Dialog introduction

The dialog is GUI An integral part of the process . Many function components that cannot or are not suitable for the main window must be set in the dialog box . A dialog box is usually a top-level window , At the top of the program , For short-term tasks or simple user interaction . Even though Ribbon The appearance of the interface reduces the probability of using the dialog box to a certain extent , however , We can still use the latest version Office Found many dialog boxes in . therefore , For the foreseeable future , Dialog boxes will always exist in our program .

Qt Use in QDialog Class implementation dialog . Just like the main window , We usually design a class inheritance QDialog.QDialog（ And its subclasses , And all Qt::Dialog Class of type ） For its parent Pointers have extra explanations ： If parent by NULL, The dialog box will act as a top-level window , Otherwise, as the child dialog of its parent component （ here , The default location is parent Center of ）. The difference between a top-level window and a non top-level window is , The top-level window will have its own place in the taskbar , The non top level window shares the location of its parent component .

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    setWindowTitle(tr("Main Window"));
 
    openAction = new QAction(QIcon(":/images/doc-open"), tr("&Open..."), this);
    openAction->setShortcuts(QKeySequence::Open);
    openAction->setStatusTip(tr("Open an existing file"));
    connect(openAction, &QAction::triggered, this, &MainWindow::open);
 
    QMenu *file = menuBar()->addMenu(tr("&File"));
    file->addAction(openAction);
 
    QToolBar *toolBar = addToolBar(tr("&File"));
    toolBar->addAction(openAction);
}
 
MainWindow::~MainWindow()
{
}
 
void MainWindow::open()
{
    QDialog dialog;
    dialog.setWindowTitle(tr("Hello, dialog!"));
    dialog.exec();
}

Above we use the previous example code . Notice that open() What's in the function . We use QDialog Created a dialog , Set its title to “Hello, dialog!”, And then call exec() Show it . Notice the icon of the taskbar , Because we didn't set the dialog parent The pointer , We will see where the dialog box appears in the taskbar ：

So let's revise that open() The contents of the function ：

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

Run it again , A comparison will show parent Whether the pointer is right QDialog The impact of the instance .

Dialog box is divided into modal dialog box and non modal dialog box . So called modal dialog , It will block the input of other windows in the same application . Modal dialog boxes are common , such as “ Open file ” function . You can try Notepad's open file , When the open file dialog box appears , We can't operate the window except this dialog box . In contrast, modeless dialog boxes , For example, the Find dialog , We can display the search dialog while , Continue to edit the contents of Notepad .

Qt Support modal dialog and non modal dialog . among ,Qt There are two levels of modal dialog ： Application level mode and window level mode , The default is application level mode . Application level mode refers to , When the dialog box of this mode appears , The user must first interact with the dialog , Until you close the dialog , Then you can access other windows in the program . Window level mode refers to , This mode only blocks the window associated with the dialog , But it still allows users to interact with other windows in the program . Window level mode is especially suitable for multi window mode , A more detailed discussion can be seen Previously published articles .

Qt Use QDialog::exec() Implement application level modal dialog , Use QDialog::open() Realize the modal dialog box at the window level , Use QDialog::show() Implementation of non modal dialog . Review our code , In the example above , We call exec() Show the dialog box , So this is a modal dialog . When the dialog box appears , We can't have any interaction with the main window , Until we close the dialog .

Now let's try to exec() It is amended as follows show(), Look at the modeless dialog ：

void MainWindow::open()
{
    QDialog dialog(this);
    dialog.setWindowTitle(tr("Hello, dialog!"));
    dialog.show();
}

Is it the opposite of what you want ？ The dialog box just flashed by ！ This is because ,show() Function does not block the current thread , The dialog box will be displayed , Then the function immediately returns , The code continues to execute . Be careful ,dialog It's built on the stack ,show() The function returns ,MainWindow::open() End of the function ,dialog Out of scope is destructed , So the dialog disappears . It's easy to change if you know the reason , We will dialog It's built on the pile , Of course, there is no such problem ：

void MainWindow::open()
{
    QDialog *dialog = new QDialog;
    dialog->setWindowTitle(tr("Hello, dialog!"));
    dialog->show();
}

Compare this modeless dialog with the previous modal dialog . We can interact with the main window when the dialog box appears , So we can create multiple identical dialog boxes ：

If you are careful enough , It should be found that there is something wrong with the code above ：dialog There is a memory leak ！dialog Use new Allocate space on the heap , But never delete. The solution is simple ： take MainWindow The pointer to the dialog that will do . Remember what we said before Qt Object system of ？

however , There's a problem with doing this ： If our dialog does not appear in an interface class ？ because QWidget Of parent Must be QWidget The pointer , That limits us from putting an ordinary C++ Class pointer to Qt Dialog box . in addition , If there are strict limits on memory usage , When we use the main window as parent when , The main window doesn't close , The dialog will not be destroyed , So it takes up memory all the time . In this situation , We can set dialog Of WindowAttribute：

void MainWindow::open()
{
    QDialog *dialog = new QDialog;
    dialog->setAttribute(Qt::WA_DeleteOnClose);
    dialog->setWindowTitle(tr("Hello, dialog!"));
    dialog->show();
}

setAttribute() When the function settings dialog box closes , Auto destroy dialog . in addition ,QObject One more deleteLater() function , This function destroys the dialog box at the end of the current event loop （ That's it , Need to use exec() Start a new event cycle ）. About the cycle of events , We will explain in detail in the following article .