Development/Tutorials/Saving and loading/KDE4

Loading and saving files

Tutorial Series	Beginner Tutorial
Previous	Tutorial 3 - KActions
What's Next	Tutorial 5 - Using KCmdLineArgs
Further Reading	Tutorial: Using KIO Slaves in your Program KIO::NetAccess QFile

Abstract

Now that we have a basic text editor interface, it's time to make it do something useful. At the most basic, a text editor needs to be able to load files from disc, save files that you've created/edited and create new files.

KDE provides a number of classes for working with files which make life a lot easier for developers. The KIO library allows you to easily access files through network-transparent protocols as well as providing standard file dialogs.

The source code on this page applies only to KDE Development Platform 4 ("KDE 4"). For the KDE Frameworks 5 ("KF5") version, see Development/Tutorials/Saving_and_loading/KF5

The Code

main.cpp

#include <KApplication>
#include <KAboutData>
#include <KCmdLineArgs>
 
#include "mainwindow.h"
 
int main (int argc, char *argv[])
{
  KAboutData aboutData( "tutorial4", "tutorial4",
      ki18n("Tutorial 4"), "1.0",
      ki18n("A simple text area which can load and save."),
      KAboutData::License_GPL,
      ki18n("Copyright (c) 2007 Developer") );
  KCmdLineArgs::init( argc, argv, &aboutData );
  KApplication app;
 
  MainWindow* window = new MainWindow();
  window->show();
  return app.exec();
}

main.cpp hasn't changed from tutorial 3 except to change any reference from tutorial 3 to tutorial 4.

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <KXmlGuiWindow>
#include <KTextEdit>

class MainWindow : public KXmlGuiWindow
{
  Q_OBJECT //new from tutorial3
  
  public:
    MainWindow(QWidget *parent=0);
  
  private:
    KTextEdit* textArea;
    void setupActions();
    QString fileName; //new

  private slots: //new
    void newFile(); //new
    void openFile(); //new
    void saveFile(); //new
    void saveFileAs(); //new
    void saveFileAs(const QString &outputFileName); //new
};

#endif

Since we want to add the ability to load and save files, we must add the functions which will do the work. Since the functions will be called through Qt's signal/slot mechanism we must specify that these functions are slots as we do on line 19. Since we are using slots in this header file, we must also add the Q_OBJECT macro.

We also want to keep track of the filename of the currently opened file so we declare a QString fileName.

mainwindow.cpp

#include "mainwindow.h"

#include <KApplication>
#include <KAction>
#include <KLocale>
#include <KActionCollection>
#include <KStandardAction>
#include <KFileDialog> //new
#include <KMessageBox> //new
#include <KIO/NetAccess> //new
#include <KSaveFile> //new
#include <QTextStream> //new
 
MainWindow::MainWindow(QWidget *parent)
    : KXmlGuiWindow(parent),
      fileName(QString()) //new
{
  textArea = new KTextEdit;
  setCentralWidget(textArea);
 
  setupActions();
}
 
void MainWindow::setupActions()
{
  KAction* clearAction = new KAction(this);
  clearAction->setText(i18n("Clear"));
  clearAction->setIcon(KIcon("document-new"));
  clearAction->setShortcut(Qt::CTRL + Qt::Key_W);
  actionCollection()->addAction("clear", clearAction);
  connect(clearAction, SIGNAL(triggered(bool)),
          textArea, SLOT(clear()));
 
  KStandardAction::quit(kapp, SLOT(quit()),
                        actionCollection());
 
  KStandardAction::open(this, SLOT(openFile()),
                        actionCollection()); //new
 
  KStandardAction::save(this, SLOT(saveFile()),
                        actionCollection()); //new
 
  KStandardAction::saveAs(this, SLOT(saveFileAs()),
                        actionCollection()); //new
 
  KStandardAction::openNew(this, SLOT(newFile()),
                        actionCollection()); //new
 
  setupGUI();
}

//New from here on

void MainWindow::newFile()
{
  fileName.clear();
  textArea->clear();
}

void MainWindow::saveFileAs(const QString &outputFileName)
{
  KSaveFile file(outputFileName);
  file.open();
  
  QByteArray outputByteArray;
  outputByteArray.append(textArea->toPlainText().toUtf8());
  file.write(outputByteArray);
  file.finalize();
  file.close();
  
  fileName = outputFileName;
}

void MainWindow::saveFileAs()
{
  saveFileAs(KFileDialog::getSaveFileName());
}

void MainWindow::saveFile()
{
  if(!fileName.isEmpty())
  {
    saveFileAs(fileName);
  }
  else
  {
    saveFileAs();
  }
}

void MainWindow::openFile()
{
  QString fileNameFromDialog = KFileDialog::getOpenFileName();

  QString tmpFile;
  if(KIO::NetAccess::download(fileNameFromDialog, tmpFile, this))
  {
    QFile file(tmpFile);
    file.open(QIODevice::ReadOnly);
    textArea->setPlainText(QTextStream(&file).readAll());
    fileName = fileNameFromDialog;

    KIO::NetAccess::removeTempFile(tmpFile);
  }
  else
  {
    KMessageBox::error(this, 
        KIO::NetAccess::lastErrorString());
  }
}

tutorial4ui.rc

<?xml version="1.0" encoding="UTF-8"?>
<gui name="tutorial4"
     version="1"
     xmlns="http://www.kde.org/standards/kxmlgui/1.0"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://www.kde.org/standards/kxmlgui/1.0
                         http://www.kde.org/standards/kxmlgui/1.0/kxmlgui.xsd" >

  <MenuBar>
    <Menu name="file" >
      <Action name="clear" />
    </Menu>
  </MenuBar>

  <ToolBar name="mainToolBar" >
    <text>Main Toolbar</text>
    <Action name="clear" />
  </ToolBar>

</gui>

This is identical to tutorial3ui.rc from tutorial 3 except the name has changed to 'tutorial4'. We do not need to add any information about any of the KStandardActions since the placement of those actions is handled automatically by KDE.

Explanation

Okay, now to implement the code that will do the loading and saving. This will all be happening in mainwindow.cpp

The first thing we do is add fileName(QString()) to the MainWindow constructor list on line 16. This makes sure that fileName is empty right from the beginning.

Adding the actions

The first thing we are going to do is provide the outward interface for the user so they can tell the application to load and save. Like with the quit action in tutorial 3, we will use KStandardActions. On lines 37 to 47 we add the actions in the same way as for the quit action. For each one, we connect it to the appropriate slot that we declared in the header file.

Creating a new document

The first function we create is the newFile() function.

void MainWindow::newFile()
{
  fileName.clear();
  textArea->clear();
}

fileName.clear() sets the fileName QString to be empty to reflect the fact that this document does not yet have a presence on disc. textArea->clear() then clears the central text area using the same function that we connected the clear KAction to in tutorial 3.

Saving a file

saveFileAs(QString)

Now we get onto our first file handling code. We're going to implement a function which will save the contents of the text area to the file name given as a parameter. KDE provides a class for safely saving a file called KSaveFile which is derived from Qt's QFile.

The function's prototype is

void MainWindow::saveFileAs(const QString &outputFileName)

We then create our KSaveFile object and open it with

KSaveFile file(outputFileName);
file.open();

Now that we have our file to write to, we need to format the text in the text area to a format which can be written to file. For this, we create a QByteArray and fill it with the plain text version of whatever is in the text area:

QByteArray outputByteArray;
outputByteArray.append(textArea->toPlainText().toUtf8());

Now that we have our QByteArray, we use it to write to the file with KSaveFile::write(). If we were using a normal QFile, this would make the changes immediately. However, if a problem occurred partway through writing, the file would become corrupted. For this reason, KSaveFile works by first writing to a temporary file and then, when you call KSaveFile::finalize() the changes are made to the actual file.

file.write(outputByteArray);
file.finalize();
file.close();

Finally, we set MainWindows's fileName member to point to the file name we just saved to.

fileName = outputFileName;

saveFileAs()

This is the function that the saveAs slot is connected to. It simply calls the generic saveFileAs(QString) function and passes the file name returned by KFileDialog::getSaveFileName().

void MainWindow::saveFileAs()
{
  saveFileAs(KFileDialog::getSaveFileName());
}

This is our first actual use of the KIO library. KFileDialog provides a number of static functions for displaying the common file dialog that is used by all KDE applications. Calling KFileDialog::getSaveFileName() will display a dialog where the user can select the name of the file to save to or choose a new name. The function returns the full file name, which we then pass to saveFileAs(QString).

saveFile()

void MainWindow::saveFile()
{
  if(!fileName.isEmpty())
  {
    saveFileAs(fileName);
  }
  else
  {
    saveFileAs();
  }
}

There's nothing exciting or new in this function, just the logic to decide whether or not to show the save dialog. If fileName is not empty, then the file is saved to fileName. But if it is, then the dialog is shown to allow the user to select a file name.

Loading a file

Finally, we get round to being able to load a file from disc. The code for this is all contained in MainWindow::openFile().

First we must ask the user for the name of the file they wish to open. We do this using another one of the KFileDialog functions, this time getOpenFileName():

QString fileNameFromDialog = KFileDialog::getOpenFileName();

Then we use the KIO library to retrieve our file. This allows us to open the file with QFile even if it's stored in a remote location like an FTP site. We make the following call to NetAccess's download() function

KIO::NetAccess::download(fileNameFromDialog, tmpFile, this)

The first argument is the name of the file you wish to download. The second is a QString which, after the download is complete, will contain the location of the temporary copy of the file. It is this tmpFile we will work with from now on.

The function returns true or false depending on whether the transfer was successful. If it failed, we display a message box giving the error:

KMessageBox::error(this, KIO::NetAccess::lastErrorString());

Otherwise, we continue with opening the file.

We create a QFile by passing the temporary file created by NetAccess::download() to its constructor and then open it in read-only mode

QFile file(tmpFile);
file.open(QIODevice::ReadOnly);

In order to display the contents of the file, we must use a QTextStream. We create one by passing the contents of our file to its constructor and then call QFile's readAll() function to get the text from the file. This is then passed to the setPlainText() function of our text area.

textArea->setPlainText(QTextStream(&file).readAll());

We then store the path of the file we just opened:

fileName = fileNameFromDialog;

and finally, we remove the temporary file that was created by NetAccess::download():

KIO::NetAccess::removeTempFile(tmpFile);

Make, Install And Run

CMakeLists.txt

project(tutorial4)
 
find_package(KDE4 REQUIRED)
include_directories(${KDE4_INCLUDES})
 
set(tutorial4_SRCS 
  main.cpp
  mainwindow.cpp
)
 
kde4_add_executable(tutorial4 ${tutorial4_SRCS})
 
target_link_libraries(tutorial4 ${KDE4_KDEUI_LIBS} 
                                ${KDE4_KIO_LIBS})
 
install(TARGETS tutorial4 DESTINATION ${BIN_INSTALL_DIR})
install(FILES tutorial4ui.rc 
        DESTINATION ${DATA_INSTALL_DIR}/tutorial4)

Since we are now using the KIO library, we must tell CMake to link against it. We do this by passing ${KDE4_KIO_LIBS} to the target_link_libraries() function.

With this file, the tutorial can be built and run in the same way as tutorial 3. For more information, see tutorial 3.

mkdir build && cd build
cmake .. -DCMAKE_INSTALL_PREFIX=$HOME
make install
$HOME/bin/tutorial4

Note

Changed settings are saved in your KDE directory, in this case into $HOME/.kde/share/apps/tutorial4.

Moving On

Now you can move on to the KCmdLineArgs tutorial.