|GooCanvas Reference Manual|
Underlying Architecture — how the canvas fits together.
The GooCanvas Widget
GooCanvas is a GtkWidget (it is actually a subclass of GtkContainer), and so can be placed in an interface just like any normal widget. Usually a GooCanvas widget would be placed inside a GtkScrolledWindow in order to enable scrolling of the canvas.
The size of the canvas can be set explicitly using
goo_canvas_set_bounds(), or if the "automatic-bounds"
property is set to
TRUE the bounds will be automatically calculated
to include all of the canvas items. The units used in the canvas can
be set with the "units" property. The canvas units can be
pixels, points, inches or millimeters and apply to the canvas and
The Structure of the Simple Canvas
The simple canvas consists of a hierarchy of canvas items.
The root item is automatically created by the canvas and can be
goo_canvas_get_root_item(). New items and groups can
then be created and added to the root item.
Each item in the canvas keeps a GooCanvasBounds structure which stores the bounding rectangle of the item and all of its descendants. This makes it easy to find out which items in the canvas need repainting or which item the mouse is over. (The bounds are stored in the canvas coordinate space, which is the coordinate space of the entire canvas, after any item transformation matrices have been applied.)
The Structure of the Model/View Canvas
The model/view canvas consists of a hierarchy of item models, and an identical hierarchy of canvas items, with each canvas item corresponding to one item model.
The root item model is set with
The canvas will automatically create canvas items to display
the hierarchy of item models, and will automatically add and
remove canvas items as the item model hierarchy is changed.
The Update Procedure
When items are added to the canvas or their properties are changed
they may need to recalculate their bounds. To do this they set an
internal flag such as
need_update, and make a call to
GooCanvas handles all the update requests at once, to avoid multiple
redraws of the same parts of the canvas. To do this it installs
an idle handler,
goo_canvas_idle_handler(), which is called as soon
as the application is idle (and before any part of the canvas is
The idle handler calls
goo_canvas_item_update() on the root item,
which recursively calls
goo_canvas_item_update() on any items as
necessary, recalculating their bounds and requesting redraws as
If a container item (e.g. GooCanvasGroup) is changed it needs to
ensure that all descendants recalculate their bounds so it
goo_canvas_item_update() for all of its children with the
entire_tree argument set to
How Changes to Items are Handled
When an item is changed (e.g. if the "x" property of
a GooCanvasRect is changed), the item calls
goo_canvas_item_simple_changed() with a flag indicating if the
bounds of the item need to be recalculated.
If the bounds don't need to be recalculated, then
goo_canvas_request_redraw() is called to simply request that the
item is redrawn. This results in a call to
and the redraw proceeds just like a normal GtkWidget.
However, if the bounds do need to be recalculated then
goo_canvas_item_request_update() is called to request that the item
be updated the next time the canvas performs an update.
How Changes are Handled in the Model/View Canvas
In the Model/View canvas it is the underlying item models which are
initially changed. The item models emit "changed" signals which the
items respond to. For the standard canvas items the
goo_canvas_item_model_simple_changed() signal handler is called,
goo_canvas_item_simple_changed() and the
procedure continues as in the simple canvas case above.