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With this, the expand_content_files list has been completely converted to markdown files. Whats left in content_files is man pages, and a few special cases.
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Markdown
558 lines
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Markdown
# Common Questions {#gtk-question-index}
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This is an "index" of the reference manual organized by common "How do
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I..." questions. If you aren't sure which documentation to read for
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the question you have, this list is a good place to start.
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## General Questions
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1. How do I get started with GTK?
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The GTK [website](https://www.gtk.org) offers some
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[tutorials](https://www.gtk.org/documentation.php) and other documentation
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(most of it about GTK 2.x and 3.x, but still somewhat applicable). This
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reference manual also contains a introductory
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[Getting Started](#gtk-getting-started) part.
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More documentation ranging from whitepapers to online books can be found at
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the [GNOME developer's site](https://developer.gnome.org). After studying these
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materials you should be well prepared to come back to this reference manual for details.
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2. Where can I get help with GTK, submit a bug report, or make a feature request?
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See the [documentation](#gtk-resources) on this topic.
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3. How do I port from one GTK version to another?
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See the [migration guide](#migrating). You may also find useful information in
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the documentation for specific widgets and functions. If you have a question not
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covered in the manual, feel free to ask, and please
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[file a bug report](https://gitlab.gnome.org/GNOME/gtk/issues/new) against the
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documentation.
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4. How does memory management work in GTK? Should I free data returned from functions?
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See the documentation for #GObject and #GInitiallyUnowned. For #GObject note
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specifically g_object_ref() and g_object_unref(). #GInitiallyUnowned is a
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subclass of #GObject so the same points apply, except that it has a "floating"
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state (explained in its documentation).
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For strings returned from functions, they will be declared "const" if they should
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not be freed. Non-const strings should be freed with g_free(). Arrays follow the
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same rule. If you find an undocumented exception to the rules, please
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[file a bug report.](https://gitlab.gnome.org/GNOME/gtk/issues/new).
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The transfer annotations for gobject-introspection that are part of the
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documentation can provide useful hints for memory handling semantics as well.
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5. Why does my program leak memory, if I destroy a widget immediately
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after creating it ?
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If `GtkFooi` isn't a toplevel window, then
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foo = gtk_foo_new ();
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g_object_unref (foo);
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is a memory leak, because no one assumed the initial floating reference
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(you will get a warning about this too). If you are using a widget and
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you aren't immediately packing it into a container, then you probably
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want standard reference counting, not floating reference counting.
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To get this, you must acquire a reference to the widget and drop the
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floating reference (_ref and sink_ in GObject parlance) after creating it:
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foo = gtk_foo_new ();
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g_object_ref_sink (foo);
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When you immediately add a widget to a container, it takes care of assuming
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the initial floating reference and you don't have to worry about reference
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counting at all ... just remove the widget from the container to get rid of it.
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6. How do I use GTK with threads?
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GTK requires that all GTK API calls are made from the same thread in which
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gtk_init() was called (the _main thread_).
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If you want to take advantage of multi-threading in a GTK application,
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it is usually best to send long-running tasks to worker threads, and feed
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the results back to the main thread using g_idle_add() or GAsyncQueue. GIO
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offers useful tools for such an approach such as GTask.
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7. How do I internationalize a GTK program?
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Most people use <[GNU gettext](https://www.gnu.org/software/gettext/),
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already required in order to install GLib. On a UNIX or Linux system with
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gettext installed, type `info gettext` to read the documentation.
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The short checklist on how to use gettext is: call bindtextdomain() so
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gettext can find the files containing your translations, call textdomain()
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to set the default translation domain, call bind_textdomain_codeset() to
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request that all translated strings are returned in UTF-8, then call
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gettext() to look up each string to be translated in the default domain.
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`gi18n.h` provides the following shorthand macros for convenience.
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Conventionally, people define macros as follows for convenience:
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#define _(x) gettext (x)
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#define N_(x) x
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#define C_(ctx,x) pgettext (ctx, x)
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You use N_() (N stands for no-op) to mark a string for translation in
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a location where a function call to gettext() is not allowed, such as
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in an array initializer. You eventually have to call gettext() on the
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string to actually fetch the translation. _() both marks the string for
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translation and actually translates it. The C_() macro (C stands for
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context) adds an additional context to the string that is marked for
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translation, which can help to disambiguate short strings that might
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need different translations in different parts of your program.
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Code using these macros ends up looking like this:
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#include <gi18n.h>
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static const char *global_variable = N_("Translate this string");
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static void
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make_widgets (void)
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{
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GtkWidget *label1;
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GtkWidget *label2;
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label1 = gtk_label_new (_("Another string to translate"));
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label2 = gtk_label_new (_(global_variable));
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...
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Libraries using gettext should use dgettext() instead of gettext(),
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which allows them to specify the translation domain each time they
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ask for a translation. Libraries should also avoid calling textdomain(),
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since they will be specifying the domain instead of using the default.
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With the convention that the macro `GETTEXT_PACKAGE` is defined to hold
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your libraries translation domain, `gi18n-lib.h` can be included to provide
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the following convenience:
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#define _(x) dgettext (GETTEXT_PACKAGE, x)
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8. How do I use non-ASCII characters in GTK programs ?
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GTK uses [Unicode](http://www.unicode.org) (more exactly UTF-8) for all text.
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UTF-8 encodes each Unicode codepoint as a sequence of one to six bytes and
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has a number of nice properties which make it a good choice for working with
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Unicode text in C programs:
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- ASCII characters are encoded by their familiar ASCII codepoints.
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- ASCII characters never appear as part of any other character.
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- The zero byte doesn't occur as part of a character, so that UTF-8
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string can be manipulated with the usual C library functions for
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handling zero-terminated strings.
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More information about Unicode and UTF-8 can be found in the
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[UTF-8 and Unicode FAQ](https://www.cl.cam.ac.uk/~mgk25/unicode.html).
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GLib provides functions for converting strings between UTF-8 and other
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encodings, see g_locale_to_utf8() and g_convert().
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Text coming from external sources (e.g. files or user input), has to be
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converted to UTF-8 before being handed over to GTK. The following example
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writes the content of a IS0-8859-1 encoded text file to `stdout`:
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char *text, *utf8_text;
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gsize length;
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GError *error = NULL;
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if (g_file_get_contents (filename, &text, &length, NULL))
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{
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utf8_text = g_convert (text, length, "UTF-8", "ISO-8859-1",
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NULL, NULL, &error);
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if (error != NULL)
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{
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fprintf ("Couldn't convert file %s to UTF-8\n", filename);
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g_error_free (error);
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}
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else
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g_print (utf8_text);
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}
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else
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fprintf (stderr, "Unable to read file %s\n", filename);
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For string literals in the source code, there are several alternatives
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for handling non-ASCII content:
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- Direct UTF-8
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If your editor and compiler are capable of handling UTF-8 encoded
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sources, it is very convenient to simply use UTF-8 for string literals,
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since it allows you to edit the strings in "wysiwyg". Note that choosing
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this option may reduce the portability of your code.
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- Escaped UTF-8
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Even if your toolchain can't handle UTF-8 directly, you can still
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encode string literals in UTF-8 by using octal or hexadecimal escapes
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like `\\212` or `\\xa8` to encode each byte. This is portable, but
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modifying the escaped strings is not very convenient. Be careful when
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mixing hexadecimal escapes with ordinary text; `"\\xa8abcd" is a string
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of length 1 !
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- Runtime conversion
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If the string literals can be represented in an encoding which your
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toolchain can handle (e.g. IS0-8859-1), you can write your source
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files in that encoding and use g_convert() to convert the strings
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to UTF-8 at runtime. Note that this has some runtime overhead, so
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you may want to move the conversion out of inner loops.
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Here is an example showing the three approaches using the copyright
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sign © which has Unicode and ISO-8859-1 codepoint 169 and is represented
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in UTF-8 by the two bytes 194, 169, or `"\\302\\251"` as a string literal:
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g_print ("direct UTF-8: ©");
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g_print ("escaped UTF-8: \302\251");
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text = g_convert ("runtime conversion: ©", -1,
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"ISO-8859-1", "UTF-8", NULL, NULL, NULL);
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g_print (text);
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g_free (text);
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If you are using gettext() to localize your application, you need
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to call bind_textdomain_codeset() to ensure that translated strings
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are returned in UTF-8 encoding.
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9. How do I use GTK with C++?
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There are two ways to approach this. The GTK header files use the subset
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of C that's also valid C++, so you can simply use the normal GTK API
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in a C++ program. Alternatively, you can use a "C++ binding" such as
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[gtkmm](https://www.gtkmm.org/) which provides a native C++ API.
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When using GTK directly, keep in mind that only functions can be
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connected to signals, not methods. So you will need to use global
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functions or "static" class functions for signal connections.
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Another common issue when using GTK directly is that C++ will not
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implicitly convert an integer to an enumeration. This comes up when
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using bitfields; in C you can write the following code:
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gdk_surface_set_events (gdk_surface,
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GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
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while in C++ you must write:
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gdk_surface_set_events (gdk_surface,
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(GdkEventMask) GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
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There are very few functions that require this cast, however.
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10. How do I use GTK with other non-C languages?
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See the list of [language bindings](https://www.gtk.org/language-bindings.php)
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on the GTK [website](https://www.gtk.org).
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11. How do I load an image or animation from a file?
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To load an image file straight into a display widget, use
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gtk_image_new_from_file(). To load an image for another purpose, use
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gdk_texture_new_from_file(). To load a video from a file, use
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gtk_media_file_new_for_file().
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12. How do I draw text?
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To draw a piece of text onto a cairo surface, use a Pango layout and
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pango_cairo_show_layout().
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layout = gtk_widget_create_pango_layout (widget, text);
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fontdesc = pango_font_description_from_string ("Luxi Mono 12");
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pango_layout_set_font_description (layout, fontdesc);
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pango_cairo_show_layout (cr, layout);
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pango_font_description_free (fontdesc);
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g_object_unref (layout);
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See also the [Cairo Rendering](https://developer.gnome.org/pango/stable/pango-Cairo-Rendering.html)
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section of the [Pango documentation](https://developer.gnome.org/pango/stable/).
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To draw a piece of text in a widget snapshot() implementation, use
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gtk_snapshot_append_layout().
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13. How do I measure the size of a piece of text?
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To obtain the size of a piece of text, use a Pango layout and
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pango_layout_get_pixel_size(), using code like the following:
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layout = gtk_widget_create_pango_layout (widget, text);
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fontdesc = pango_font_description_from_string ("Luxi Mono 12");
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pango_layout_set_font_description (layout, fontdesc);
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pango_layout_get_pixel_size (layout, &width, &height);
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pango_font_description_free (fontdesc);
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g_object_unref (layout);
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See also the [Layout Objects](https://developer.gnome.org/pango/stable/pango-Layout-Objects.html)
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section of the [Pango documentation](https://developer.gnome.org/pango/stable/).
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14. Why are types not registered if I use their `GTK_TYPE_BLAH` macro?
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The %GTK_TYPE_BLAH macros are defined as calls to gtk_blah_get_type(), and
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the `_get_type()` functions are declared as %G_GNUC_CONST which allows the
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compiler to optimize the call away if it appears that the value is not
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being used.
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GLib provides the g_type_ensure() function to work around this problem.
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g_type_ensure (GTK_TYPE_BLAH);
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15. How do I create a transparent toplevel window?
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Any toplevel window can be transparent. It is just a matter of setting a
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transparent background in the CSS style for it.
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## Which widget should I use...
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16. ...for lists and trees?
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This question has different answers, depending on the size of the dataset
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and the required formatting flexibility.
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If you want to display a large amount of data in a uniform way, your best
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option is a #GtkTreeView widget. See the [tree widget overview](#TreeWidget).
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A list is just a tree with no branches, so the treeview widget is used for
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lists as well.
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If you want to display a small amount of items, but need flexible formatting
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and widgetry inside the list, then you probably want to use a #GtkListBox,
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which uses regular widgets for display.
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17. ...for multi-line text display or editing?
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See the [text widget overview](#TextWidget) -- you should use the
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#GtkTextView widget.
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If you only have a small amount of text, #GtkLabel may also be appropriate
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of course. It can be made selectable with gtk_label_set_selectable(). For a
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single-line text entry, see #GtkEntry.
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18. ...to display an image or animation?
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GTK has two widgets that are dedicated to displaying images. #GtkImage, for
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small, fixed-size icons and #GtkPicture for content images.
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Both can display images in just about any format GTK understands.
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You can also use #GtkDrawingArea if you need to do something more complex,
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such as draw text or graphics over the top of the image.
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Both GtkImage and GtkPicture can display animations and videos as well.
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To show an webm file, load it with the GtkMediaFile API and then use
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it as a paintable:
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mediafile = gtk_media_file_new_for_filename ("example.webm");
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picture = gtk_picture_new_for_paintable (GDK_PAINTABLE (mediafile));
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19. ...for presenting a set of mutually-exclusive choices, where Windows
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would use a combo box?
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With GTK, a #GtkComboBox is the recommended widget to use for this use case.
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If you need an editable text entry, use the #GtkComboBox:has-entry property.
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## Questions about GtkWidget
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20. How do I change the color of a widget?
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The background color of a widget is determined by the CSS style that applies
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to it. To change that, you can set style classes on the widget, and provide
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custom CSS to change the appearance. Such CSS can be loaded with
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gtk_css_provider_load_from_file() and its variants.
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See gtk_style_context_add_provider().
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21. How do I change the font of a widget?
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If you want to make the text of a label larger, you can use
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gtk_label_set_markup():
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gtk_label_set_markup (label, "<big>big tex</big>");
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This is preferred for many apps because it's a relative size to the
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user's chosen font size. See g_markup_escape_text() if you are
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constructing such strings on the fly.
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You can also change the font of a widget by putting
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.my-widget-class {
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font: Sans 30;
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}
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in a CSS file, loading it with gtk_css_provider_load_from_file(), and
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adding the provider with gtk_style_context_add_provider_for_display().
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To associate this style information with your widget, set a style class
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on its #GtkStyleContext using gtk_style_context_add_class(). The advantage
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of this approach is that users can then override the font you have chosen.
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See the #GtkStyleContext documentation for more discussion.
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22. How do I disable/ghost/desensitize a widget?
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In GTK a disabled widget is termed _insensitive_.
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See gtk_widget_set_sensitive().
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## GtkTextView questions
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23. How do I get the contents of the entire text widget as a string?
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See gtk_text_buffer_get_bounds() and gtk_text_buffer_get_text()
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or gtk_text_iter_get_text().
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GtkTextIter start, end;
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GtkTextBuffer *buffer;
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char *text;
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buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (text_view));
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gtk_text_buffer_get_bounds (buffer, &start, &end);
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text = gtk_text_iter_get_text (&start, &end);
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/* use text */
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g_free (text);
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24. How do I make a text widget display its complete contents in a specific font?
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If you use gtk_text_buffer_insert_with_tags() with appropriate tags to
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select the font, the inserted text will have the desired appearance, but
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text typed in by the user before or after the tagged block will appear in
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the default style.
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To ensure that all text has the desired appearance, use
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gtk_widget_override_font() to change the default font for the widget.
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25. How do I make a text view scroll to the end of the buffer automatically ?
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A good way to keep a text buffer scrolled to the end is to place a
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[mark](#GtkTextMark) at the end of the buffer, and give it right gravity.
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The gravity has the effect that text inserted at the mark gets inserted
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*before*, keeping the mark at the end.
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To ensure that the end of the buffer remains visible, use
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gtk_text_view_scroll_to_mark() to scroll to the mark after
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inserting new text.
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The gtk-demo application contains an example of this technique.
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## GtkTreeView questions
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26. How do I associate some data with a row in the tree?
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Remember that the #GtkTreeModel columns don't necessarily have to be
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displayed. So you can put non-user-visible data in your model just
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like any other data, and retrieve it with gtk_tree_model_get().
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See the [tree widget overview](#TreeWidget).
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27. How do I put an image and some text in the same column?
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You can pack more than one #GtkCellRenderer into a single #GtkTreeViewColumn
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using gtk_tree_view_column_pack_start() or gtk_tree_view_column_pack_end().
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So pack both a #GtkCellRendererPixbuf and a #GtkCellRendererText into the
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column.
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28. I can set data easily on my #GtkTreeStore/#GtkListStore models using
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gtk_list_store_set() and gtk_tree_store_set(), but can't read it back?
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Both the #GtkTreeStore and the #GtkListStore implement the #GtkTreeModel
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interface. Consequentially, you can use any function this interface
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implements. The easiest way to read a set of data back is to use
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gtk_tree_model_get().
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29. How do I change the way that numbers are formatted by #GtkTreeView?
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Use gtk_tree_view_insert_column_with_data_func() or
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gtk_tree_view_column_set_cell_data_func() and do the conversion
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from number to string yourself (with, say, g_strdup_printf()).
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The following example demonstrates this:
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enum
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{
|
|
DOUBLE_COLUMN,
|
|
N_COLUMNS
|
|
};
|
|
|
|
GtkListStore *mycolumns;
|
|
|
|
GtkTreeView *treeview;
|
|
|
|
void
|
|
my_cell_double_to_text (GtkTreeViewColumn *tree_column,
|
|
GtkCellRenderer *cell,
|
|
GtkTreeModel *tree_model,
|
|
GtkTreeIter *iter,
|
|
gpointer data)
|
|
{
|
|
GtkCellRendererText *cell_text = (GtkCellRendererText *)cell;
|
|
double d;
|
|
char *text;
|
|
|
|
/* Get the double value from the model. */
|
|
gtk_tree_model_get (tree_model, iter, (int)data, &d, -1);
|
|
/* Now we can format the value ourselves. */
|
|
text = g_strdup_printf ("%.2f", d);
|
|
g_object_set (cell, "text", text, NULL);
|
|
g_free (text);
|
|
}
|
|
|
|
void
|
|
set_up_new_columns (GtkTreeView *myview)
|
|
{
|
|
GtkCellRendererText *renderer;
|
|
GtkTreeViewColumn *column;
|
|
GtkListStore *mycolumns;
|
|
|
|
/* Create the data model and associate it with the given TreeView */
|
|
mycolumns = gtk_list_store_new (N_COLUMNS, G_TYPE_DOUBLE);
|
|
gtk_tree_view_set_model (myview, GTK_TREE_MODEL (mycolumns));
|
|
|
|
/* Create a GtkCellRendererText */
|
|
renderer = gtk_cell_renderer_text_new ();
|
|
|
|
/* Create a new column that has a title ("Example column"),
|
|
* uses the above created renderer that will render the double
|
|
* value into text from the associated model's rows.
|
|
*/
|
|
column = gtk_tree_view_column_new ();
|
|
gtk_tree_view_column_set_title (column, "Example column");
|
|
renderer = gtk_cell_renderer_text_new ();
|
|
gtk_tree_view_column_pack_start (column, renderer, TRUE);
|
|
|
|
/* Append the new column after the GtkTreeView's previous columns. */
|
|
gtk_tree_view_append_column (GTK_TREE_VIEW (myview), column);
|
|
/* Since we created the column by hand, we can set it up for our
|
|
* needs, e.g. set its minimum and maximum width, etc.
|
|
*/
|
|
/* Set up a custom function that will be called when the column content
|
|
* is rendered. We use the func_data pointer as an index into our
|
|
* model. This is convenient when using multi column lists.
|
|
*/
|
|
gtk_tree_view_column_set_cell_data_func (column, renderer,
|
|
my_cell_double_to_text,
|
|
(gpointer)DOUBLE_COLUMN, NULL);
|
|
}
|
|
|
|
30. How do I hide the expander arrows in my tree view?
|
|
|
|
Set the expander-column property of the tree view to a hidden column.
|
|
See gtk_tree_view_set_expander_column() and gtk_tree_view_column_set_visible().
|
|
|
|
## Using cairo with GTK
|
|
|
|
31. How do I use cairo to draw in GTK applications?
|
|
|
|
Use gtk_snapshot_append_cairo() in your #GtkWidgetClass.snapshot() vfunc
|
|
to obtain a cairo context and draw with that.
|
|
|
|
32. Can I improve the performance of my application by using another backend
|
|
of cairo (such as GL)?
|
|
|
|
No. Most drawing in GTK is not done via cairo anymore (but instead
|
|
by the GL or Vulkan renderers of GSK).
|
|
|
|
If you use cairo for drawing your own widgets, gtk_snapshot_append_cairo()
|
|
will choose the most appropriate surface type for you.
|
|
|
|
If you are interested in using GL for your own drawing, see #GtkGLArea.
|
|
|
|
33. Can I use cairo to draw on a #GdkPixbuf?
|
|
|
|
No. The cairo image surface does not support the pixel format used by GdkPixbuf.
|
|
|
|
If you need to get cairo drawing into a format that can be displayed efficiently
|
|
by GTK, you may want to use an image surface and gdk_memory_texture_new().
|