Qt 创建 MDI 文档窗口

Question

I am trying to create a MDI document program. I have a question on creating the subwindow.

This is my mainwindow constructor:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    setWindowTitle(tr("MDI"));

    workspace = new QMdiArea;
    setCentralWidget(workspace);
    //fileNew();

    createActions();
    createMenus();
    createToolbars();

    statusBar()->showMessage(tr("Done"));

    enableActions();
}

The interesting point is the fileNew(); function. It is a private slot function actually which I want to invoke when "New File" button is triggered. Here is the private slot fileNew() function:

void MainWindow::fileNew()
{
    DocumentWindows* document = new DocumentWindows;
    workspace->addSubWindow(document);
}

This function works perfectly when I call from the mainwindow constructor. However, there is a problem when I call it from the createActions(); function which uses a signal-slot mechanism.

Here is my createActions()

void MainWindow::createActions()
{
    newAction = new QAction(QIcon(":/Image/NewFile.png"),tr("&New"),this);
    newAction->setShortcut(tr("Ctrl+N"));
    newAction->setToolTip(tr("Open new document"));
    connect(newAction, SIGNAL(triggered(bool)), this, SLOT(fileNew()));
}

No subwindow is created even the SLOT is triggered. Subsequently, I find out that if I add document->show(); , everything works well.

void MainWindow::fileNew()
{
    DocumentWindows* document = new DocumentWindows;
    workspace->addSubWindow(document);
    document->show();
}

My question is: Why the show() function is needed in a SLOT but not in the constructor?

PS. DocumentWindows is just a class inherits QTextEdit .

Answer 1

This problem has nothing to do with the class of the widgets one is using. It is unrelated to documents, MDI, or the main window. After you add a child widget to a widget that is already visible, you must explicitly show it. Otherwise, the widget will remain hidden.

All widgets are hidden by default. When you initially show the MainWindow , all of its children are recursively shown too. When you later add a child MDI widget, it remains hidden. When widgets are added to layouts, they are shown by default - but your widget is managed by the MDI area, not by a layout.

This is a minimal test case demonstrating your issue:

// https://github.com/KubaO/stackoverflown/tree/master/questions/widget-show-32534931
#include <QtWidgets>

int main(int argc, char ** argv) {
    QApplication app{argc, argv};
    QWidget w;
    w.setMinimumSize(200, 50);
    QLabel visible{"Visible", &w};
    w.show();
    QLabel invisible{"Invisible", &w};
    invisible.move(100, 0);
    return app.exec();
}