Bewerken

Share via

Register a background task

  • Artikel

Important APIs

Learn how to create a function that can be re-used to safely register most background tasks.

This topic is applicable to both in-process background tasks and out-of-process background tasks. This topic assumes that you already have a background task that needs to be registered. (See Create and register a background task that runs out-of-process or Create and register an in-process background task for information about how to write a background task).

This topic walks through a utility function that registers background tasks. This utility function checks for existing registrations first before registering the task multiple times to avoid problems with multiple registrations, and it can apply a system condition to the background task. The walkthrough includes a complete, working example of this utility function.

Note  

Universal Windows apps must call RequestAccessAsync before registering any of the background trigger types.

To ensure that your Universal Windows app continues to run properly after you release an update, you must call RemoveAccess and then call RequestAccessAsync when your app launches after being updated. For more information, see Guidelines for background tasks.

Define the method signature and return type

This method takes in the task entry point, task name, a pre-constructed background task trigger, and (optionally) a SystemCondition for the background task. This method returns a BackgroundTaskRegistration object.

Important

taskEntryPoint - for background tasks that run in out of process, this must be constructed as the namespace name, '.', and the name of the class containing your background class. The string is case-sensitive. For example, if you had a namespace "MyBackgroundTasks" and a class "BackgroundTask1" that contained your background class code, the string for taskEntryPoint would be "MyBackgroundTasks.BackgroundTask1". If your background task runs in the same process as your app (i.e., an in-process background task) taskEntryPoint should not be set.

public static BackgroundTaskRegistration RegisterBackgroundTask(
                                                string taskEntryPoint,
                                                string name,
                                                IBackgroundTrigger trigger,
                                                IBackgroundCondition condition)
{
    
    // We'll add code to this function in subsequent steps.

}

BackgroundTaskRegistration^ MainPage::RegisterBackgroundTask(
                                             Platform::String ^ taskEntryPoint,
                                             Platform::String ^ taskName,
                                             IBackgroundTrigger ^ trigger,
                                             IBackgroundCondition ^ condition)
{
    
    // We'll add code to this function in subsequent steps.

}

Check for existing registrations

Check whether the task is already registered. It's important to check this because if a task is registered multiple times, it will run more than once whenever it’s triggered; this can use excess CPU and may cause unexpected behavior.

You can check for existing registrations by querying the BackgroundTaskRegistration.AllTasks property and iterating on the result. Check the name of each instance – if it matches the name of the task you’re registering, then break out of the loop and set a flag variable so that your code can choose a different path in the next step.

Note  Use background task names that are unique to your app. Ensure each background task has a unique name.

The following code registers a background task using the SystemTrigger we created in the last step:

public static BackgroundTaskRegistration RegisterBackgroundTask(
                                                string taskEntryPoint,
                                                string name,
                                                IBackgroundTrigger trigger,
                                                IBackgroundCondition condition)
{
    //
    // Check for existing registrations of this background task.
    //

    foreach (var cur in BackgroundTaskRegistration.AllTasks)
    {

        if (cur.Value.Name == name)
        {
            //
            // The task is already registered.
            //

            return (BackgroundTaskRegistration)(cur.Value);
        }
    }
    
    // We'll register the task in the next step.
}

BackgroundTaskRegistration^ MainPage::RegisterBackgroundTask(
                                             Platform::String ^ taskEntryPoint,
                                             Platform::String ^ taskName,
                                             IBackgroundTrigger ^ trigger,
                                             IBackgroundCondition ^ condition)
{
    //
    // Check for existing registrations of this background task.
    //
    
    auto iter   = BackgroundTaskRegistration::AllTasks->First();
    auto hascur = iter->HasCurrent;
    
    while (hascur)
    {
        auto cur = iter->Current->Value;
        
        if(cur->Name == name)
        {
            //
            // The task is registered.
            //
            
            return (BackgroundTaskRegistration ^)(cur);
        }
        
        hascur = iter->MoveNext();
    }
    
    // We'll register the task in the next step.
}

Register the background task (or return the existing registration)

Check to see if the task was found in the list of existing background task registrations. If so, return that instance of the task.

Then, register the task using a new BackgroundTaskBuilder object. This code should check whether the condition parameter is null, and if not, add the condition to the registration object. Return the BackgroundTaskRegistration returned by the BackgroundTaskBuilder.Register method.

Note  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. Note If you are registering a background task that runs in the same process as your app, send String.Empty or null for the taskEntryPoint parameter.

The following example either returns the existing task, or adds code that registers the background task (including the optional system condition if present):

public static BackgroundTaskRegistration RegisterBackgroundTask(
                                                string taskEntryPoint,
                                                string name,
                                                IBackgroundTrigger trigger,
                                                IBackgroundCondition condition)
{
    //
    // Check for existing registrations of this background task.
    //

    foreach (var cur in BackgroundTaskRegistration.AllTasks)
    {

        if (cur.Value.Name == taskName)
        {
            //
            // The task is already registered.
            //

            return (BackgroundTaskRegistration)(cur.Value);
        }
    }

    //
    // Register the background task.
    //

    var builder = new BackgroundTaskBuilder();

    builder.Name = name;

    // in-process background tasks don't set TaskEntryPoint
    if ( taskEntryPoint != null && taskEntryPoint != String.Empty)
    {
        builder.TaskEntryPoint = taskEntryPoint;
    }
    builder.SetTrigger(trigger);

    if (condition != null)
    {
        builder.AddCondition(condition);
    }

    BackgroundTaskRegistration task = builder.Register();

    return task;
}

BackgroundTaskRegistration^ MainPage::RegisterBackgroundTask(
                                             Platform::String ^ taskEntryPoint,
                                             Platform::String ^ taskName,
                                             IBackgroundTrigger ^ trigger,
                                             IBackgroundCondition ^ condition)
{

    //
    // Check for existing registrations of this background task.
    //

    auto iter   = BackgroundTaskRegistration::AllTasks->First();
    auto hascur = iter->HasCurrent;

    while (hascur)
    {
        auto cur = iter->Current->Value;

        if(cur->Name == name)
        {
            //
            // The task is registered.
            //

            return (BackgroundTaskRegistration ^)(cur);
        }

        hascur = iter->MoveNext();
    }

    //
    // Register the background task.
    //

    auto builder = ref new BackgroundTaskBuilder();

    builder->Name = name;
    builder->TaskEntryPoint = taskEntryPoint;
    builder->SetTrigger(trigger);

    if (condition != nullptr) {
        
        builder->AddCondition(condition);
    }

    BackgroundTaskRegistration ^ task = builder->Register();

    return task;
}

Complete background task registration utility function

This example shows the completed background task registration function. This function can be used to register most background tasks, with the exception of networking background tasks.

//
// Register a background task with the specified taskEntryPoint, name, trigger,
// and condition (optional).
//
// taskEntryPoint: Task entry point for the background task.
// taskName: A name for the background task.
// trigger: The trigger for the background task.
// condition: Optional parameter. A conditional event that must be true for the task to fire.
//
public static BackgroundTaskRegistration RegisterBackgroundTask(string taskEntryPoint,
                                                                string taskName,
                                                                IBackgroundTrigger trigger,
                                                                IBackgroundCondition condition)
{
    //
    // Check for existing registrations of this background task.
    //

    foreach (var cur in BackgroundTaskRegistration.AllTasks)
    {

        if (cur.Value.Name == taskName)
        {
            //
            // The task is already registered.
            //

            return (BackgroundTaskRegistration)(cur.Value);
        }
    }

    //
    // Register the background task.
    //

    var builder = new BackgroundTaskBuilder();

    builder.Name = taskName;
    builder.TaskEntryPoint = taskEntryPoint;
    builder.SetTrigger(trigger);

    if (condition != null)
    {

        builder.AddCondition(condition);
    }

    BackgroundTaskRegistration task = builder.Register();

    return task;
}

//
// Register a background task with the specified taskEntryPoint, name, trigger,
// and condition (optional).
//
// taskEntryPoint: Task entry point for the background task.
// taskName: A name for the background task.
// trigger: The trigger for the background task.
// condition: Optional parameter. A conditional event that must be true for the task to fire.
//
BackgroundTaskRegistration^ MainPage::RegisterBackgroundTask(Platform::String ^ taskEntryPoint,
                                                             Platform::String ^ taskName,
                                                             IBackgroundTrigger ^ trigger,
                                                             IBackgroundCondition ^ condition)
{

    //
    // Check for existing registrations of this background task.
    //

    auto iter   = BackgroundTaskRegistration::AllTasks->First();
    auto hascur = iter->HasCurrent;

    while (hascur)
    {
        auto cur = iter->Current->Value;

        if(cur->Name == name)
        {
            //
            // The task is registered.
            //

            return (BackgroundTaskRegistration ^)(cur);
        }

        hascur = iter->MoveNext();
    }


    //
    // Register the background task.
    //

    auto builder = ref new BackgroundTaskBuilder();

    builder->Name = name;
    builder->TaskEntryPoint = taskEntryPoint;
    builder->SetTrigger(trigger);

    if (condition != nullptr) {

        builder->AddCondition(condition);
    }

    BackgroundTaskRegistration ^ task = builder->Register();

    return task;
}