function [X,Y] = move(Constr, action, X, Y, X0, Y0)
%MOVE  Moves constraint
% 
%   [X0,Y0] = CONSTR.MOVE('init',X0,Y0) initialize the move. The 
%   pointer may be slightly moved to sit on the constraint edge
%   and thus eliminate distorsions due to patch thickness.
%
%   [X,Y] = CONSTR.MOVE('restrict',X,Y,X0,Y0) limits the displacement
%   to locations reachable by CONSTR.
%
%   STATUS = CONSTR.MOVE('update',X,Y,X0,Y0) moves the constraint.
%   For constraints indirectly driven by the mouse, the update is
%   based on a displacement (X0,Y0) -> (X,Y) for a similar constraint 
%   initially sitting at (X0,Y0).

%   Author(s): P. Gahinet, Bora Eryilmaz
%   Copyright 1986-2002 The MathWorks, Inc. 
%   $Revision: 1.1.6.2 $  $Date: 2004/12/10 19:31:55 $

% RE: Incremental move is not practical because the constraint cannot
%     track arbitrary mouse motion (e.g., enter negative gain zone)

HostAx  = handle(Constr.Parent);
XUnits  = Constr.PhaseUnits;
YUnits  = Constr.MagnitudeUnits;
Ylinabs = strcmp(YUnits, 'abs') & strcmp(HostAx.Yscale, 'linear');

switch action
 case 'init'
  % If the mouse is selecting this constraint, move pointer to margin origin 
  HostFig = HostAx.Parent;
  hChildren = Constr.Children;
  Tags = get(hChildren,'Tag');
  idx = strcmp(Tags,'ConstraintPatch');
  if hChildren(idx) == hittest(HostFig)
    % X = unitconv(Constr.OriginPha, 'deg', XUnits);
    % Y = unitconv(0, 'dB', YUnits);
    % Move pointer to new (X0,Y0)
    moveptr(HostAx, 'init')
    moveptr(HostAx, 'move',X,Y);
  end
  
  % Model is
  % Origin = -180 + 360k, k = ...,-1,0,1,... (in deg)
  % Save initial margin data
  Constr.AppData = struct('Origin0', Constr.OriginPha);
  
 case 'restrict'
  % Restrict displacement Y0 -> Y to account for constraints.
  if Ylinabs
    Y = max(Y, 1e-3*max(HostAx.Ylim));  % prevent negative gain
  end
  
 case 'update'
  % Phase margin origin should sit at -180 + 360k (in deg) for some k,
  % so that the initial phase margin origin is closest to the mouse position.
  dX = unitconv(X-X0, XUnits, 'deg') + Constr.AppData.Origin0;
  sgnPhaOrig = (dX >= 0) - (dX < 0);
  
  % Update phase margin origin (in deg)
  Constr.OriginPha = sgnPhaOrig * (abs(dX) + 180 - rem(abs(dX),360));
  
  % Update graphics and notify observers
  update(Constr)
  
  % Status
  Origin0 = unitconv(Constr.AppData.Origin0, 'deg', XUnits);
  Origin  = unitconv(Constr.OriginPha, 'deg', XUnits);
  LocStr = sprintf('Phase margin moved from %0.3g to %0.3g %s.', ...
		   Origin0, Origin, XUnits);
  X = sprintf('Move constraint to desired location and release the mouse.\n%s',LocStr); 
end