Tag Archives: PeriodicTask

EvenTiles Until Now (A Windows Phone Development Series)

Looking at all the different topics that the EvenTiles series covers, I think it makes sense to at least have a table of contents for the series so far. For instance, if you are interested in reading something about tombstoning, it is not necessary to read through the entire series (although of course I invite you to do so anyway). At this moment, EvenTiles explains how to use Windows Phone Tiles, including Secondary Tiles. It also show how you can update content on your Tiles locally by using a Background Agent. Also don’t forget that all source code is available and each individual episode of EvenTiles contains a demo on video as well. The latest sample source code can be downloaded.

Here is the table of contents for the first 14 parts of EvenTiles:

Introducing the EvenTiles application

  • Creating a new project in Visual Studio 2010 Express for Windows Phone
  • Defining an Application Tile from within the WMAppManifest.xml file
  • Using the emulator to run the application

PageNavigation and ApplicationBar

  • Using Expression Blend to create an Application Bar
  • Navigating to different pages by using the NavigationService

Using the Silverlight for Windows Phone Toolkit in your application

  • Defining page transitions in XAML and using them in your application
  • Using styles in XAML
  • Declaring and using a ToggleSwitch control

Creating the Settings Page

  • Designing a Windows Phone page using Expression Blend
  • Executing code as a result of page navigation (using OnNavigatingTo and OnNavigatingFrom)
  • Using event handlers to execute code on user interaction with UI elements

Using IsolatedStorage

  • Storing data in between different times an application executes
  • Introducing the application life cycle
  • Using IsolatedStorageSettings to persist information

Using the Isolated Storage Explorer Tool

  • Examining the contents of our application’s IsolatedStorage

Fast Application Switching and Tombstoning

  • A Windows Phone Application’s life cycle
  • Saving application state when moved to the background
  • Restoring application state when returning to the foreground

More on Tombstoning

  • Maintaining Page State
  • Restoring a page to exactly the same situation it was when the application moved to the background
  • Restoring focus of input UI elements

Creating a Secondary Tile

  • Creating a Secondary Tile inside your application
  • Navigating from the Secondary Tile to the application
  • Determine if a Secondary Tile is currently visible on the Start screen of a Windows Phone through a lambda expression
  • Setting the front and back contents of a Secondary Tile

Background Agents

  • Different types of Background Agents
  • Properly creating a PeriodicTask
  • Starting and stopping a PeriodicTask
  • Relation between an Application and a Background Agent

Debugging Background Agents

  • Using conditional compilation to test Background Agents
  • Properly using the LaunchForTest method
  • Using the debugger to debug a PeriodicTask

The lifetime of PeriodicTask

  • Rescheduling a PeriodicTask
  • Exceptions that might be thrown when scheduling a PeriodicTask

Communication between an Application and its PeriodicTask

  • Passing data between an application and a PeriodicTask
  • Protecting variables against simultaneous access by different threads
  • Using a Mutex to synchronize threads

Exchanging data between an Application and its PeriodicTask

  • Using a file in IsolatedStorage to exchange data between an application and a PeriodicTask

Even though there are many more episodes of EvenTiles planned, attention will now move beyond tiles as we focus on using built-in applications and accessing device hardware.

EvenTiles from Start to Finish–Part 14

In the previous episode of this series about how to develop a Windows Phone application from scratch we found out that special care must be taken when data must be passed between an application and its PeriodicTask. You learned how data can be protected against mutual access by using a Mutex object. You also learned that you cannot pass data directly from one to another, because the application and its PeriodicTask execute inside different processes.

In this episode of EvenTiles you will learn how you can make use of IsolatedStorage to pass data from the application to the PeriodicTask. Since we already created a separate project in the EvenTiles solution that is taking care of passing data, we can simply modify functionality in that project (EvenTilesComm) to use a file to pass data between the application and its PeriodicTask. Data protection against mutual access is already in place, so we can concentrate on file access. Hopefully the design decision in part 13 to make use of private methods that are called each time we access a public property inside the TileData class starts to make sense now.

What we want to achieve is the following:

  • From inside the application we can store a string containing content for the backside of a Secondary Tile in a file at any time
  • Our PeriodicTask executes approximately once per 30 minutes and it either displays the string passed by the application on the backside of the Secondary Tile or it displays a default string that is defined inside the PeriodicTask
  • The content of the backside of the Secondary Tile needs to toggle each time the PeriodicTask executes
  • If the EvenTiles user modifies the string to be displayed inside the Settings page, it will immediately be displayed on the Secondary Tile (if it exists)

In order to pass data between the application and the PeriodicTask we will extend the private retrieve / store methods inside the TileData class by adding a call to a couple of other private methods. Those new private methods will use a file in IsolatedStorage to read / write data from, meaning we can effectively pass data between the application and its PeriodicTask.

Properties to access Tile Data
  1. private static string RetrieveSecondaryBackContent()
  2. {
  3.     mtx.WaitOne();
  4.  
  5.     try
  6.     {
  7.         RetrieveTileContent();
  8.         return secondaryBackContent;
  9.     }
  10.     finally
  11.     {
  12.         mtx.ReleaseMutex();
  13.     }
  14. }
  15.  
  16. private static void StoreSecondaryBackContent(string content)
  17. {
  18.     mtx.WaitOne();
  19.  
  20.     try
  21.     {
  22.         secondaryBackContent = content;
  23.         PersistTileContent();
  24.     }
  25.     finally
  26.     {
  27.         mtx.ReleaseMutex();
  28.     }
  29. }

A new string to be displayed on the back side of a Secondary Tile will be stored by the application each time users modify their own tile text in the Settings Page of the EvenTiles application (something that will also be done initially when the application starts). The PeriodicTask simply retrieves that data (since now the data is persisted in a file this will work properly) and displays it on the back side of the Secondary Tile. Each time data needs to be stored a new file is created in IsolatedStorage or an existing file is overwritten. Each time data needs to be retrieved, we check if a file containing that data exists. There are small ways to optimize data access in the TileData class, because right now we simply read / write all file content when retrieving / storing single property values. However, this approach simplifies the code and the overhead with only two different variables is very small.

Using IsolatedStorage
  1. private const string contentFileName = "EvenTileContent.txt";
  2.  
  3. private static void PersistTileContent()
  4. {
  5.     using (var store = IsolatedStorageFile.GetUserStoreForApplication())
  6.     {
  7.         using (var contentStream = new StreamWriter(store.CreateFile(contentFileName)))
  8.         {
  9.             contentStream.WriteLine(showDefaultSecondaryBackContent.ToString());
  10.             contentStream.WriteLine(secondaryBackContent);
  11.         }
  12.     }
  13. }
  14.  
  15. private static void RetrieveTileContent()
  16. {
  17.     using (var store = IsolatedStorageFile.GetUserStoreForApplication())
  18.     {
  19.         if (store.FileExists(contentFileName))
  20.         {
  21.             using (var contentStream = new StreamReader(store.OpenFile(contentFileName, FileMode.Open)))
  22.             {
  23.                 showDefaultSecondaryBackContent = Convert.ToBoolean(contentStream.ReadLine());
  24.                 secondaryBackContent = contentStream.ReadToEnd();
  25.             }
  26.         }
  27.     }
  28. }

With the way we organized the functionality inside the TileData class in the previous episode of this development series, these are the only new methods necessary in order to pass data. When the user installs a Secondary Tile for the EvenTiles application and modifies its back string content through the Settings page, this is the result (changing the back content of the Secondary Tile every 30 minutes):

image

The following video shows EvenTiles in action with proper transfer of data between the application and its PeriodicTask.

Data transfer between an Application and a PeriodicTask through IsolatedStorage

To be able to experiment with this working implementation of EvenTiles, especially to understand how the application interacts with the PeriodicTask through a file in IsolatedStorage, the sample code is available for dowload here.

Right now we have the basic functionality of EvenTiles more or less ready, although the About Page still needs to get some content. That is something we will work on in the next part of EvenTiles. After that we will cover much more in the upcoming episodes of EvenTiles including but not limited to

  • using ads in the application
  • retrieving location information inside the application
  • taking pictures
  • modifying pictures
  • using alarms and notifications
  • using Visual Studio’s integrated Performance Analysis to find performance bottlenecks inside the application
  • submitting the application for certification

EvenTiles will continue soon so stay tuned for the next episode.

EvenTilesIf you want to see EvenTiles already in action on your Windows Phone, you can also install the latest version from Marketplace. Remember that this application is not meant to be extremely useful, although it contains similar functionality that “serious” applications have. Just go ahead and get your free copy of EvenTiles from Marketplace at this location: http://www.windowsphone.com/en-US/search?q=EvenTiles (or search on your phone for EvenTiles in the Marketplace application).

EvenTiles from Start to Finish–Part 13

In the previous episode of this series about how to develop a Windows Phone application from scratch we fixed some issues to properly start and renew a PeriodicTask. Now we are ready to find out how an application can communicate with its PeriodicTask.

In today’s EvenTiles episode, you will learn why you cannot simply use a variable to communicate between the application and the PeriodicTask. In my opinion that is important, because understanding why some things are not working makes you a better Windows Phone developer and decreases the chance of having hard to trace bugs inside your own applications. In order to separate the functionality to transfer data between the application and its PeriodicTask, we are going to create a new project inside the EvenTiles solution.

Dump1

The project will be of type Windows Phone Class Library and we will call it EvenTilesComm. In its initial version, this class library will contain one single class with the name TileData. It is used to store the text that the user can enter in the Settings Page of the EvenTiles application in order to allow the PeriodicTask to display this string on the back of the Secondary Tile once it is created. In order to vary the text on the back of the Secondary Tile, TileData will also contain a boolean variable indicating which string needs to be displayed on the Secondary Tile’s backside (either the string passed by the application or a default string that is defined inside the PeriodicTask).

screenTo be able to access TileData from within both the EvenTiles application and the PeriodicTask, it contains two static properties. A string property to store / retrieve the message that is defined through the Settings Page and a boolean property that defines which string needs to be displayed from within the PeriodicTask. The string property is always written by the application and read by the PeriodicTask. The boolean variable is both written by the application and the PeriodicTask but only read by the PeriodicTask. Because of the fact that the PeriodicTask and the application are running inside their own, separate process in different threads, and since they are running completely unsynchronized from each other, there is a possibility that they both want to access one of the properties of TileData at the same time. To prevent this potential dangerous situation (dangerous because it can leave the properties in an undefined state when one of the two different threads want to access them while the other is updating them), both properties are protected by a Mutex synchronization object.

NOTE: The following (initial) implementation of the TileData class will not work properly, but it is shown here to help you understand what it means that an application and its PeriodicTask belong to each other, but are completely separated from each other at the same time. You will have to wait a little while for a working solution, because that will be introduced in the next episode of EvenTiles.

Threadsafe Data Access
  1. public class TileData
  2. {
  3.     private static Mutex mtx = new Mutex(false, "MtxSync");
  4.  
  5.     private static string secondaryBackContent;
  6.     private static bool showDefaultSecondaryBackContent;
  7.  
  8.  
  9.     public static string SecondaryBackContent
  10.     {
  11.         get
  12.         {
  13.             return RetrieveSecondaryBackContent();
  14.         }
  15.         set
  16.         {
  17.             StoreSecondaryBackContent(value);
  18.         }
  19.     }
  20.  
  21.     public static bool ShowDefaultSecondaryBackContent
  22.     {
  23.         get
  24.         {
  25.             return RetrieveShowDefaultSecondaryBackContent();
  26.         }
  27.         set
  28.         {
  29.             StoreShowDefaultSecondaryBackContent(value);
  30.         }
  31.     }
  32.  
  33.     private static string RetrieveSecondaryBackContent()
  34.     {
  35.         mtx.WaitOne();
  36.  
  37.         try
  38.         {
  39.             return secondaryBackContent;
  40.         }
  41.         finally
  42.         {
  43.             mtx.ReleaseMutex();
  44.         }
  45.     }
  46.  
  47.     private static void StoreSecondaryBackContent(string content)
  48.     {
  49.         mtx.WaitOne();
  50.  
  51.         try
  52.         {
  53.             secondaryBackContent = content;
  54.         }
  55.         finally
  56.         {
  57.             mtx.ReleaseMutex();
  58.         }
  59.     }
  60.  
  61.     private static bool RetrieveShowDefaultSecondaryBackContent()
  62.     {
  63.         mtx.WaitOne();
  64.  
  65.         try
  66.         {
  67.             return showDefaultSecondaryBackContent;
  68.         }
  69.         finally
  70.         {
  71.             mtx.ReleaseMutex();
  72.         }
  73.     }
  74.  
  75.     private static void StoreShowDefaultSecondaryBackContent(bool showDefaultContent)
  76.     {
  77.         mtx.WaitOne();
  78.  
  79.         try
  80.         {
  81.             showDefaultSecondaryBackContent = showDefaultContent;
  82.         }
  83.         finally
  84.         {
  85.             mtx.ReleaseMutex();
  86.         }
  87.     }
  88. }

If you look at the source code of the TileData class, you can see that we are using a two properties through which we are calling a few private methods to actually store or retrieve data. You will also see that we define a single Mutex inside the TileData class. This Mutex has a name, meaning it can be used to synchronize threads over multiple processes. This is important, since EvenTiles and its PeriodicTask will execute in separate processes. If we take a look at one of the individual retrieval methods, you will see how the single Mutex that is defined in TileData is used.

Retrieving data
  1. private static string RetrieveSecondaryBackContent()
  2. {
  3.     mtx.WaitOne();
  4.  
  5.     try
  6.     {
  7.         return secondaryBackContent;
  8.     }
  9.     finally
  10.     {
  11.         mtx.ReleaseMutex();
  12.     }
  13. }

In this method, we access the variable secondaryBackContent only when the method WaitOne on our Mutex returns. When WaitOne returns, our method has exclusive access to the variable secondaryBackContent until we call the method ReleaseMutex on our Mutex. At that moment, another thread that is waiting for the same Mutex will be granted access. Every method that uses of a Mutex to get access to protected data must under all circumstances call ReleaseMutex when it is done. That is the reason why we are adding a try / finally block. The code in the finally block will execute, even when exceptions are thrown. In this way, we prevent our application from a potential deadlock situations (where multiple threads are waiting on each other to free up a synchronization object that is never being freed).

The way data is protected for EvenTiles works properly, which can be verified by debugging the application and deliberately skipping a call to ReleaseMutex from within the EvenTiles application (which is fully explained in the accompanying video as well). Doing so will result in the PeriodicTask waiting forever for the Mutex, because it will never be released by the application. This at least proves that both EvenTiles and its PeriodicTask are making use of the same Mutex that they both need to own before being able to access variables.

There is still a potential problem, because we are calling the WaitOne method without parameters, which means that we will wait indefinitely until the Mutex becomes available. In our simple example this is sufficient, but in a real application you most likely want to wait for a specific time and take action upon timeouts.

However, there is a way bigger problem with the current implementation of our newly created TileData class. We already saw that the Mutex is properly shared between EvenTiles and its PeriodicTask, but what about the public properties. The following screen dump shows you what is wrong with the TileData class right now. Even though the variable secondaryBackContent is set by the EvenTiles application, it seems that it is not initialized when the PeriodicTask retrieves the variable (both through the appropriate properties).

image image

Since our EvenTiles application and its PeriodicTask are running in separate processes, they are completely separated from each other, meaning that they get their own copies of variables they both want to access, even though these variables are defined in a separate project. This is a good thing (even though we have to solve the problem), because otherwise all variables used by every process on the phone would need a unique name. The following video shows EvenTiles in action in combination with its PeriodicTask. It shows how the Mutex works and how passing data fails.

Protecting variables against simultaneous access by multiple threads

To be able to experiment with this wrong implementation of EvenTiles, especially to understand how the application interacts with the PeriodicTask, the sample code is available for dowload here.

Instead of using ‘simple’ variables to pass data between EvenTiles and its PeriodicTask, we will have to make use of IsolatedStorage and have them share data through a file. That will be the topic of the next episode of EvenTiles.

EvenTiles from Start to Finish–Part 12

In the previous episode of this series about how to develop a Windows Phone application from scratch we talked about debugging a PeriodicTask. Before digging deeper into exchanging data between an application and its PeriodicTask, we need to talk a little more about scheduling the PeriodicTask and about expiration of a PeriodicTask.

Going back to part 10 of the EvenTiles series, the PeriodicTask was created inside the MainPage.xaml.cs file in a method called StartPeriodicAgent. This private method takes care of stopping a currently active agent before creating a new one. It also handles an exception that might be thrown when an attempt is made to start a new PeriodicTask while the user has disabled background processing for the EvenTiles application, which they can do from inside the Phone Settings.

However, after submitting EvenTiles to Marketplace, I noticed an exceptionally high crash count in the application (as you can see from my AppHub dashboard). Since I want to share all details about developing this application, but also about the application’s life cycle, it is only fair to share the number of crashes to date, a number I am not proud off.

image

For starters, the first version of EvenTiles called LaunchForTest in release mode which results in a thrown exception as shows up in the call stack of various crash incidents. Also, the application does not count on other applications making use of background agents on a particular device. To preserve memory, but mainly battery power on the device, the number of background agents that can run at the same time is limited. It varies depending on the hardware capabilities of the device and might differ between devices, but it is a small number and therefore the limit can easily be reached. When this limit is reached, a SchedulerServiceException will be thrown whenever you attempt to add a periodic task. Of course you much handle this exception to prevent your application against crashing, since you are adding your periodic task from inside your application.

So, to create a periodic task, the following code should be used (which is an extension to the code that we used in part 10 of the series):

Correctly adding the agent
  1. private void StartPeriodicAgent()
  2. {
  3.     RemovePeriodicAgent();
  4.  
  5.     var evenTilesPeriodicTask = new PeriodicTask(evenTilesPeriodicTaskName);
  6.     evenTilesPeriodicTask.Description = "EvenTilesPeriodicTask";
  7.  
  8.     try
  9.     {
  10.         ScheduledActionService.Add(evenTilesPeriodicTask);
  11. #if DEBUG_TASK
  12.         ScheduledActionService.LaunchForTest(evenTilesPeriodicTaskName, TimeSpan.FromSeconds(30));
  13. #endif
  14.         App.PeriodicTaskScheduled = true;
  15.     }
  16.     catch (InvalidOperationException ex)
  17.     {
  18.         if (ex.Message.Contains("BNS Error: The action is disabled"))
  19.         {
  20.             MessageBox.Show("Background agents for this application are disabled.");
  21.         }
  22.     }
  23.     catch (SchedulerServiceException)
  24.     {
  25.         MessageBox.Show("Can't schedule your background agent. There might be too many background agents enabled. You can disable background agents through the phone's settings application.");
  26.     }
  27. }

If you take a look at how a periodic task is created you will also see that a App.PeriodicTaskScheduled is set to true. This static property is maintained in the App.xaml.cs file, its value is stored / retrieved from the IsolatedStorageSettings (see part 5 of EvenTiles for more information).

The reason to add this boolean property is to find out if the application did schedule a PeriodicTask. If a PeriodicTask was added for the application, you must realize that the PeriodicTask will age and eventually it will expire unless it is rescheduled by the application. A PeriodicTask will expire if it has not been rescheduled for 14 days from the time it was scheduled. Since the application is responsible for adding a PeriodicTask, it means that the PeriodicTask will only continue to run if the application is activated at least once every two weeks and if the application makes sure to reschedule the PeriodicTask. The approach that EvenTiles takes right now is to check if at some time a PeriodicTask was scheduled. If so, it will be rescheduled as soon as the EvenTiles’ MainPage is created.

NOTE: We could have used another way to determine if a PeriodicTask was scheduled, for instance by verifying if a SecondaryTile is currently available on the StartScreen.

Making use of IsolatedStorageSettings, we will add logic to the app.xaml.cs file to properly store and retrieve the App.PeriodicTaskScheduled property when the application is started, activated, deactivated and terminated.

To begin, a new property and a new const string to identify the value in the IsoloatedStorageSettings are created in app.xaml.cs as follows:

  1. public const string keyPTScheduled = "K_PTS";
  2.  
  3. public static bool PeriodicTaskScheduled { get; set; }

In the constructor, the PeriodicTaskScheduled property is explicitly initialized to false. Finally, the value of PeriodicTaskScheduled is stored / retrieved in the various application life cycle events:

Store/Retrieve property value
  1. // Code to execute when the application is launching (eg, from Start)
  2. // This code will not execute when the application is reactivated
  3. private void Application_Launching(object sender, LaunchingEventArgs e)
  4. {
  5.     if (appSettings.Contains(keyPTScheduled))
  6.     {
  7.         PeriodicTaskScheduled = (bool)appSettings[keyPTScheduled];
  8.     }
  9.  
  10.     if (!appSettings.Contains(keyActSecBackContent))
  11.     {
  12.         appSettings[keyActSecBackContent] = DefaultSecBackContent;
  13.     }
  14.     ActualSecBackContent = (string)appSettings[keyActSecBackContent];
  15. }
  16.  
  17. // Code to execute when the application is activated (brought to foreground)
  18. // This code will not execute when the application is first launched
  19. private void Application_Activated(object sender, ActivatedEventArgs e)
  20. {
  21.     if (!e.IsApplicationInstancePreserved)
  22.     {
  23.         PeriodicTaskScheduled = (bool)appSettings[keyPTScheduled];
  24.         ActualSecBackContent = (string)appSettings[keyActSecBackContent];
  25.     }
  26. }
  27.  
  28. // Code to execute when the application is deactivated (sent to background)
  29. // This code will not execute when the application is closing
  30. private void Application_Deactivated(object sender, DeactivatedEventArgs e)
  31. {
  32.     appSettings[keyPTScheduled] = PeriodicTaskScheduled;
  33.     appSettings[keyActSecBackContent] = ActualSecBackContent;
  34. }
  35.  
  36. // Code to execute when the application is closing (eg, user hit Back)
  37. // This code will not execute when the application is deactivated
  38. private void Application_Closing(object sender, ClosingEventArgs e)
  39. {
  40.     appSettings[keyPTScheduled] = PeriodicTaskScheduled;
  41.     appSettings[keyActSecBackContent] = ActualSecBackContent;
  42. }

Finally, we will use this property inside the MainPage.xaml.cs file when the page is constructed to determine if we need to reschedule a PeriodicTask:

Rescheduling PeriodicTask?
  1. public MainPage()
  2. {
  3.     InitializeComponent();
  4.     if (App.PeriodicTaskScheduled)
  5.     {
  6.         StartPeriodicAgent();
  7.     }
  8. }

Each time the PeriodicTask is scheduled, the App.PeriodicTaskProperty is set to true and a Secondary Tile is created. If the Secondary Tile is removed, the App.PeriodicTaskProperty is set to false as well.

The following video shows you how to modify the existing EvenTiles project to improve the way the PeriodicTask is scheduled as well as the functionality needed to reschedule the PeriodicTask:

Rescheduling a PeriodicTask

If you want to take a look at the source code that we have available for EvenTiles so far, you can download the entire solution belonging to this episode:

Download EvenTiles Episode 12 Source Code

After downloading and unzipping the sample code, you can open the EvenTiles solution in Visual Studio 2010 Express for Windows Phone. You must have the Windows Phone SDK 7.1 installed on your development system to do so. If you have a developer unlocked phone you can deploy the application to your phone, if you don’t have a developer unlocked phone you can still experiment with this application inside the emulator.

EvenTilesIf you want to see EvenTiles already in action on your Windows Phone, you can also install the latest version from Marketplace. Remember that this application is not meant to be extremely useful, although it contains similar functionality that “serious” applications have. Just go ahead and get your free copy of EvenTiles from Marketplace at this location: http://www.windowsphone.com/en-US/search?q=EvenTiles (or search on your phone for EvenTiles in the Marketplace application).

When EvenTiles continues with part 13, you will learn how to exchange data between an application and its background agent.

EvenTiles from Start to Finish–Part 11

In the previous episode of this series about how to develop a Windows Phone application from scratch we started talking about using a PeriodicTask to perform background processing for our application. You learned how to create a Windows Phone Scheduled Task Agent, a separate project that became part of the EvenTiles solution to host a PeriodicTask. You also learned how the PeriodicTask relates to the EvenTiles to enable background processing for the application. In this episode of EvenTiles we will take a look at how to debug the PeriodicTask.

When you have added a PeriodicTask to your application you may have noticed that the behavior of Visual Studio’s debugger did change. Instead of detaching the debugger if you terminate the application, the debugger remains attached. This allows you to debug the PeriodicTask when the application is no longer executing. This is of course important because typically the PeriodicTask executes without our application being active. Under normal circumstances, a PeriodicTask will run once every 30 minutes for a few seconds. This schedule can not be changed for released applications and is determined by the operating system. Of course debugging a PeriodicTask would be unproductive in this way. Therefore there is a possibility to run your PeriodicTask more frequent under test.

If you take a look at the ScheduledActionService class, you will see that it contains a static method called LaunchForTest. This method allows you to schedule a Periodic Task to run at the time you specify. Since you will only use this method for testing Periodic Tasks, draining the battery by frequently executing code in the background is not really an issue. However, you will must not forget to remove the call to LaunchForTest in your production code. You can for instance do this by making use of conditional compilation. What I like to do is to define a separate symbol (to be used in combination with DEBUG) to control execution of LaunchForTest. Usually in Debug mode you want to test your Periodic Task frequently, but by defining a separate symbol, you can also easily disable calling LaunchForTest, even in Debug mode. The following code fragment defines the DEBUG_TASK constant:

Conditionally defined constant
  1. #if DEBUG
  2. #define DEBUG_TASK
  3. #endif

Since this constant is not defined in Release mode, you don’t have to worry about accidentally adding a call to LaunchForTest in your released application. If you want to test your Periodic Agent with a normal schedule in Debug mode, you can simply change the name of the constant, for instance into NDEBUG_TASK. To make sure that the Periodic Agent will be called more frequent then usual, you can make use of the following code (inside the #if and #endif directives):

Calling Periodic Agent faster
  1. private void StartPeriodicAgent()
  2. {
  3.     RemovePeriodicAgent();
  4.  
  5.     evenTilesPeriodicTask = new PeriodicTask(evenTilesPeriodicTaskName);
  6.     evenTilesPeriodicTask.Description = "EvenTilesPeriodicTask";
  7.  
  8.     try
  9.     {
  10.         ScheduledActionService.Add(evenTilesPeriodicTask);
  11. #if DEBUG_TASK
  12.         ScheduledActionService.LaunchForTest(evenTilesPeriodicTaskName, TimeSpan.FromSeconds(30));
  13. #endif
  14.     }
  15.     catch (InvalidOperationException ex)
  16.     {
  17.         if (ex.Message.Contains("BNS Error: The action is disabled"))
  18.         {
  19.             MessageBox.Show("Background agents for this application are disabled.");
  20.         }
  21.     }
  22. }

As a result, the Periodic Task will now be executed approximately 30 seconds after this statement is executed. After that, the Periodic Task will execute on its normal 30 minute schedule, unless you add the call to LaunchForTest in the DoInvoke method of the Periodic Task as well:

Relaunching Periodic Agent
  1. protected override void OnInvoke(ScheduledTask task)
  2. {
  3.     //TODO: Add code to perform your task in background
  4.  
  5. #if DEBUG_TASK
  6.     ScheduledActionService.LaunchForTest(task.Name, TimeSpan.FromSeconds(30));
  7. #endif
  8.  
  9.     NotifyComplete();
  10. }

The following video shows you how to use the LaunchForTest method inside the EvenTiles application and how to make use of the Visual Studio debugger to test the Periodic Agent:

Debugging a Windows Phone Application’s PeriodicTask using LaunchForTest.

Of course the functionality of the Periodic Agent is still to be determined. In part 12 of EvenTiles we will work on adding functionality for the Periodic Agent.

EvenTiles from Start to Finish–Part 10

In the previous episode of this series about how to develop a Windows Phone application from scratch we talked about creating a Secondary Tile programmatically inside our application and showing different content on both the front and the back of the Secondary Tile.

This time we will introduce the concept of Background Agents for Windows Phone. A Background Agent is a piece of application code that can execute, even when the application is not running. Background Agents can be used to periodically perform some actions. Depending on the type of action and the type of scheduling, Background Agents can be of type PeriodicTask (which runs regularly for short amounts of time) or of type ResourceIntensiveTask (which runs for a longer amount of time, but not on a regular interval). Each application can have at most one PeriodicTask and one ResourceIntensiveTask. There are restrictions in using both PeriodicTask and ResourceIntensiveTask. One of the more important things to keep in mind is that a PeriodicTask will run approximately every 30 minutes for at most a couple of seconds, and it will continue running roughly twice an hour for 14 days in a row (assuming the PeriodicTask does not crash). If the application has not renewed the PeriodicTask within those 14 days, the PeriodicTask will be removed from the schedule. Thinking about it, that makes sense. If a user is not using your application for such a long time, it is probably not necessary to continue executing code for that application in the background.

For EvenTiles we will make use of a PeriodicTask, to allow updating of our Secondary Tile on a regular interval. Since updating the Secondary Tile is a relatively simple action, this is a perfect candidate for a PeriodicTask. In this way we can bring our Secondary Tile to life (after all, things will change to it on the start screen of the user’s phone), regardless of the execution state of our application. This technique can for instance be used to update weather information, or number of newly available emails to simply keep the user informed. In our sample application, we will use a PeriodicTask to simply display different strings on the back of the Secondary Tile. The reason to chose something so simple is to be able to concentrate on the bare functionality of the PeriodicTask, its relation to the application and ways to exchange data between the application and its Periodic Task.

To make use of a PeriodicTask, we need to add a new project to our solution. Visual Studio already contains a template for a Windows Phone Scheduled Task Agent, as shown in the next figure:

image

In the newly created PeriodicTask you will find a method called OnInvoke. That is the method into which you will add the functionality you like to have executed in the background. It is important to call the NotifyComplete method as last statement inside your OnInvoke method. Each time your code executes, it will execute in a separate thread. Since the PeriodicTask runs independent from the application (once the application has started the PeriodicTask), it can not update the User Interface of the application it belongs to.

To be able to start the PeriodicTask from inside your application, the first thing you will do is add a reference to the PeriodicTask in your application’s project:

image

When you have added the reference to the PeriodicTask, you have effectively made a connection between it and the application. This is visible inside the WPAppManifest.xml file:

PeriodicTask in Manifest
  1. <Tasks>
  2.   <DefaultTask Name="_default" NavigationPage="MainPage.xaml" />
  3.   <ExtendedTask Name="BackgroundTask">
  4.     <BackgroundServiceAgent Specifier="ScheduledTaskAgent"
  5.                             Name="EvenTilesScheduledTaskAgent"
  6.                             Source="EvenTilesScheduledTaskAgent"
  7.                             Type="EvenTilesScheduledTaskAgent.ScheduledAgent" />
  8.   </ExtendedTask>
  9. </Tasks>

The next thing that needs to be done is starting the PeriodicTask from inside the application. Of course it is good practice to keep the user in control, so we are going to create the PeriodicTask as a result of a user action. In episode 9 of EvenTiles we added a button to the MainPage to create a Secondary Tile. Since we want this Secondary Tile to change its backside text regularly (even without the application being active), it makes sense to create the PeriodicTask as part of the same Click event handler. The code to create and to remove the PeriodicTask looks like this:

Create and Remove PeriodicTask
  1. private void StartPeriodicAgent()
  2. {
  3.     RemovePeriodicAgent();
  4.  
  5.     evenTilesPeriodicTask = new PeriodicTask(evenTilesPeriodicTaskName);
  6.     evenTilesPeriodicTask.Description = "EvenTilesPeriodicTask";
  7.  
  8.     try
  9.     {
  10.         ScheduledActionService.Add(evenTilesPeriodicTask);
  11.     }
  12.     catch (InvalidOperationException ex)
  13.     {
  14.         if (ex.Message.Contains("BNS Error: The action is disabled"))
  15.         {
  16.             MessageBox.Show("Background agents for this application are disabled.");
  17.         }
  18.     }
  19. }
  20.  
  21. private void RemovePeriodicAgent()
  22. {
  23.     var runningPeriodicTask = ScheduledActionService.Find(evenTilesPeriodicTaskName) as PeriodicTask;
  24.     if (runningPeriodicTask != null)
  25.     {
  26.         ScheduledActionService.Remove(evenTilesPeriodicTaskName);
  27.     }
  28. }

Note how we make use of ExceptionHandling to catch an InvalidOperationException. This exception will typically be thrown if the user has disabled background processing for our application, which they can do from inside the Phone Settings. The following picture shows how the user can enable / disable background processing for our application:

image

Calling the methods to create / remove our PeriodicTask from inside our Click event handler is of course a simple action, although you should note that we create the PeriodicTask before creating a Secondary Tile. The reason for that is to allow the PeriodicTask to display a MessageBox to the user in case of problems. By first creating the Secondary Tile we force our application to immediately go to the background, and with that, not being able to show the MessageBox to the user.

Starting/Stopping Agent
  1. private void btnInstall_Click(object sender, RoutedEventArgs e)
  2. {
  3.     if (secondaryTileInstalled)
  4.     {
  5.         var secondaryTile = ShellTile.ActiveTiles.FirstOrDefault(x => x.NavigationUri.ToString().Contains("TileId=Secondary"));
  6.         if (secondaryTile != null)
  7.         {
  8.             RemovePeriodicAgent();
  9.             secondaryTile.Delete();
  10.             btnInstall.Content = txtInstallTile;
  11.             secondaryTileInstalled = false;
  12.         }
  13.     }
  14.     else
  15.     {
  16.         StartPeriodicAgent();
  17.         StandardTileData NewTileData = new StandardTileData
  18.         {
  19.             BackgroundImage = new Uri("Background.png", UriKind.Relative),
  20.             Title = "EvenTiles",
  21.             Count = 0,
  22.             BackBackgroundImage = new Uri("BackBackTile.png", UriKind.Relative),
  23.             BackTitle = "EvenTiles",
  24.             BackContent = App.ActualSecBackContent
  25.         };
  26.         ShellTile.Create(new Uri("/MainPage.xaml?TileId=Secondary", UriKind.Relative), NewTileData);
  27.     }
  28. }

Since we did not add functionality to our PeriodicTask, it will wake up every 30 minutes to immediately terminate again, because the only thing we are doing inside the PeriodicTask’s OnInvoke method is calling the NotifyComplete method:

An ‘Empty’ PeriodicTask
  1. protected override void OnInvoke(ScheduledTask task)
  2. {
  3.     //TODO: Add code to perform your task in background
  4.     NotifyComplete();
  5. }

I strongly encourage you to take a look at the following video, that shows all the functionality we have so far in action:

PeriodicTask for Background Processing inside a Windows Phone Application

One thing is annoying in our solution so far. If we want to debug our PeriodicTask, we would have to wait 30 minutes each time until code in the PeriodicTask will be executed. How to overcome this problem will be topic of part 11 of our ongoing series around EvenTiles.