contents
This page is a place to collect pieces of information regarding the visual appearance of the Lumiera GTK UI.
Further (more technical) aspects regarding the inner workings of the toolkit can be found
in the »Code Base« section regarding GTK. Details about the concrete
arrangement and conventions within some parts of the UI are given on the sub pages…
Timeline
· Navigation
· Property Editors
Styling of GTK-3 interfaces is based on CSS, with some specific conventions about the selectors and some additional macro functions to generate colours and gradients. When GTK actually renders a widget, it consults a strategy object known as Theme Engine, passing it the region to draw in a abstracted way. The Theme Engine in turn uses a “style provider” to retrieve the generic style properties it uses for drawing. Thus, the Theme Engine defines the actual meaning of any style and is in the position to understand and thus introduce additional engine specific styles and settings.
GTK-3 supports the powerful cascading and contextual selectors from CSS. Thus the nesting of elements
in the GUI forms the base for creating styling rules. Hereby, widget class names translate into “tag” names
in the CSS selectors.
[The “tag” name is the widget’s class name without the Gtk:: prefix, and transformed
into all lower caps, single word, no underscores. E.g. Gtk::TextView → textview.
However — these names are established programmatically, unfortunately within the C implementation of the style
class constructor, which makes them more or less hard wired. The Gtk::Widget subclasses from Gtkmm (C++ language) use
their own naming scheme, apart from the basic GTK+ (C language) names, and it is basically not possible for custom widgets
to expose their distinct type names — rather they will show up under the name of the base class used from Gtkmm.]
Widgets may also expose CSS classes for styling — the standard widgets define a generic set
of predefined CSS style classes,
which can be used to establish the foundation for theming. Obviously it is preferable to keep styling rules as
concise, generic and systematic as possible; yet we may still refer to individual GUI elements by name (#ID) though.
for technically precise coverage, consult the pages GtkCssProvider and predefined style classes in the GTK-3 reference manual.
there is an overview page in the developer manual, and a reference of supported properties.
to start, look at this introductory text, or the more hands-on series of articles from world of gnome
this post from fedora forum features a concise description of the task of theme creation
to understand the old (now obsolete) GTK-2 stylesheets, you might look here
Unfortunately, documentation about creating GTK-3 themes is still fragmentary. Most people seem to learn by studying existing themes. To make matters worse, CSS allows to address every widget under various contextual constraints — and people tend to approach such abundant possibilities with a case-by-case attitude, instead of a systematic approach, and this leads to incredible large and redundant stylesheets.
Often we’ll also face the perils of over-constrained settings. More so, since every system contains several
style sheets, and settings from those are combined (“cascaded”). When things are specified multiple times
redundantly at different levels, we can never be sure as to which change actually caused a visible effect.
A good recommendation is really to “probe” settings by changing them temporarily to a really obvious value
(e.g. background-color: red). It is just too easy to learn wrong techniques based on false conclusions.
An essential tool when working with styles and Gtk widgets in general is the
GTK+ inspector, which is part of the standard GTK distribution.
It allows to inspect all GTK objects with their properties, and to see the actual tree of CSS nodes and the
corresponding selectors. You can even add a style class or state flag (like “hover”) dynamically, and you
may add style rules and verify the effect on the application immediately. To use this ispector, launch the
application like GTK_DEBUG=interactive target/lumiera
GTK-3 supports binary theme bundles, which combine CSS style sheets and accompanying images and vector graphics
into a single archive file. See this blog entry
for a tutorial. But when it comes to investigating an existing theme, we need a way to extract the original sources
from such a distribution bundle. This can be achieved with the help of the gresource command. The following bash
script
[published by Peter Gordon to the Public Domain
at his blog in 2012]
simplifies this process, allowing
to extract all resource files in a given GResource file, with the given base URL. For example, if a GResource file
contained the resource with the URL /org/foo/bar/baz.txt, and the base URL defined as "/org/foo/", then the resource
named /org/foo/bar/baz.txt in that file would be extracted and written to bar/baz.txt in the current directory.
#!/bin/bash # The GResource file name GR_FILE="gtk.gresource" # base URL of the extracted resources GR_BASEDIR="/org/gnome/" which gresource &>/dev/null if [ $? -ne 0 ]; then echo "Unable to find gresource program in PATH." exit 1 fi for RSRC in $(gresource list $GR_FILE) do RSRC_FILE=$(echo "${RSRC#$GR_BASEDIR}") mkdir -p $(dirname "$RSRC_FILE") ||: gresource extract "$GR_FILE" "$RSRC" > "$RSRC_FILE" done
The Lumiera UI achieves some degree of uniformity by adhering to common schemes and conventions; moreover, we ship a dedicated Application Style Sheet to establish a distinct look and feel(planned as of 2022) — additionally, as alternative, we ship “theme complement” style sheets to add just those CSS definitions necessary for Lumiera to properly work together with a typical GTK desktop theme.
More elaborate websites as well as larger applications tend to run into problems with naming of components.
Unintended matches of CSS rules in other segments of the site may cause side effects, thereby hampering the ability to
adapt a given part to new requirements. While the true cause for such problems is rooted in an unsystematic and “pragmatic”
attitude, treating the style as an assortment of local tweaks, a coherent naming scheme helps to indicate which setting
is meant to be generic and global, and which one addresses a bounded scope.
As an ubiquitous naming scheme, the BEM notation(TODO find a good authoritative website to link) has gained
some traction in the context of naming of CSS selectors. Yet at the same time, also an unfortunate trend could be observed
to neglect or even defeat the cascading nature of CSS and to apply redundant and repetitive fine grained styles to
individual elements.
[Some people even think they need style generators to cope with the resulting combinatoric
explosion. A similar tendency can be observed in the realm of publication, where people frequently disregard or even directly
reject the idea of a style sheet, and then create elaborate schemes of work organisation, combined with text processor macros,
to cope with the nonsensical task of having to format each paragraph individually. Those disdaining the use of abstractions
have to suffer and deserve no mercy.]
In Lumiera, we use a combination of style classes and element-IDs to define the anchor points to attach specific styling; typically, we also have to designate sub elements below such an anchor point, and here the usage of BEM notation is recommended:
block[__element][--modifier]the first part of a name designates the enclosing block — which must be a building block to be used in varying context and without the tie to a single specific hierarchy
the second part of the name, separated by "__", designates the individual element within this block. Such an element
should be tied logically to the block; if this use is just an optional combination of building blocks and the element
can also be used elsewhere, then it should be treated as another block rather.
sometimes we want to modify some aspects of the presentation by a fixed set of options, and in this case, the name can
be extended by a modifier component, attached with a "--" separator.
A flexible and open structure model is one of the cornerstones of the Lumiera architecture: Instead of relying on a common global data model, the application rather defines some structural conventions, assuming that individual Entities will be arranged in accordance to standardised patterns.
Those “Entities” are conceived as objects, which are placed into a context and expose properties and methods. Within the UI, this structure finds its correspondence in the ElementBoxWidget(TODO dedicated page and link), which is used pervasively as a building block in several contexts and for various purposes. All these usages visually share some kind of common denominator:
the »Entity« is represented as a box with a name
it features an icon as a leading handle
followed by a expander/collapser or menu button
This structure is implemented as a specialised Gtk::Frame — which uses a label widget comprised of a box with
three elements: two icon buttons, and a text field. The content area of the Frame widget is used to represent the
content of this “Element”; be it just a background box coloured in accordance to the type or kind of object, or
be it a dedicated content renderer (for Clips in the Timeline) or even a canvas further exposing inner structures.
frame#element
box#caption
both icons are placed into a Gtk::Button to receive events