Drag and Drop of files

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Drag and Drop of files on QMainWindows

Background / technical stuff

Drag and drop is done via some events:

• void dragEnterEvent(QDragEnterEvent* event);
• void dragMoveEvent(QDragMoveEvent* event);
• void dragLeaveEvent(QDragLeaveEvent* event);
• void dropEvent(QDropEvent* event);

They are called in order to let the application decide if it could accept a drag/drop or not. If the application says: do not accept this dragEnter event, then the OS will deny dropping of the content for the application. Same applies to the dragMoveEvent (perhaps, dropping is only allowed in special places, like toolbar etc.).

The drop event is called when the user drops their data. Inside the drop event, the widget must react on the different drop actions and the content type. e.g. (here we react on all actions):

 void DocumentWindow::dropEvent(QDropEvent* event)
 {
   const QMimeData* mimeData = event->mimeData();
 
   // check for our needed mime type, here a file or a list of files
   if (mimeData->hasUrls())
   {
     QStringList pathList;
     QList<QUrl> urlList = mimeData->urls();
 
     // extract the local paths of the files
     for (int i = 0; i < urlList.size() && i < 32; +i)
     {
       pathList.append(urlList.at(i).toLocalFile());
     }
 
     // call a function to open the files
     openFiles(pathList);
   }
 }

Use the DocumentWindow class

If you want to create an (MDI) editor application, you can derive your main window class from DocumentWindow instead of QMainWindow. If you implement an MDI application, you should also reimplement the virtual method

 ddeOpenFile

The example code is based on the Qt MDI Example

The only changes needed are:

• derive from DocumentWindow instead of QMainWindow
• implement the functions virtual bool openFiles(const QStringList& pathList);

 bool MainWindow::openFiles(const QStringList& pathList)
 {
   bool success = true;
   for (int i = 0; i < pathList.size() && i < 32;i)
   {
     MdiChild child = createMdiChild();
     if (child->loadFile(pathList.at(i)))
     {
       statusBar()->showMessage(tr("File loaded"), 2000);
       child->show();
     }
     else
     {
       child->close();
     }
   }
   return success;
 }

Use the DocumentWindow sources

DocumentWindow.h

 // ————————————————————————————————-
 /*
  * author Gerolf Reinwardt
  * date 30. march 2011
  *
  * Copyright © 2011, Gerolf Reinwardt. All rights reserved.
  *
  * Simplified BSD License
  *
  * Redistribution and use in source and binary forms, with or without modification, are
  * permitted provided that the following conditions are met:
  *
  * 1. Redistributions of source code must retain the above copyright notice, this list of
  * conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright notice, this list
  * of conditions and the following disclaimer in the documentation and/or other materials
  * provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY <COPYRIGHT HOLDER> ``AS IS AND ANY EXPRESS OR IMPLIED
  * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
  * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * The views and conclusions contained in the software and documentation are those of the
  * authors and should not be interpreted as representing official policies, either expressed
  * or implied, of Gerolf Reinwardt.
  */
 // ————————————————————————————————-
 
 #ifndef WIDGET_H
 #define WIDGET_H
 
 // —— general includes —————————————————————————
 #include <QMainWindow>
 #include <QStringList>
 
 // —— local includes —————————————————————————-
 // —— pre defines ——————————————————————————-
 
 // —— class definition —————————————————————————
 /*
  * The usage is fairly easy. Derive your own MainWindow class from DocumentWindow instead of QMainWindow
  * and implement the pure virtual function openFiles.
  *
  */
 MainWindow::MainWindow(QWidget* parent, Qt::WindowFlags flags) : DocumentWindow(parent, flags)
 {
   setWindowTitle(tr("MDI"));
   setUnifiedTitleAndToolBarOnMac(true);
 }
 
 /*
  * Aditionally, the openFiles must be implermented:
  */
 bool MyClass::openFiles(const QStringList& pathList)
 {
   bool success = true;
   for (int i = 0; i < pathList.size() && i < 32;i)
   {
     MdiChild child = createMdiChild();
     if (child->loadFile(pathList.at(i)))
     {
       statusBar()->showMessage(tr("File loaded"), 2000);
       child->show();
     }
     else
     {
       child->close();
     }
   }
   return success;
 }
 
 class DocumentWindow : public QMainWindow
 {
   Q_OBJECT
   public:
   // —— enums ———————————————————————————
   // —— construction —————————————————————————
   /*
    * Constructor.
    *
    * Creates a DocumentWindow with a given arg parent and arg flags.
    */
   explicit DocumentWindow(QWidget parent = 0, Qt::WindowFlags flags = 0);
 
   /**
   * Destructor
   */
   ~DocumentWindow();
 
  // —— operators ——————————————————————————
  // —— methods ——————————————————————————-
  // —— accessors ——————————————————————————
  // —— members ——————————————————————————-
 
   protected:
   // —— events ———————————————————————————
   /*
    * this event is called when the mouse enters the widgets area during a drag/drop operation
    */
   void dragEnterEvent(QDragEnterEvent* event);
 
   /**
    * this event is called when the mouse moves inside the widgets area during a drag/drop operation
    */
   void dragMoveEvent(QDragMoveEvent* event);
 
   /**
    * this event is called when the mouse leaves the widgets area during a drag/drop operation
    */
   void dragLeaveEvent(QDragLeaveEvent* event);
 
   /**
    * this event is called when the drop operation is initiated at the widget
    */
   void dropEvent(QDropEvent* event);
 
   // —— helpers ——————————————————————————-
   /**
    * This method must be implemented by the client. It is used to opened the dropped files.
    * param pathList list of urls given by the drop event
    * retval true if successfull otherwise false
    *
    * Here is an example implementation:
    */
   bool MyClass::openFiles(const QStringList& pathList)
   {
     bool success = true;
     for (int i = 0; i < pathList.size() && i < 32;i)
     {
       MdiChild child = createMdiChild();
       if (child->loadFile(pathList.at(i)))
       {
         statusBar()->showMessage(tr("File loaded"), 2000);
         child->show();
       }
       else
       {
         child->close();
       }
     }
     return success;
   }
 
   virtual bool openFiles(const QStringList& pathList) = 0;
 
   private:
   // —— privat helpers ————————————————————————
   // —— members ——————————————————————————-
   // —— not allowed members ——————————————————————-
 };
 #endif  WIDGET_H

DocumentWindow.cpp

 // ————————————————————————————————-
 /**
  * author Gerolf Reinwardt
  * date 30.01.2011
  *
  * Copyright © 2011, Gerolf Reinwardt. All rights reserved.
  *
  * Simplified BSD License
  *
  * Redistribution and use in source and binary forms, with or without modification, are
  * permitted provided that the following conditions are met:
  *
  * 1. Redistributions of source code must retain the above copyright notice, this list of
  * conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright notice, this list
  * of conditions and the following disclaimer in the documentation and/or other materials
  * provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY <COPYRIGHT HOLDER> ``AS IS AND ANY EXPRESS OR IMPLIED
  * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
  * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * The views and conclusions contained in the software and documentation are those of the
  * authors and should not be interpreted as representing official policies, either expressed
  * or implied, of Gerolf Reinwardt.
  */
 // ————————————————————————————————-
 
 // —— general includes —————————————————————————
 #include <QDragEnterEvent>
 #include <QDragLeaveEvent>
 #include <QDragMoveEvent>
 #include <QDropEvent>
 #include <QMimeData>
 #include <QUrl>
 #include <QList>
  
 // —— local includes —————————————————————————-
 #include "documentwindow.h"
 
 // —— construction ——————————————————————————
 DocumentWindow::DocumentWindow(QWidget parent, Qt::WindowFlags flags) : QMainWindow(parent, flags)
 {
   setAcceptDrops(true);
 }
 
 DocumentWindow::~DocumentWindow()
 {
 }
 
 // —— operators ———————————————————————————
 // —— methods ————————————————————————————
 // —— accessors ———————————————————————————
 // —— public slots ——————————————————————————
 // —— protected slots —————————————————————————
 // —— events ————————————————————————————
 void DocumentWindow::dragEnterEvent(QDragEnterEvent* event)
 {
   // if some actions should not be usable, like move, this code must be adopted
   event->acceptProposedAction();
 }
 
 void DocumentWindow::dragMoveEvent(QDragMoveEvent* event)
 {
   // if some actions should not be usable, like move, this code must be adopted
   event->acceptProposedAction();
 }
 
 void DocumentWindow::dragLeaveEvent(QDragLeaveEvent* event)
 {
   event->accept();
 }
 
 void DocumentWindow::dropEvent(QDropEvent* event)
 {
   const QMimeData* mimeData = event->mimeData();
 
   if (mimeData->hasUrls())
   {
     QStringList pathList;
     QList<QUrl> urlList = mimeData->urls();
 
     for (int i = 0; i < urlList.size() && i < 32;+i)
     {
       pathList.append(urlList.at(i).toLocalFile());
     }
 
     if(openFiles(pathList))
       event->acceptProposedAction();
   }
 }
 
 // —— private slots ——————————————————————————
 // —— private helpers —————————————————————————

You may use this code without any warranty.

Future extensions / combinations

typically I would use this together with a file type assignment on windows

The code of the class and a demo project are located on gitorious in the following repository: qtdevnet-dropfiles

Versions

The current code is version 1 of the code.

Version 1

This marks the first public release.

Feedback

I would really welcome feedback on this class. If you spot big weaknesses, please let me know.