This proposal is aimed to provide:

• sane theming implementation, with all the features that could be expected in a modern toolkit
• a coherent migration path for GTK+, not everything has to be ported at once, also providing a more feasible schedule wrt GTK+ 3.0

Participants

GtkStyleContext

• One per widget
• Has API to add several style providers: each provider would have a priority
• The style context merges the style from all providers, from lesser to more priority, provides API for widgets to query the resulting set.
• Allows widgets to define named regions, such regions could have different style settings than the widget itself
  • notebook tabs
  • treeview rows
• Enables the use of animations (only for state changes)
  • Provides API to notify on a state change
  • Handles animations
    • Bookkeeping of animation resources
    • Keeps track of which regions need invalidating
  • Theme engines get to know the progress [0..1] of the animation for a given state.
  • Enables widgets to add further context information
    • Specific API calls, widgets aren't supposed to change style information this way
    • Placing info: CONNECTS_[TOP|BOTTOM|LEFT|RIGHT] so the engine has a sense of interconnected elements
      • Spin buttons
      • Path bar / Breadcrumbs
    • Contextual information for named regions
      • First/last
      • Even/Odd
• Allows widgets to query style settings

  • Initial set of properties, should mimic GtkStyle contents, plus a few more settings

  • widget style properties

GtkStyleProvider

• Interface to be implemented by several objects
  • CSS or whatever is the replacement to GtkRC: Future plans

  • GtkStyle: Legacy settings until GtkRC is replaced

  • GtkSettings?: user font settings, custom colors

  • GtkStyleContext itself: Proper parent->child style inheritance

• Their purpose is to feed GtkStyleContext with style info

GtkTheme

• Theme engine implementations

• Methods to paint elements, pretty much like in GtkStyleClass

• Cairo as first class citizen
• Limited access to application/toolkit information

  • Its own set of getters so the GtkStyleContext is not exposed, no way to change anything in it.

  • Little to no knowledge about the actual widget, all necessary info should be already in the context

Miscellaneous objects/structs

• GtkWidgetPath: Object to represent a full widget path with all its details: widget names, named regions, class name. Would be used by the style context in order to query information from providers.

• GtkPadding: boxed struct to replace x/y thickness, could provide different padding for left/right or top/bottom

• GdkColor should have alpha info

  • Need to check backwards compat, maybe could be done like GdkEvent private data.

Migration plan

• We only need GtkStyleContext , GtkTheme and the GtkStyle implementation of GtkStyleProvider in place

• Make gtk_paint_*() methods use new API underneath
  • Widgets would automatically start using the most basic set of features of the new API

At this point, we'd have a GTK+ using the new API, but taking style information from the old places.

Further changes, such as doing a CSS GtkStyleProvider implementation, or actually porting widgets to use the new API could be done on a one by one basis, and in the least disrupting way

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