If this is a problem for your apps, there are a couple ways you can work around it.

Use Fixed/Locked Control Padding

For many apps, the primary concern is to not have controls change their size or position depending on the theme being used. You can ensure specific styling for controls in your apps by using a static amount of padding that won’t change regardless of the user’s theme selection. This has the benefit of letting your app use other aspects of the theme, such as colors, while maintaining control layout better.

Have Your App Load and Use a Specific Theme

If you would like your app to use a specific theme so that your layout works exactly as you want it, you can bundle a theme and load it when your app starts. Your app will use this loaded theme regardless of what the user has selected as a theme for the distribution.

Both of these solutions can be handled by Declares that you put in your App.Open event handler.

We’ve put together a couple sample projects to demonstrate these two solutions:

Download GTK3ThemeExamples

The examples should be sufficient for most apps. But if you require more control of themes and how they render widgets, you will have to dig deeper into Linux. The GTK+ Inspector can be a great tool for this as it lets you see property and widget information in real time.

For more information and details, refer to GTK3 Themes in the User Guide.

The Linux API is largely based on the C/C++ programming language and makes heavy use of structures.

As a simple example, the gtk_widget_set_opacity method in libgtk-3 can be used to change the window opacity so that it appears more transparent. This is what the method declaration looks like in the Gnome docs:

void
gtk_widget_set_opacity (GtkWidget *widget,
 double opacity);

This tells you it is a method (sub) because the “void” at the beginning indicates it does not return a value. The first parameter is a pointer to a GtkWidget, which in this case is the handle to the Xojo window. The opacity is a Double in the range of 0 to 1.

So the above method call translates to a Declare that looks like this:

Declare Sub gtk_widget_set_opacity Lib "libgtk-3" (handle As Integer, opacity As Double)

To set the window to 75% opacity you can call it like this:

gtk_widget_set_opacity(Self.Handle, 0.75)

Because this method is called for a window, you can put it in a Xojo Extension Method to make it easier to call. To do this, create a global method on a module like this:

Public Sub Opacity(Extends w As Window, value As Double)
  #If TargetLinux Then
    Declare Sub gtk_widget_set_opacity Lib "libgtk-3" (handle As Integer, opacity As Double)
     gtk_widget_set_opacity(w, value)
  #EndIf
End Sub

Note the use of the “#If TargetLinux” line. This prevents this code from being compiled into Windows or macOS builds of your app where the code could possible crash your app.

This now allows you to have code like this on a window’s Open event to center the window:

Self.Opacity(0.75)

To learn more in the Xojo Documentation:

• Calling Native Linux APIs

If there are Linux-related or other topics, regarding Xojo Programming or general technology, you are interested in reading about on the Xojo Blog about please let us know!

Let’s take a closer look at what this means for your Linux apps:

Besides the benefit of not having to tell your end users to install GTK+ 2 on their particular Linux distro, here are some other notable items affected by the GTK+ 3 transition:

While there’s a short list of actual differences, there have been numerous changes underneath made during this transition. For example, features that we had to custom code because GTK+ 2 did not offer them but GTK+ 3 did. Updating to GTK+ 3 is the final piece to our cross-platform HiDPI support for Desktop apps. Until the day comes when we have to say goodbye to GTK+ 3 (GTK+ 4 is already in the works), we welcome its inclusion in Xojo with a long overdue “Hello”!