While dragging we don't need to perform picking to determine the actor
underneath the pointer, for two reasons:
• we use a capture on the stage to determine the motion delta.
• we know the actor underneath the pointer because that's the
actor we are dragging around.
This change should make dragging actors in complex scenes a bit faster.
Both ::drag-begin and ::drag-end have a "button" argument - even though
we assume internally, and externally, that dragging can only be the
result of a primary button operation.
The marshallers we use for the signals are declared in a private header,
and it stands to reason that they should also be hidden in the shared
object by using the common '_' prefix. We are also using some direct
g_cclosure_marshal_* symbol from GLib, instead of consistently use the
clutter_marshal_* symbol.
Since emit_drag_end() can be called from a MOTION event capture we
cannot call clutter_event_get_button(). We should, instead, use the
press_button value because if we're emitting ::drag-end it means we
also emitted ::drag-begin and the value is valid.
The DragAction should, by default, drag the actor to which it has been
applied, instead of delegating what to do to the developer. If custom
code need to override it, g_signal_stop_emission_by_name() can be called
to stop the default handler to ever running.
DragAction is an Action sub-class that provides dragging capabilities to
any actor. DragAction has:
• drag-begin, drag-motion and drag-end signals, relaying the event
information like coordinates, button and modifiers to user code;
• drag-threshold property, for delaying the drag start by a given
amount of pixels;
• drag-handle property, to allow using other actors as the drag
handle.
• drag-axis property, to allow constraining the dragging to a specific
axis.
An interactive test demonstrating the various features is also provided.
