Drag and drop files in Matlab GUI

Question

I am trying to find a way to use drag and drop in my matlab GUI. The closest I've found is this.

However, I would like the result to look like this:

When a file has been dropped, all I need is the path of the file and a call to my load function.

All suggestions are much appreciated!

Answer 1

This submission by Maarten van der Seijs on the file exchange seems to solve it.

It creates a callback function which can be coupled to java swing GUI components, as shown in the attached demo.

It uses a java class, which is a thin wrapper around java.awt.dnd.DropTarget:

import java.awt.dnd.*;
import java.awt.datatransfer.*;
import java.util.*;
import java.io.File;
import java.io.IOException;

public class MLDropTarget extends DropTarget
{
    /**
     * Modified DropTarget to be used for drag & drop in MATLAB UI control.
     */
    private static final long serialVersionUID = 1L;
    private int droptype;
    private Transferable t;
    private String[] transferData;

    public static final int DROPERROR = 0;
    public static final int DROPTEXTTYPE = 1;
    public static final int DROPFILETYPE = 2;

    @SuppressWarnings("unchecked")
    @Override
    public synchronized void drop(DropTargetDropEvent evt) {

        // Make sure drop is accepted
        evt.acceptDrop(DnDConstants.ACTION_COPY_OR_MOVE);

        // Set droptype to zero
        droptype = DROPERROR;        

        // Get transferable and analyze
        t = evt.getTransferable();

        try {
            if (t.isDataFlavorSupported(DataFlavor.javaFileListFlavor)) {
                // Interpret as list of files
                List<File> fileList = (ArrayList<File>) t.getTransferData(DataFlavor.javaFileListFlavor);
                transferData = new String[fileList.size()];
                for (int i = 0; i < fileList.size(); i++) 
                    transferData[i] = fileList.get(i).getAbsolutePath();
                droptype = DROPFILETYPE;
            } 
            else if (t.isDataFlavorSupported(DataFlavor.stringFlavor)) {
                // Interpret as string              
                transferData[0] = (String) t.getTransferData(DataFlavor.stringFlavor);
                droptype = DROPTEXTTYPE;
            }

        } catch (UnsupportedFlavorException e) {
            droptype = DROPERROR;
            super.drop(evt);            
            return;
        } catch (IOException e) {
            droptype = DROPERROR;
            super.drop(evt);
            return;
        }

        // Call built-in drop method (fire MATLAB Callback)       
        super.drop(evt);
    }

    public int getDropType() {
        return droptype;
    }   
    public Transferable getTransferable() {
        return t;
    }
    public String[] getTransferData() {
        return transferData;
    }
}

which is then initialized and called by a MATLAB class:

classdef (CaseInsensitiveProperties) dndcontrol < handle
%DNDCONTROL Class for Drag & Drop functionality.
%   obj = DNDCONTROL(javaobj) creates a dndcontrol object for the specified
%   Java object, such as 'javax.swing.JTextArea' or 'javax.swing.JList'. Two
%   callback functions are available: obj.DropFileFcn and obj.DropStringFcn, 
%   that listen to drop actions of respectively system files or plain text.
%
%   The Drag & Drop control class relies on a Java class that need to be
%   visible on the Java classpath. To initialize, call the static method
%   dndcontrol.initJava(). The Java class can be adjusted and recompiled if
%   desired.
%
%   DNDCONTROL Properties:
%       Parent            - The associated Java object.
%       DropFileFcn       - Callback function for system files.
%       DropStringFcn     - Callback function for plain text.
%
%   DNDCONTROL Methods:
%       dndcontrol        - Constructs the DNDCONTROL object.
%
%   DNDCONTROL Static Methods:
%       defaultDropFcn    - Default callback function for drop events.
%       demo              - Runs the demonstration script.
%       initJava          - Initializes the Java class.
%       isInitialized     - Checks if the Java class is visible.
%
%   A demonstration is available from the static method dndcontrol.demo().
%
%   Example:
%       dndcontrol.initJava();
%       dndcontrol.demo();
%
%   See also:
%       uicontrol, javaObjectEDT.    
%
%   Written by: Maarten van der Seijs, 2015.
%   Version: 1.0, 13 October 2015.


    properties (Hidden)
        dropTarget;                
    end

    properties (Dependent)
        %PARENT The associated Java object.
        Parent;
    end

    properties
        %DROPFILEFCN Callback function executed upon dropping of system files.
        DropFileFcn;        
        %DROPSTRINGFCN Callback function executed upon dropping of plain text.
        DropStringFcn;        
    end

    methods (Static)
        function initJava()
        %INITJAVA Initializes the required Java class.

            %Add java folder to javaclasspath if necessary
            if ~dndcontrol.isInitialized();
                classpath = fileparts(mfilename('fullpath'));                
                javaclasspath(classpath);                
            end 
        end

        function TF = isInitialized()            
        %ISINITIALIZED Returns true if the Java class is initialized.

            TF = (exist('MLDropTarget','class') == 8);
        end                           
    end

    methods
        function obj = dndcontrol(Parent,DropFileFcn,DropStringFcn)
        %DNDCONTROL Drag & Drop control constructor.
        %   obj = DNDCONTROL(javaobj) contstructs a DNDCONTROL object for 
        %   the given parent control javaobj. The parent control should be a 
        %   subclass of java.awt.Component, such as most Java Swing widgets.
        %
        %   obj = DNDCONTROL(javaobj,DropFileFcn,DropStringFcn) sets the
        %   callback functions for dropping of files and text.

            % Check for Java class
            assert(dndcontrol.isInitialized(),'Javaclass MLDropTarget not found. Call dndcontrol.initJava() for initialization.')

            % Construct DropTarget            
            obj.dropTarget = handle(javaObjectEDT('MLDropTarget'),'CallbackProperties');
            set(obj.dropTarget,'DropCallback',{@dndcontrol.DndCallback,obj});
            set(obj.dropTarget,'DragEnterCallback',{@dndcontrol.DndCallback,obj});

            % Set DropTarget to Parent
            if nargin >=1, Parent.setDropTarget(obj.dropTarget); end

            % Set callback functions
            if nargin >=2, obj.DropFileFcn = DropFileFcn; end 
            if nargin >=3, obj.DropStringFcn = DropStringFcn; end
        end

        function set.Parent(obj, Parent)
            if isempty(Parent)
                obj.dropTarget.setComponent([]);
                return
            end
            if isa(Parent,'handle') && ismethod(Parent,'java')
                Parent = Parent.java;
            end
            assert(isa(Parent,'java.awt.Component'),'Parent is not a subclass of java.awt.Component.')
            assert(ismethod(Parent,'setDropTarget'),'DropTarget cannot be set on this object.')

            obj.dropTarget.setComponent(Parent);
        end

        function Parent = get.Parent(obj)
            Parent = obj.dropTarget.getComponent();
        end
    end

    methods (Static, Hidden = true)
        %% Callback functions
        function DndCallback(jSource,jEvent,obj)

            if jEvent.isa('java.awt.dnd.DropTargetDropEvent')
                % Drop event     
                try
                    switch jSource.getDropType()
                        case 0
                            % No success.
                        case 1
                            % String dropped.
                            string = char(jSource.getTransferData());
                            if ~isempty(obj.DropStringFcn)
                                evt = struct();
                                evt.DropType = 'string';
                                evt.Data = string;                                
                                feval(obj.DropStringFcn,obj,evt);
                            end
                        case 2
                            % File dropped.
                            files = cell(jSource.getTransferData());                            
                            if ~isempty(obj.DropFileFcn)
                                evt = struct();
                                evt.DropType = 'file';
                                evt.Data = files;                                
                                feval(obj.DropFileFcn,obj,evt);
                            end
                    end

                    % Set dropComplete
                    jEvent.dropComplete(true);  
                catch ME
                    % Set dropComplete
                    jEvent.dropComplete(true);  
                    rethrow(ME)
                end                              

            elseif jEvent.isa('java.awt.dnd.DropTargetDragEvent')
                 % Drag event                               
                 action = java.awt.dnd.DnDConstants.ACTION_COPY;
                 jEvent.acceptDrag(action);
            end            
        end
    end

    methods (Static)
        function defaultDropFcn(src,evt)
        %DEFAULTDROPFCN Default drop callback.
        %   DEFAULTDROPFCN(src,evt) accepts the following arguments:
        %       src   - The dndcontrol object.
        %       evt   - A structure with fields 'DropType' and 'Data'.

            fprintf('Drop event from %s component:\n',char(src.Parent.class()));
            switch evt.DropType
                case 'file'
                    fprintf('Dropped files:\n');
                    for n = 1:numel(evt.Data)
                        fprintf('%d %s\n',n,evt.Data{n});
                    end
                case 'string'
                    fprintf('Dropped text:\n%s\n',evt.Data);
            end
        end            

        function [dndobj,hFig] = demo()
        %DEMO Demonstration of the dndcontrol class functionality.
        %   dndcontrol.demo() runs the demonstration. Make sure that the
        %   Java class is visible in the Java classpath.

            % Initialize Java class
            dndcontrol.initJava();

            % Create figure
            hFig = figure();

            % Create Java Swing JTextArea
            jTextArea = javaObjectEDT('javax.swing.JTextArea', ...
                sprintf('Drop some files or text content here.\n\n'));

            % Create Java Swing JScrollPane
            jScrollPane = javaObjectEDT('javax.swing.JScrollPane', jTextArea);
            jScrollPane.setVerticalScrollBarPolicy(jScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

            % Add Scrollpane to figure
            [~,hContainer] = javacomponent(jScrollPane,[],hFig);
            set(hContainer,'Units','normalized','Position',[0 0 1 1]);

            % Create dndcontrol for the JTextArea object
            dndobj = dndcontrol(jTextArea);

            % Set Drop callback functions
            dndobj.DropFileFcn = @demoDropFcn;
            dndobj.DropStringFcn = @demoDropFcn;

            % Callback function
            function demoDropFcn(~,evt)
                switch evt.DropType
                    case 'file'
                        jTextArea.append(sprintf('Dropped files:\n'));
                        for n = 1:numel(evt.Data)
                            jTextArea.append(sprintf('%d %s\n',n,evt.Data{n}));
                        end
                    case 'string'
                        jTextArea.append(sprintf('Dropped text:\n%s\n',evt.Data));
                end
                jTextArea.append(sprintf('\n'));
            end
        end
    end    
end