java_dnd.m

function java_dnd(jObj, dropFcn)
% Set Matlab dropFcn for java object, like JavaFrame or JTextField.

% 170421 Xiangrui Li adapted from dndcontrol class by Maarten van der Seijs:
% https://www.mathworks.com/matlabcentral/fileexchange/53511

% Required: MLDropTarget.class under the same folder

if ~exist('MLDropTarget', 'class')
    pth = fileparts(mfilename('fullpath'));
    javaaddpath(pth); % dynamic for this session
    fid = fopen(fullfile(prefdir, 'javaclasspath.txt'), 'a+');
    if fid>0 % static path for later sessions: work for 2013+?
        cln = onCleanup(@() fclose(fid));
        fseek(fid, 0, 'bof');
        classpth = fread(fid, inf, '*char')';
        if isempty(strfind(classpth, pth)) %#ok<*STREMP> % avoid multiple write
            fseek(fid, 0, 'bof');
            fprintf(fid, '%s\n', pth);
        end
    end
end

dropTarget = handle(javaObjectEDT('MLDropTarget'), 'CallbackProperties');
set(dropTarget, 'DragEnterCallback', @DragEnterCallback, ...
                'DragExitCallback', @DragExitCallback, ...
                'DropCallback', {@DropCallback, dropFcn});
jObj.setDropTarget(dropTarget);
%%

function DropCallback(jSource, jEvent, dropFcn)
setComplete = onCleanup(@()jEvent.dropComplete(true));
% Following DropAction is for ~jEvent.isLocalTransfer, such as dropping file.
% For LocalTransfer, Linux seems consistent with other OS.
% DropAction: Neither ctrl nor shift Dn, PC/MAC 2, Linux 1
% All OS: ctrlDn 1, shiftDn 2, both Dn 1073741824 (2^30)
if ispc || ismac
    evt.ControlDown = jEvent.getDropAction() ~= 2;
else % fails to report CtrlDn if user releases shift between DragEnter and Drop
    evt.ControlDown = bitget(jEvent.getDropAction,31)>0; % ACTION_LINK 1<<30
    java.awt.Robot().keyRelease(16); % shift up
end
% evt.Location = [jEvent.getLocation.x jEvent.getLocation.y]; % top-left [0 0]
if jSource.getDropType() == 1 % String dropped
    evt.DropType = 'string';
    evt.Data = char(jSource.getTransferData());
    if strncmp(evt.Data, 'file://', 7) % files identified as string
        evt.DropType = 'file';
        evt.Data = regexp(evt.Data, '(?<=file://).*?(?=\r?\n)', 'match')';
    end
elseif jSource.getDropType() == 2 % file(s) dropped
    evt.DropType = 'file';
    evt.Data = cell(jSource.getTransferData());
else, return; % No success
end

if iscell(dropFcn), feval(dropFcn{1}, jSource, evt, dropFcn{2:end});
else, feval(dropFcn, jSource, evt);
end
%%

function DragEnterCallback(~, jEvent)
try jEvent.acceptDrag(1); catch, return; end % ACTION_COPY
if ~ispc && ~ismac, java.awt.Robot().keyPress(16); end % shift down
%%

function DragExitCallback(~, jEvent)
if ~ispc && ~ismac, java.awt.Robot().keyRelease(16); end % shift up
try jEvent.rejectDrag(1); catch, end % ACTION_COPY
%%