Matthias Clasen f561000124 docs: Clean up many dangling links

In many cases, these were references to no longer
existing api, so some rewording was necessary.

2020-08-04 22:11:34 -04:00

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Common Questions

This is an "index" of the reference manual organized by common "How do I..." questions. If you aren't sure which documentation to read for the question you have, this list is a good place to start.

General Questions

How do I get started with GTK?

The GTK website offers some tutorials and other documentation (most of it about GTK 2.x and 3.x, but still somewhat applicable). This reference manual also contains a introductory Getting Started part.

More documentation ranging from whitepapers to online books can be found at the GNOME developer's site. After studying these materials you should be well prepared to come back to this reference manual for details.
Where can I get help with GTK, submit a bug report, or make a feature request?

See the documentation on this topic.
How do I port from one GTK version to another?

See the migration guide. You may also find useful information in the documentation for specific widgets and functions. If you have a question not covered in the manual, feel free to ask, and please file a bug report against the documentation.
How does memory management work in GTK? Should I free data returned from functions?

See the documentation for #GObject and #GInitiallyUnowned. For #GObject note specifically g_object_ref() and g_object_unref(). #GInitiallyUnowned is a subclass of #GObject so the same points apply, except that it has a "floating" state (explained in its documentation).

For strings returned from functions, they will be declared "const" if they should not be freed. Non-const strings should be freed with g_free(). Arrays follow the same rule. If you find an undocumented exception to the rules, please file a bug report..

The transfer annotations for gobject-introspection that are part of the documentation can provide useful hints for memory handling semantics as well.
Why does my program leak memory, if I destroy a widget immediately after creating it ?

If GtkFoo isn't a toplevel window, then
```
foo = gtk_foo_new ();
g_object_unref (foo);
```
is a memory leak, because no one assumed the initial floating reference (you will get a warning about this too). If you are using a widget and you aren't immediately packing it into a container, then you probably want standard reference counting, not floating reference counting.

To get this, you must acquire a reference to the widget and drop the floating reference (ref and sink in GObject parlance) after creating it:
```
foo = gtk_foo_new ();
g_object_ref_sink (foo);
```
When you immediately add a widget to a container, it takes care of assuming the initial floating reference and you don't have to worry about reference counting at all ... just remove the widget from the container to get rid of it.
How do I use GTK with threads?

GTK requires that all GTK API calls are made from the same thread in which gtk_init() was called (the main thread).

If you want to take advantage of multi-threading in a GTK application, it is usually best to send long-running tasks to worker threads, and feed the results back to the main thread using g_idle_add() or GAsyncQueue. GIO offers useful tools for such an approach such as GTask.
How do I internationalize a GTK program?

Most people use GNU gettext, already required in order to install GLib. On a UNIX or Linux system with gettext installed, type info gettext to read the documentation.

The short checklist on how to use gettext is: call bindtextdomain() so gettext can find the files containing your translations, call textdomain() to set the default translation domain, call bind_textdomain_codeset() to request that all translated strings are returned in UTF-8, then call gettext() to look up each string to be translated in the default domain.

gi18n.h provides the following shorthand macros for convenience. Conventionally, people define macros as follows for convenience:
```
#define  _(x)     gettext (x)
#define N_(x)     x
#define C_(ctx,x) pgettext (ctx, x)
```
You use N_() (N stands for no-op) to mark a string for translation in a location where a function call to gettext() is not allowed, such as in an array initializer. You eventually have to call gettext() on the string to actually fetch the translation. () both marks the string for translation and actually translates it. The C() macro (C stands for context) adds an additional context to the string that is marked for translation, which can help to disambiguate short strings that might need different translations in different parts of your program.

Code using these macros ends up looking like this:
```
#include <gi18n.h>

static const char *global_variable = N_("Translate this string");

static void
make_widgets (void)
{
  GtkWidget *label1;
  GtkWidget *label2;

  label1 = gtk_label_new (_("Another string to translate"));
  label2 = gtk_label_new (_(global_variable));
...
```
Libraries using gettext should use dgettext() instead of gettext(), which allows them to specify the translation domain each time they ask for a translation. Libraries should also avoid calling textdomain(), since they will be specifying the domain instead of using the default.

With the convention that the macro GETTEXT_PACKAGE is defined to hold your libraries translation domain, gi18n-lib.h can be included to provide the following convenience:
```
#define _(x) dgettext (GETTEXT_PACKAGE, x)
```
How do I use non-ASCII characters in GTK programs ?

GTK uses Unicode (more exactly UTF-8) for all text. UTF-8 encodes each Unicode codepoint as a sequence of one to six bytes and has a number of nice properties which make it a good choice for working with Unicode text in C programs:
- ASCII characters are encoded by their familiar ASCII codepoints.
- ASCII characters never appear as part of any other character.
- The zero byte doesn't occur as part of a character, so that UTF-8 string can be manipulated with the usual C library functions for handling zero-terminated strings.
More information about Unicode and UTF-8 can be found in the UTF-8 and Unicode FAQ. GLib provides functions for converting strings between UTF-8 and other encodings, see g_locale_to_utf8() and g_convert().

Text coming from external sources (e.g. files or user input), has to be converted to UTF-8 before being handed over to GTK. The following example writes the content of a IS0-8859-1 encoded text file to stdout:
```
char *text, *utf8_text;
gsize length;
GError *error = NULL;

if (g_file_get_contents (filename, &amp;text, &amp;length, NULL))
  {
    utf8_text = g_convert (text, length, "UTF-8", "ISO-8859-1",
                           NULL, NULL, &error);
    if (error != NULL)
      {
        fprintf ("Couldn't convert file %s to UTF-8\n", filename);
        g_error_free (error);
      }
    else
      g_print (utf8_text);
  }
else
  fprintf (stderr, "Unable to read file %s\n", filename);
```
For string literals in the source code, there are several alternatives for handling non-ASCII content:
- Direct UTF-8
  
  If your editor and compiler are capable of handling UTF-8 encoded sources, it is very convenient to simply use UTF-8 for string literals, since it allows you to edit the strings in "wysiwyg". Note that choosing this option may reduce the portability of your code.
- Escaped UTF-8
  
  Even if your toolchain can't handle UTF-8 directly, you can still encode string literals in UTF-8 by using octal or hexadecimal escapes like \212 or \xa8 to encode each byte. This is portable, but modifying the escaped strings is not very convenient. Be careful when mixing hexadecimal escapes with ordinary text; `"\xa8abcd" is a string of length 1 !
- Runtime conversion
  
  If the string literals can be represented in an encoding which your toolchain can handle (e.g. IS0-8859-1), you can write your source files in that encoding and use g_convert() to convert the strings to UTF-8 at runtime. Note that this has some runtime overhead, so you may want to move the conversion out of inner loops.
Here is an example showing the three approaches using the copyright sign © which has Unicode and ISO-8859-1 codepoint 169 and is represented in UTF-8 by the two bytes 194, 169, or "\302\251" as a string literal:
```
g_print ("direct UTF-8: ©");
g_print ("escaped UTF-8: \302\251");
text = g_convert ("runtime conversion: ©", -1,
                  "ISO-8859-1", "UTF-8", NULL, NULL, NULL);
g_print (text);
g_free (text);
```
If you are using gettext() to localize your application, you need to call bind_textdomain_codeset() to ensure that translated strings are returned in UTF-8 encoding.
How do I use GTK with C++?

There are two ways to approach this. The GTK header files use the subset of C that's also valid C++, so you can simply use the normal GTK API in a C++ program. Alternatively, you can use a "C++ binding" such as gtkmm which provides a native C++ API.

When using GTK directly, keep in mind that only functions can be connected to signals, not methods. So you will need to use global functions or "static" class functions for signal connections.

Another common issue when using GTK directly is that C++ will not implicitly convert an integer to an enumeration. This comes up when using bitfields; in C you can write the following code:
```
gdk_surface_set_events (gdk_surface,
                        GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
```
while in C++ you must write:
```
gdk_surface_set_events (gdk_surface,
                        (GdkEventMask) GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
```
There are very few functions that require this cast, however.
How do I use GTK with other non-C languages?

See the list of language bindings on the GTK website.
How do I load an image or animation from a file?

To load an image file straight into a display widget, use gtk_image_new_from_file(). To load an image for another purpose, use gdk_texture_new_from_file(). To load a video from a file, use gtk_media_file_new_for_file().

How do I draw text?

To draw a piece of text onto a cairo surface, use a Pango layout and pango_cairo_show_layout().

layout = gtk_widget_create_pango_layout (widget, text);
fontdesc = pango_font_description_from_string ("Luxi Mono 12");
pango_layout_set_font_description (layout, fontdesc);
pango_cairo_show_layout (cr, layout);
pango_font_description_free (fontdesc);
g_object_unref (layout);

See also the Cairo Rendering section of the Pango documentation.

To draw a piece of text in a widget snapshot() implementation, use gtk_snapshot_append_layout().

How do I measure the size of a piece of text?

To obtain the size of a piece of text, use a Pango layout and pango_layout_get_pixel_size(), using code like the following:

layout = gtk_widget_create_pango_layout (widget, text);
fontdesc = pango_font_description_from_string ("Luxi Mono 12");
pango_layout_set_font_description (layout, fontdesc);
pango_layout_get_pixel_size (layout, &amp;width, &amp;height);
pango_font_description_free (fontdesc);
g_object_unref (layout);

See also the Layout Objects section of the Pango documentation.

Why are types not registered if I use their GTK_TYPE_BLAH macro?

The %GTK_TYPE_BLAH macros are defined as calls to gtk_blah_get_type(), and the _get_type() functions are declared as %G_GNUC_CONST which allows the compiler to optimize the call away if it appears that the value is not being used.

GLib provides the g_type_ensure() function to work around this problem.
```
g_type_ensure (GTK_TYPE_BLAH);
```
How do I create a transparent toplevel window?

Any toplevel window can be transparent. It is just a matter of setting a transparent background in the CSS style for it.

...for lists and trees?

This question has different answers, depending on the size of the dataset and the required formatting flexibility.

If you want to display a large amount of data in a uniform way, your best option is a #GtkTreeView widget. See the tree widget overview. A list is just a tree with no branches, so the treeview widget is used for lists as well.

If you want to display a small amount of items, but need flexible formatting and widgetry inside the list, then you probably want to use a #GtkListBox, which uses regular widgets for display.
...for multi-line text display or editing?

See the text widget overview -- you should use the #GtkTextView widget.

If you only have a small amount of text, #GtkLabel may also be appropriate of course. It can be made selectable with gtk_label_set_selectable(). For a single-line text entry, see #GtkEntry.
...to display an image or animation?

GTK has two widgets that are dedicated to displaying images. #GtkImage, for small, fixed-size icons and #GtkPicture for content images.

Both can display images in just about any format GTK understands. You can also use #GtkDrawingArea if you need to do something more complex, such as draw text or graphics over the top of the image.

Both GtkImage and GtkPicture can display animations and videos as well. To show an webm file, load it with the GtkMediaFile API and then use it as a paintable:
```
mediafile = gtk_media_file_new_for_filename ("example.webm");
picture = gtk_picture_new_for_paintable (GDK_PAINTABLE (mediafile));
```
...for presenting a set of mutually-exclusive choices, where Windows would use a combo box?

With GTK, a #GtkComboBox is the recommended widget to use for this use case. If you need an editable text entry, use the #GtkComboBox:has-entry property.

Questions about GtkWidget

How do I change the color of a widget?

The background color of a widget is determined by the CSS style that applies to it. To change that, you can set style classes on the widget, and provide custom CSS to change the appearance. Such CSS can be loaded with gtk_css_provider_load_from_file() and its variants. See gtk_style_context_add_provider().
How do I change the font of a widget?

If you want to make the text of a label larger, you can use gtk_label_set_markup():
```
gtk_label_set_markup (label, "<big>big tex</big>");
```
This is preferred for many apps because it's a relative size to the user's chosen font size. See g_markup_escape_text() if you are constructing such strings on the fly.

You can also change the font of a widget by putting
```
.my-widget-class {
  font: Sans 30;
}
```
in a CSS file, loading it with gtk_css_provider_load_from_file(), and adding the provider with gtk_style_context_add_provider_for_display(). To associate this style information with your widget, set a style class on its #GtkStyleContext using gtk_style_context_add_class(). The advantage of this approach is that users can then override the font you have chosen. See the #GtkStyleContext documentation for more discussion.
How do I disable/ghost/desensitize a widget?

In GTK a disabled widget is termed insensitive. See gtk_widget_set_sensitive().

GtkTextView questions

How do I get the contents of the entire text widget as a string?

See gtk_text_buffer_get_bounds() and gtk_text_buffer_get_text() or gtk_text_iter_get_text().

GtkTextIter start, end;
GtkTextBuffer *buffer;
char *text;

buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (text_view));
gtk_text_buffer_get_bounds (buffer, &start, &end);
text = gtk_text_iter_get_text (&start, &end);
/* use text */
g_free (text);

How do I make a text widget display its complete contents in a specific font?

If you use gtk_text_buffer_insert_with_tags() with appropriate tags to select the font, the inserted text will have the desired appearance, but text typed in by the user before or after the tagged block will appear in the default style.
How do I make a text view scroll to the end of the buffer automatically ?

A good way to keep a text buffer scrolled to the end is to place a mark at the end of the buffer, and give it right gravity. The gravity has the effect that text inserted at the mark gets inserted before, keeping the mark at the end.

To ensure that the end of the buffer remains visible, use gtk_text_view_scroll_to_mark() to scroll to the mark after inserting new text.

The gtk-demo application contains an example of this technique.

GtkTreeView questions

How do I associate some data with a row in the tree?

Remember that the #GtkTreeModel columns don't necessarily have to be displayed. So you can put non-user-visible data in your model just like any other data, and retrieve it with gtk_tree_model_get(). See the tree widget overview.
How do I put an image and some text in the same column?

You can pack more than one #GtkCellRenderer into a single #GtkTreeViewColumn using gtk_tree_view_column_pack_start() or gtk_tree_view_column_pack_end(). So pack both a #GtkCellRendererPixbuf and a #GtkCellRendererText into the column.
I can set data easily on my #GtkTreeStore or #GtkListStore models using gtk_list_store_set() and gtk_tree_store_set(), but can't read it back?

Both the #GtkTreeStore and the #GtkListStore implement the #GtkTreeModel interface. Consequentially, you can use any function this interface implements. The easiest way to read a set of data back is to use gtk_tree_model_get().

How do I change the way that numbers are formatted by #GtkTreeView?

Use gtk_tree_view_insert_column_with_data_func() or gtk_tree_view_column_set_cell_data_func() and do the conversion from number to string yourself (with, say, g_strdup_printf()).

The following example demonstrates this:

enum
{
  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);
}

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

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.
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.
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().

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