Bundling AppImages with Themes.

One of the projects which I have been undertaking in recent weeks has been, to teach myself GUI programming using the Qt5 GUI Library, of which I have version 5.7.1 installed on a good, tower computer, along with the IDE “Qt Creator”. What can be observed about this already is, that under Debian 9 / Stretch, which is a specific build of Linux, in addition to just a few packages, it’s really necessary to install many additional packages, before one is ready to develop Qt Applications, because of the way Debian breaks the facility into many smaller packages. Hypothetically, if a person was using the Windows, Qt SDK, then he or she would have many of the resources all in one package.

Beyond just teaching myself the basics of how to design GUIs with this, I’ve also explored what the best way is, to deploy the resulting applications, so that other people – technically, my users – may run them. This can be tricky because, with Qt especially, libraries tend to be incompatible, due to even minor version differences. So, an approach which can be taken is, to bundle the main libraries required into an AppImage, such that, when the developer has compiled everything, the resulting AppImage – a binary – is much more likely actually to run, on different versions of Linux specifically.

The tool which I’ve been using, to turn my compiled binaries into AppImage’s, is called ‘linuxdeployqt‘, and is not available in the Debian / Stretch repositories. However, it does run under …Stretch.

But a developer may have questions that go beyond just this basic capability, such as, what he or she can do, so that the application will have a predictable appearance – a “Style” or “Theme” – on the end-user’s Linux computer. And essentially, I can think of two ways to approach that question: The ‘official but slightly quirky way’, and ‘a dirty fix, that seems to get used often’…

The official, but slightly quirky way:

Within the AppImage, there will be a ‘plugins’ directory, within which there will be a ‘platformthemes’ as well as a ‘styles’ subdirectory. It’s important to note, that these subdirectories serve officially different purposes:

  • The ‘platformthemes’ subdirectory will contain plugins, that allow the application to connect with whatever theme engine the end-user’s computer has. Its plugins need to match libraries that the eventual user has, determining his desktop theme, And
  • The ‘styles’ subdirectory may contain plugins, which the end-user does not have installed, but were usually compiled by upstream developers, to make use of one specific platform-engine each.

Thus, what I had in these directories, for better or worse, was as shown:

 

dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/platformthemes$ ls
KDEPlasmaPlatformTheme.so  libqgtk2.so  libqgtk3.so
dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/platformthemes$ 


dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/styles$ ls
breeze.so  libqgtk2style.so
dirk@Phosphene:~/Programs/build-Dirk_Roots_GUI_1-Desktop-Release/plugins/styles$ 

 

The reader may already get, that this was a somewhat amateurish way, to satisfy themes on the end-user’s machine. But in reality, what this set of contents, of the AppImage, does rather well is, to make sure that the 3 main theme engines on an end-user’s computer are recognized:

  1. Gtk2,
  2. Gtk3,
  3. Plasma 5.

And, if the application tries to make no attempts to set its own theme or style, it will most probably run with the same theme, that the end-user has selected for his desktop. But, what the point of this posting really is, is to give a hint to the reader, as to how his AppImage could set its own theme eventually. And so, according to what I just cited above, my application could choose to select “Breeze” as the Style with which to display itself, or “Gtk2″. But, here is where the official way gets undermined, at least as the state of the art was, with v5.7.1 of Qt:

  • ‘Breeze’ can only be set (by the application), if the end-user’s machine is running Plasma 5 (:1), And
  • ‘Gtk2′ can only be set (by the application), if the end-user’s machine supports Gtk2 themes, which many Plasma 5 computers have the additional packages installed, to do.

What this means is that, even though I could try to create a predictable experience for the end-user, what the end-user will see can still vary, depending on what, exactly, his platform is. And beyond that, even though I could set the ‘Gtk2′ Style with better reliability in the outcome, I could also just think, that the classical, ‘Gtk2′ style is a boring style, not worthy of my application. Yet, in this situation, I can only select the “Breeze” theme from within my application successfully, if the end-user is based on Plasma 5. If the end-user is not, then my application’s attempt to set “Breeze” will actually cause Qt v5.7.1 to choose the “Fusion” theme, that Qt5 always supports, that might look okay, but that is not “Breeze”…

 

So, what other options does the application developer have?

(Updated 9/12/2020, 18h15… )

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