File dialog box

In the previous chapter , We talked about Qt Standard dialog box QMessageBox Use . The so-called standard dialog box , In fact, it is an ordinary dialog box . therefore , We can also put QDialog Other features provided apply to this standard dialog . today , Let's move on to another standard dialog ：QFileDialog, The file dialog box . In this section , We will try to write a simple text file editor , We will use QFileDialog To open a text file , And save the modified file to the hard disk . This is probably the first example with practical functions that we provide in this series .

First , We need to create a window with text editing function . To borrow from our previous program code , It should be easy to do ：

openAction = new QAction(QIcon(":/images/file-open"), tr("&Open..."), this);
openAction->setShortcuts(QKeySequence::Open);
openAction->setStatusTip(tr("Open an existing file"));
 
saveAction = new QAction(QIcon(":/images/file-save"), tr("&Save..."), this);
saveAction->setShortcuts(QKeySequence::Save);
saveAction->setStatusTip(tr("Save a new file"));
 
QMenu *file = menuBar()->addMenu(tr("&File"));
file->addAction(openAction);
file->addAction(saveAction);
 
QToolBar *toolBar = addToolBar(tr("&File"));
toolBar->addAction(openAction);
toolBar->addAction(saveAction);
 
textEdit = new QTextEdit(this);
setCentralWidget(textEdit);

We added two actions to the menu and toolbar ： Open and save . Next is a QTextEdit class , This class is used to display rich text files . in other words , It's not just for displaying text , It can also show pictures 、 Forms, etc . however , Now we only use it to display plain text files .QMainWindow There is one setCentralWidget() function , You can use a component as the central component of the window , In the center of the window . obviously , In a text editor , The text editing area is the central component , So we will QTextEdit As such a component .

We use connect() function , For these two QAction Object to add a response action ：

/// !!!Qt5
connect(openAction, &QAction::triggered, this, &MainWindow::openFile);
connect(saveAction, &QAction::triggered, this, &MainWindow::saveFile);
 
/// !!!Qt4
connect(openAction, SIGNAL(triggered()), this, SLOT(openFile()));
connect(saveAction, SIGNAL(triggered()), this, SLOT(saveFile()));

None of this should be a problem . We should be able to clearly understand the meaning of these codes . Here's the main openFile() and saveFile() The code for these two functions ：

void MainWindow::openFile()
{
    QString path = QFileDialog::getOpenFileName(this,
                                                tr("Open File"),
                                                ".",
                                                tr("Text Files(*.txt)"));
    if(!path.isEmpty()) {
        QFile file(path);
        if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) {
            QMessageBox::warning(this, tr("Read File"),
                                 tr("Cannot open file:\n%1").arg(path));
            return;
        }
        QTextStream in(&file);
        textEdit->setText(in.readAll());
        file.close();
    } else {
        QMessageBox::warning(this, tr("Path"),
                             tr("You did not select any file."));
    }
}
 
void MainWindow::saveFile()
{
    QString path = QFileDialog::getSaveFileName(this,
                                                tr("Open File"),
                                                ".",
                                                tr("Text Files(*.txt)"));
    if(!path.isEmpty()) {
        QFile file(path);
        if (!file.open(QIODevice::WriteOnly | QIODevice::Text)) {
            QMessageBox::warning(this, tr("Write File"),
                                       tr("Cannot open file:\n%1").arg(path));
            return;
        }
        QTextStream out(&file);
        out << textEdit->toPlainText();
        file.close();
    } else {
        QMessageBox::warning(this, tr("Path"),
                             tr("You did not select any file."));
    }
}

stay openFile() Function , We use QFileDialog::getOpenFileName() To get the path of the file to be opened . This function has a long signature ：

QString getOpenFileName(QWidget * parent = 0,
                        const QString & caption = QString(),
                        const QString & dir = QString(),
                        const QString & filter = QString(),
                        QString * selectedFilter = 0,
                        Options options = 0)

But pay attention to , All of its parameters are optional , So to a certain extent , This function is also simple . These six parameters are ：

• parent： The parent window . We introduced ,Qt The standard dialog box provides static functions , Used to return a modal dialog （ To some extent, this is an embodiment of the appearance mode ）;
• caption： Dialog title ;
• dir： The default directory when the dialog box opens ,“.” Represents the program running directory ,“/” Represents the root directory of the current drive letter （ especially Windows platform ;Linux The platform, of course, is the root directory ）, This parameter can also be platform related , such as “C:\” etc. ;
• filter： filter . We can browse many kinds of files by using the file dialog box , however , Most of the time we just want to open certain types of files . such as , The text editor wants to open a text file , The image browser wants to open the image file . Filters are used to filter specific suffixes . If we use “Image Files(.jpg .png)”, Only the suffix is jpg perhaps png The file of . If you need more than one filter , Use “;;” Division , such as “JPEG Files(.jpg);;PNG Files(.png)”;
• selectedFilter： The filter selected by default ;
• options： Some parameter settings of dialog box , For example, only display the folder and so on , It's going to be theta enum QFileDialog::Option, Each option can use | The combination of operations .

QFileDialog::getOpenFileName() The return value is the selected file path . We assign it to path. By judgment path Is it empty , You can determine whether the user has selected a file . Only when the user selects a file , We just do the following . stay saveFile() Used in QFileDialog::getSaveFileName() It's the same thing . Using this static function , stay Windows、Mac OS The above are all direct calls to local dialog boxes , however Linux On the other hand, it's QFileDialog My own simulation . It suggests that , If you don't use these static functions , But directly QFileDialog Set it up , As we mentioned earlier QMessageBox The settings are the same , The resulting dialog box is likely to be inconsistent with the appearance of the system dialog box . This point needs to be noted .

First , We create a QFile object , Pass the file path selected by the user to this object . Then we need to open this file , It uses QFile::open(), Its parameter is the specified opening method , Here we use read-only mode and text mode to open this file （ Because we choose the suffix txt The file of , It can be considered as a text file . Of course , in application , Further judgment may be required ）.QFile::open() If it is opened successfully, it returns true, Continue with the following operation ： Use QTextStream::readAll() Read all contents of the file , Then assign it to QTextEdit Show it . Finally, don't forget to close the file . in addition ,saveFile() Functions are similar , Just the last step , We use << Redirect , take QTextEdit Output the contents of to a file . About file manipulation , We will introduce it further in the following chapters .

Here's a little bit of attention ： Our code is just for demonstration , Many necessary operations have not been carried out . such as , We didn't check whether the actual type of this file is a text file . also , We used QTextStream::readAll() Read all contents of the file directly , If this file has 100M, The program will die immediately , These are all issues that must be considered in the actual procedure . However, these contents are beyond the introduction of this chapter , There will be no more details .

thus , Our code has been introduced , You can compile and run it soon ：

The code of this chapter can be downloaded here ：