wstring entryPoint{ L"Tasks.ExampleBackgroundTaskClass" };
std::wstring taskName{ L"Example hourly background task" };

Windows::ApplicationModel::Background::BackgroundTaskRegistration task{
    RegisterBackgroundTask(entryPoint, taskName, hourlyTrigger, userCondition) };

String ^ entryPoint = "Tasks.ExampleBackgroundTaskClass";
String ^ taskName   = "Example hourly background task";

BackgroundTaskRegistration ^ task = RegisterBackgroundTask(entryPoint, taskName, hourlyTrigger, userCondition);

Background task registration parameters are validated at the time of registration. An error is returned if any of the registration parameters are invalid. Ensure that your app gracefully handles scenarios where background task registration fails - if instead your app depends on having a valid registration object after attempting to register a task, it may crash.

Manage resources for your background task

Use BackgroundExecutionManager.RequestAccessAsync to determine if the user has decided that your app’s background activity should be limited. Be aware of your battery usage and only run in the background when it is necessary to complete an action that the user wants. See Optimize background activity for more information about the ways users can control the settings for background activity.

  • Memory: Tuning your app's memory and energy use is key to ensuring that the operating system will allow your background task to run. Use the Memory Management APIs to see how much memory your background task is using. The more memory your background task uses, the harder it is for the OS to keep it running when another app is in the foreground. The user is ultimately in control of all background activity that your app can perform and has visibility on the impact your app has on battery use.
  • CPU time: Background tasks are limited by the amount of wall-clock usage time they get based on trigger type.

See Support your app with background tasks for the resource constraints applied to background tasks.

Remarks

Starting with Windows 10, it is no longer necessary for the user to add your app to the lock screen in order to utilize background tasks.

A background task will only run using a TimeTrigger if you have called RequestAccessAsync first.