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AmberIndicator - a systray app under Linux

, 27 Sep 2012 CPOL
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Implemention of an ApplicationIndicator (NotifyIcon under Ubuntu)

Introduction

This article shows how to implement a AppIndicator[^], a kind of SysTray in Ubuntu, and react to a specific RSS-feed.

Background

There's a free Dutch service called "AMBER Alert[^]", that broadcasts a nation-wide message when a child goes missing. One of the clients they provide is a Windows-app that sits in your system-tray, and notifies the user when there's an alert. It was noted on their website that there wasn't a Linux-version yet, so I tried to puzzle one together. Coming from a Windows-platform, I ran into quite some surprises. Half of this article is dedicated to the installer, as that is where I spent most of the time researching what to do next.

Basically, we're listening to two RSS-feeds, which are nicely documented (in English!) here[^]. The first one is a list of missing children, the second one is reserved for immediate alerts.

The common NotifyIcon isn't visible if you're running a default KDE-desktop; it requires a Gnome Panel called Notification Area Applet[^]. The image below shows the original NotifyIcon of the Windows-client running on Ubuntu 12.04, in the lower-left corner (using Wine). Those things on the upper-right corner, are the AppIndicators.

Using the code

C# would be my language of choice. We could use the System.Windows.Forms.NotifyIcon[^], but then we'd still end up in the Gnome panel. Luckily, the manual has a code-example on building a Application Indicators[^] in C# using Gtk#. Another usefull example can be found here[^].

Assuming that you've installed Mono and the MonoDevelop[^] IDE, you still need one more additional package. So, open a Terminal-window (the command line interface), and enter the command below;

    apt-get install libappindicator0.1-cil-dev libappindicator0.1-cil
	

When you open the solution, you'll recognize the basic framework of the sample;

  1  namespace AmberIndicator
  2  {
  3      public static class Program
  4      {		
  5          static ApplicationIndicator _indicator;  // this is the "NotifyIcon"
  6          static ImageMenuItem _menuItemShowAlert; // a menu
  7          static Window _dummyForm;                // a hidden mainform	
  8          static string[] _stringsBag;             // magic
  9          static bool _alertRaised;                // whether there's an alert 
 10                                                   //  (an entry in the second feed)
 11          static bool _blinkOn;                    // whether or not the icon is 
 12                                                   //  blinking on/off to indicate 
 13                                                   //  an alert
 14          static System.Timers.Timer _blinkTimer;  // a timer to do said blinking
 15          
 16          public static void Main ()
 17          {
 18              Application.Init ();
 19  			
 20              _stringsBag = HttpHelpers.DownloadIfModified(
 21                  AmberQueryThread.QUERY_ROOT + "amberResources.txt")
 22                  .Split(new string[] { "\n" }, System.StringSplitOptions.RemoveEmptyEntries);
 23              
 24              _dummyForm = new MainWindow();
 25              _dummyForm.Visible = false;
 26              		
 27              [...]
 28  					
 29              _indicator = new ApplicationIndicator 
 30              (
 31                  "amber-indicator",
 32                  "amber16x16x8g",
 33                  Category.ApplicationStatus,
 34                  @"/usr/share/amberindicator" // AssemblyInfo.Location would point to /usr/bin!
 35              );
 36              _indicator.Status = Status.Active;
 37                  
 38              Menu popupMenu = new Menu ();            
 39              [...]
 40              _indicator.Menu = popupMenu;
 41              
 42              new AmberQueryThread((int missingCount) => 
 43              {
 44              	UpdateAlertStatus(0 != missingCount);
 45              });
 46              
 47              Application.Run ();
 48          }

We first create a hidden main-window, then the indicator, then we create some menu's for the indicator, and finally we launch a background-thread to poll the RSS-feed.

Points of Interest

IfModifiedSince

We could choose to download the entire thing every now and then, but that would cause a huge amount of traffic. It'd mean that each user would be downloading the same feed over and over. The neat solution is to keep a local copy of the latest version that the server has, and to ask the webserver (from time to time) whether it has an updated version. By setting the IfModifiedSince[^] -property, the server will answer with a status code of 304[^], as opposed to serving the entire file. That way we can prevent causing a lot of useless traffic.

  1      HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("url to download");
  2      req.IfModifiedSince = // enter day of your local copy
  3  
  4      try
  5      {
  6          var response = req.GetResponse();
  7          string lastModified = response.Headers[HttpResponseHeader.LastModified];
  8          
  9          // since we got this far, we know that the content is outdated.
 10          // a 304 will throw an exception
 11          
 12          // save a local copy of the file.
 13      }
 14      catch (WebException ex)
 15      {
 16          // only handle protocol errors that have valid responses
 17          if (ex.Response == null || ex.Status != WebExceptionStatus.ProtocolError)
 18              throw;
 19          
 20          HttpStatusCode statusCode = ((HttpWebResponse)ex.Response).StatusCode;
 21          if (HttpStatusCode.NotModified == statusCode)  // 304, server sayin' it ain't modified
 22          {
 23              // fetch the old stored version
 24          }
 25      }
 26      // return the content
 27  

I generalized this construction in the DownloadIfModified method in the static class called HttpHelpers. Which leads us to the next point in the app; where do you keep the 'local' versions? Where do you keep local data at all?

AssemblyDatabase

Decided to re-use a tested method, and to use a Sqlite[^] database to store the data. There's another static class called AssemblyDatabase that creates a connection to a database, and that holds the logic to create and initialize a new one. If you look at it, you'll encounter some calls to AssemblyInfo.GetResourceAsString.

  1      internal static string GetResourceAsString(string AResourceName)
  2      {
  3          string pathAndResourceName = Path.Combine(
  4              @"/usr/share/amberindicator",
  5              AResourceName);
  6          return File.ReadAllText(pathAndResourceName);
  7      }

As the sqlite-documentation notes, there's no support for such a thing as a stored procedure. Storing the query as a text-file was the next-best-thing I could think of. They're not traditional resource-files, just silly plain-text files. You'll find all the Sql-commands in the solution, simply copied to the output-directory when updated.

StringsBag?

Traditionally, we keep our strings in a resource-file. That centralizes things a bit, making for easy maintenance and translation. Since I already had a simple way to download a modified text-file, it seemed like a novel idea to have those strings available online. You can actually request them here[^]. As you can see, it's a flat text-file. Updating it will cause the clients to download a new version of the file, giving me a way to "update" (part of) the app without having to distribute binaries for the update.

Creating an installer

We're not finished if there's no installer, and we can't use a MSI-based setup - we'll have to create a so-called "Debian package". There's a helpfull tutorial here[^], be sure to also read the "Next" page there. In essence, you rebuild the entire directory-tree where you want to install, starting from the root. The installer will create below structure;

  • /usr/bin/amberindicator.exe
  • /usr/share/amberindicator/amber16x16x8.png
  • /usr/share/amberindicator/amber16x16x8g.png
  • /usr/share/amberindicator/_CreateDb.sql
  • /usr/share/amberindicator/_FetchFromDownloadCache.sql
  • /usr/share/amberindicator/_InsertIntoDownloadCache.sql
  • /usr/share/amberindicator/_SelectLastServerModifiedFromDownloadCacheWhereSource.sql
  • /usr/share/amberindicator/_UpdateDownloadCache.sql
  • /usr/share/doc/amberindicator/changelog.Debian.gz
  • /usr/share/doc/amberindicator/changelog.gz
  • /usr/share/doc/amberindicator/copyright
  • /usr/share/man/man1/amberindicator.exe.1.gz
  • /DEBIAN/control

The bin folder holds the main-executable, while the rest of the app is installed in subdirectory of a shared folder. That means that the app and it's data are split over two directories, something that we don't do on Windows. The rest of the files are support-files for the installer.

/usr/share/doc contains (zipped) versions of the changelog, and a copyright notice. These files must follow a specific format, or the package-builder will present you a list of errors. The changelogs are nearly similar for this particular project. Every application under Linux also has a help-file, called the "man page". You can request the manual from the terminal by entering "man amberindicator", and it is also a required part of the installer.

/DEBIAN/control

The control file[^] is where you'll set most of the options for the installer. The cool part here are the dependencies;

Package: amberindicator
Version: 1.0.0-1
Section: misc
Priority: optional
Architecture: i386
Depends: debhelper (>= 5), libappindicator0.1-cil-dev (>=0.4.92-0), libappindicator0.1-cil (>=0.4.92-0), mono-runtime (>=2.10.8.1-1)
Installed-Size: 34
Maintainer: Eddy Vluggen <amberindicator@eddyvluggen.info>
Homepage: http://www.compu-link.net/index.php?id=amber-client-for-linux
Description: Statusindicator for the Dutch AMBER-alert service.
 The Dutch AMBER-alert service (http://www.amberalertnederland.nl)
 exposes a RSS-feed. Whenever a child is missing and an Alert is
 sent out, the feed is updated. The statusindicator queries the
 feed in the background, and provides a direct link to the
 page with further details.
 .
 Written under Mono.    
</amberindicator@eddyvluggen.info>

To build it, open the terminal-window, change to the root of the directory-structure you created, and enter the commands below;

    fakeroot dpkg-deb --build debian
    lintian debian.deb
	
The command lintian checks whether the installer is "good enough". If it doesn't include everything that lintian wants, you'll get a nasty warning when installing the application.

..finally, when all is done, the results looks like this;

History

  • 26 september 2012, version 1.0

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Eddy Vluggen
Software Developer
Netherlands Netherlands
I'm a Delphi-convert, mostly into WinForms and C#. My first article is from 2001, extending the Delphi-debugger, which is still visible on the WayBackMachine[^] and even available in Russian[^] Smile | :)

Comments and Discussions

 
GeneralMy vote of 5 PinmentorWayne Gaylard22-Oct-12 4:59 
GeneralRe: My vote of 5 PinmemberEddy Vluggen22-Oct-12 5:57 
GeneralMy vote of 1 PinmemberAadhar Joshi22-Oct-12 4:15 
GeneralRe: My vote of 1 PinmemberEddy Vluggen22-Oct-12 4:24 
GeneralMessage Removed PinmemberAadhar Joshi22-Oct-12 4:30 
GeneralRe: My vote of 1 PinmemberEddy Vluggen22-Oct-12 7:09 
GeneralRe: My vote of 1 Pinmemberjim lahey22-Oct-12 4:32 
GeneralMy vote of 3 PinmemberAl-Samman Mahmoud27-Sep-12 13:56 

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