Difference between revisions of "Development/Tutorials/Saving and loading"

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{{Template:I18n/Language Navigation Bar|Development/Tutorials/Saving_and_loading}}
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This page was moved [https://develop.kde.org/docs/getting-started/saving_and_loading/ here].
 
 
{{TutorialBrowser|
 
 
 
series=Beginner Tutorial|
 
 
 
name=Loading and saving files|
 
 
 
pre=[[Development/Tutorials/Using_KActions|Tutorial 3 - KActions]]|
 
 
 
next=TODO (milliams)|
 
 
 
reading=KIO::{{class|NetAccess}} {{qt|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 or 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.
 
 
 
[[image:introtokdetutorial4.png|frame|center]]
 
 
 
== The Code ==
 
 
 
===main.cpp===
 
<code cppqt n>
 
#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();
 
}
 
</code>
 
<tt>main.cpp</tt> hasn't changed from tutorial 3 except to change any reference to tutorial 3 to tutorial 4.
 
 
 
===mainwindow.h===
 
<code cppqt n>
 
#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
 
</code>
 
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 [http://doc.trolltech.com/latest/signalsandslots.html 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 [http://doc.trolltech.com/latest/qobject.html#Q_OBJECT <tt>Q_OBJECT</tt>] macro.
 
 
 
We also want to keep track of the filename of the currently opened file so we create a <tt>{{qt|QString}} fileName</tt>.
 
 
 
===mainwindow.cpp===
 
<code cppqt n>
 
#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());
 
  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());
 
  }
 
}
 
</code>
 
 
 
===tutorial4ui.rc===
 
<code xml n>
 
<?xml version="1.0" encoding="UTF-8"?>
 
<!DOCTYPE kpartgui SYSTEM "kpartgui.dtd">
 
<gui name="tutorial4" version="1">
 
  <ToolBar name="mainToolBar" >
 
    <text>Main Toolbar</text>
 
    <Action name="clear" />
 
  </ToolBar>
 
  <MenuBar>
 
    <Menu name="file" >
 
      <Action name="clear" />
 
    </Menu>
 
  </MenuBar>
 
</gui>
 
</code>
 
This is identical to <tt>tutorial3ui.rc</tt> from tutorial 3 except the <tt>name</tt> has changed to 'tutorial4'. We do not need to add any information about any of the <tt>KStandardAction</tt>s 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.
 
 
 
The first thing we do is add
 
<code cppqt>
 
fileName(QString())
 
</code>
 
to the <tt>MainWindow</tt> constructor list on line 16. This makes sure that <tt>fileName</tt> 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 <tt>quit</tt> action in tutorial 3, we will use <tt>KStandardActions</tt>. On lines 37 to 47 we add the actions in the same way as for the <tt>quit</tt> 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 <tt>newFile()</tt> function.
 
<code cppqt>
 
void MainWindow::newFile()
 
{
 
  fileName.clear();
 
  textArea->clear();
 
}
 
</code>
 
<tt>fileName.clear()</tt> sets the <tt>fileName</tt> QString to be empty to reflect the fact that this document does not yet have a presence on disc. <tt>textArea->clear()</tt> then clears the central text area using the same function that we connected the <tt>clear</tt> <tt>KAction</tt> to in tutorial 3.
 
 
 
===Saving a file===
 
 
 
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 {{class|KSaveFile}} which is derived from Qt's {{qt|QFile}}.
 
 
 
The function's prototype is
 
<code cppqt>
 
void MainWindow::saveFileAs(const QString &outputFileName)
 
</code>
 
 
 
We then create our <tt>KSaveFile</tt> object and open it with
 
<code cppqt>
 
KSaveFile file(outputFileName);
 
file.open();
 
</code>
 
 
 
Now that we have out 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 {{qt|QByteArray}} and fill it with the plain text version of whatever is in the text area:
 
<code cppqt>
 
QByteArray outputByteArray;
 
outputByteArray.append(textArea->toPlainText());
 
</code>
 
Now that we have our <tt>QByteArray</tt>, we use it to write to the file with <tt>KSaveFile::write()</tt>. If we were using a normal <tt>QFile</tt>, this would make the changes immediately. However, if a problem occurred partway through writing, the file would become corrupted. For this reason, <tt>KSaveFile</tt> works by first writing to a temporary file and then, when you call <tt>KSaveFile::finalize()</tt> the changes are made to the actual file.
 
<code cppqt>
 
file.write(outputByteArray);
 
file.finalize();
 
file.close();
 
</code>
 
Finally, we set <tt>MainWindows</tt>'s <tt>fileName</tt> member to point to the file name we just saved to.
 
 
 
==Make, Install And Run==
 
 
 
===CMakeLists.txt===
 
<code ini n>
 
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)
 
</code>
 
Since we are now using the KIO library, we must tell CMake to link against it. We do this by passing <tt>${KDE4_KIO_LIBS}</tt> to the <tt>target_link_libraries()</tt> function.
 
 
 
With this file, the tutorial can 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
 

Latest revision as of 14:34, 8 September 2020

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This page was last edited on 8 September 2020, at 14:34. Content is available under Creative Commons License SA 4.0 unless otherwise noted.