NAME

       canvas::drag - Manage the dragging of canvas items or item groups

SYNOPSIS

       package require Tcl  8.5

       package require Tk  8.5

       package require canvas::drag  ?0.1?

       ::canvas::drag on canvas tagOrId cmd option...

       ::canvas::drag off canvas tagOrId

       ::canvas::drag item canvas tagOrId option...

       ::canvas::drag group canvas tagOrId cmd option...

       {*}cmd start canvas item

       {*}cmd move canvas clientdata dx dy

       {*}cmd done canvas clientdata

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DESCRIPTION

       This  package provides utility commands to setup and rmeove dragging of items or item groups on a canvas,
       hiding all complexity regarding bindings from the user.

API

       ::canvas::drag on canvas tagOrId cmd option...
              This command initializes item dragging on the canvas widget, with the items used as  drag  handles
              identified  by  tagOrId.   The  command  prefix  cmd,  invoked  for  drag  start  and movement, is
              responsible for the initialization and actual execution of the drag operation.

              The signature of the command prefix is described later, in section Drag callback.

              Similarly, the accepted options and their values are described in section Options

              The result of the command is the empty string.

       ::canvas::drag off canvas tagOrId
              This command removes any drag operation set on the items of canvas canvas identified by tagOrId.

              The result of the command is the empty string.

       ::canvas::drag item canvas tagOrId option...
              This is a convenience command wrapped around method on (see above) to drag  single  items  of  the
              canvas widget, identified by tagOrId.

              It uses an internal standard callback for this.

              The result of the command is the empty string.

       ::canvas::drag group canvas tagOrId cmd option...
              This  is  a  convenience  command wrapped around method on (see above) to drag single items of the
              canvas widget, identified by tagOrId.

              It uses an internal standard callback for this. The callback cmd specified has the same  signature
              as the Drag callback, except that

              [1]    The move method is not invoked.

              [2]    The  result of the start method has to be a canvas tag refering to the whole group of items
                     to move. In other words, it must convert from drag  handle  (item  id)  to  dragged  groupt
                     (tag).

       The result of the command is the empty string.

   DRAG CALLBACK
       The drag callback is a command prefix invoked in the following two ways:

       {*}cmd start canvas item
              This  form  is invoked when has initiated dragging using drag handle identified by the canvas item
              id.  The callback now has to perform anything necessary for its type of drag operation.

              The result of the command can be anything. It is stored by the system as  client  information  and
              passed unchanged to the movement callback for its use. In this manner the drag callback is able to
              maintain custom state from start to movement.

       {*}cmd move canvas clientdata dx dy
              This  form is invoked when the mouse moved during a drag operation.  It is invoked with the client
              data from the start callback (or the previous move callback)  and  the  distances  the  mouse  has
              traveled in horizontal and vertical directions.

              The  result  of  the command can be anything. It is stored by the system as client information and
              passed unchanged to the next movement callback for its use. In this manner the  drag  callback  is
              able to maintain custom state from movement to movement.

       {*}cmd done canvas clientdata
              This  form  is  invoked when the drag operation ends.  It is invoked with the client data from the
              last movement callback (or start callback if there had been no motion).

              The result of the command is ignored.

   OPTIONS
       The commands to create drag operations (on,  item,  and  group)  all  accept  the  following  options  to
       configure the new drag.

       -event spec
              The  value  of this option specifies the mouse button used to initiate the drag operation, and the
              keyboard modifier, if any. Examples of specifications:

              To initiate a drag operation by pressing mouse button 3 on a drag handle, use:

               -event 3

              This is the default as well, if the option is not specified.

              To initiate a drag operation by pressing mouse button 2 on a drag handle while  holding  down  the
              Control key, use:

               -event Control-2

BUGS, IDEAS, FEEDBACK

       This  document,  and  the package it describes, will undoubtedly contain bugs and other problems.  Please
       report such in the category canvas of the Tklib Trackers  [http://core.tcl.tk/tklib/reportlist].   Please
       also report any ideas for enhancements you may have for either package and/or documentation.

KEYWORDS

       canvas, dragging

tklib                                                  0.1                                     canvas::drag(3tk)