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Gtk+/Gnome Application Development
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Requesting Updates

It is the canvas item's responsibility to request an update or redraw when the properties of the item are changed and the screen should be refreshed. This is straightforward. For example, here is a snippet of code from gnome_canvas_re_set_arg(), which sets the "y2" argument:

  case ARG_Y2:
    re->y2 = GTK_VALUE_DOUBLE (*arg);

    gnome_canvas_item_request_update (item);


Since "y2" modifies the shape of the rectangle, the path must be recreated and an update is necessary. Note that gnome_canvas_item_request_update() simply sets a flag and installs an idle handler if none is pending, so it can be called many times without a performance penalty.

Not all changes require an update; a redraw may be sufficient, or perhaps the argument is unrelated to the display. It depends on the canvas item and what exactly is being changed.

Gtk+/Gnome Application Development
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