Dialog data transfer

The dialog box appears to complete a simple or short-term task . The data interaction between dialog box and main window is quite important . This section will explain how to conduct data interaction between the dialog box and the main window . According to the previous explanation , Dialog boxes are divided into modal and non modal . We will also take these two as examples , Respectively .

Modal dialog uses exec() Function to display it .exec() The real meaning of function is to start a new event loop （ We will introduce the concept of events in detail in the following chapters ）. The so-called event cycle , It can be understood as an infinite loop .Qt After opening the event cycle , All kinds of events sent by the system can be monitored by the program . This event loop is equivalent to a polling function . Since it's an infinite loop , Of course, where the event cycle is turned on , The code will be blocked , The following statements will not be executed . therefore , For the exec() Modal dialog box displayed , We can do it in exec() Function, and then get the data value directly from the dialog box object .

Take a look at the following code ：

void MainWindow::open()
{
    QDialog dialog(this);
    dialog.setWindowTitle(tr("Hello, dialog!"));
    dialog.exec();
    qDebug() << dialog.result();
}

In the above code , We use exec() Display a modal dialog . Last line of code ,qDebug() Be similar to std::cout perhaps Java Of System.out.println(); sentence , Output the following information to standard output , It's usually the console . Use qDebug() Need to introduce header file . stay exec() After the function , We can get directly dialog Data value . Be careful ,exec() Start an event cycle , The code is blocked here . because exec() Function did not return , So the following result() Functions will not be executed . Until the dialog box closes ,exec() The function returns , here , We can get the data of the dialog .

One thing to note is that , If we set dialog The property of is WA_DeleteOnClose, Then when the dialog box closes , Object destroyed , We can't use this method to obtain data . under these circumstances , We can consider using parent Build the dialog box by pointer , Avoid setting WA_DeleteOnClose attribute ; Or use another way .

actually ,QDialog::exec() There is a return value , The return value is QDialog::Accepted perhaps QDialog::Rejected. Generally, we will use code similar to the following ：

QDialog dialog(this);
if (dialog.exec() == QDialog::Accepted) {
    // do something
} else {
    // do something else
}

To determine the return value of the dialog , That is, the user clicked “ determine ” still “ Cancel ”. For more details, please refer to QDialog file .

Modal dialogs are relatively simple , If it is an modeless dialog ,QDialog::show() The function immediately returns , If we also write so , It is impossible to obtain the data entered by the user . because show() Function does not block the main thread ,show() Return immediately , The user hasn't had time to input , Just execute the following code , Of course, there will be no correct result . Then we should get data in a different way , That is to use the signal slot mechanism .

Because the modeless dialog box can call QDialog::accept() perhaps QDialog::reject() Or more general QDialog::done() function , So we can send a signal here . in addition , If you can't find the right signal point , We can rewrite QDialog::closeEvent() function , Send a signal here . In the window that needs to receive data （ Here is the main window ） Connect to this signal . Similar code snippets are shown below ：

//!!! Qt 5
// in dialog:
void UserAgeDialog::accept()
{
    emit userAgeChanged(newAge); // newAge is an int
    QDialog::accept();
}
 
// in main window:
void MainWindow::showUserAgeDialog()
{
    UserAgeDialog *dialog = new UserAgeDialog(this);
    connect(dialog, &UserAgeDialog::userAgeChanged, this, &MainWindow::setUserAge);
    dialog->show();
}
 
// ...
 
void MainWindow::setUserAge(int age)
{
    userAge = age;
}

The code above is very simple , No more details here . in addition , Of the above code Qt 4 Version should also be easy to implement .

Don't worry if the dialog closes , Is it possible to obtain data . because Qt The mechanism of the signal slot ensures , When the slot function is called , We can always use sender() Function to get signal Sender of . About sender() function , More information can be found in the documentation . By the way ,sender() The existence of function enables us to use this function , To realize a modeless dialog box that can only open one （ The method is to record the mark in a dialog mapping table when the dialog box is opened , Use... When the dialog box closes sender() Function to determine whether the dialog box , Then delete it from the mapping table ）.