Communicating Dialog Outcomes in Qt: Alternatives to QDialog::setResult()

Purpose

• It sets a result code, which is an integer value, that indicates the user's choice or the dialog's execution status.
• In Qt's modal dialogs, QDialog::setResult() is used to communicate the outcome of the dialog interaction back to the code that invoked it.

How it Works

1. • You call setResult() on a QDialog object, passing an integer value as the argument.
   • Qt recommends using pre-defined values from the QDialog::DialogCode enumeration for consistency and readability. These values typically represent common dialog outcomes (e.g., Accepted, Rejected, Rejected).

   #include <QtWidgets>
   
   int main() {
       QApplication app(argc, argv);
       QMessageBox msgBox;
       msgBox.setText("Do you want to save changes?");
       msgBox.setStandardButtons(QMessageBox::Save | QMessageBox::Discard);
   
       int result = msgBox.exec(); // This shows the dialog and waits for user input
   
       if (result == QMessageBox::Save) {
           // Save changes
       } else if (result == QMessageBox::Discard) {
           // Discard changes
       } else {
           // Handle other possible results (e.g., Cancel)
       }
   
       return app.exec();
   }
   

2. Retrieving the Result Code
   

   • After the dialog is closed (either by the user or programmatically), the code that invoked the dialog can retrieve the set result code. This is typically done using the exec() method, which displays the dialog modally and returns the result code upon closure.

Key Points

• Modal dialogs block the execution of the calling code until they are closed.
• The result code is retrieved after the dialog closes using exec().
• Use QDialog::DialogCode values for clear and standardized communication.

Additional Considerations

• For non-modal dialogs, you might need to implement a mechanism (e.g., signals and slots) to communicate the outcome from the dialog back to the calling code.
• You can define custom result codes if the provided QDialog::DialogCode values aren't sufficient for your specific use case.

---

Custom Dialog with Save/Cancel Buttons

This example creates a custom dialog with "Save" and "Cancel" buttons, sets the result code based on the clicked button, and retrieves the result in the main code:

#include <QtWidgets>

class SaveCancelDialog : public QDialog {
    Q_OBJECT

public:
    explicit SaveCancelDialog(QWidget *parent = nullptr);

private slots:
    void saveButtonClicked();
    void cancelButtonClicked();

    Q_signals:
        void saveRequested();
};

SaveCancelDialog::SaveCancelDialog(QWidget *parent) : QDialog(parent) {
    QPushButton* saveButton = new QPushButton("Save");
    QPushButton* cancelButton = new QPushButton("Cancel");

    connect(saveButton, &QPushButton::clicked, this, &SaveCancelDialog::saveButtonClicked);
    connect(cancelButton, &QPushButton::clicked, this, &SaveCancelDialog::cancelButtonClicked);

    QHBoxLayout* buttonLayout = new QHBoxLayout;
    buttonLayout->addWidget(saveButton);
    buttonLayout->addWidget(cancelButton);

    QVBoxLayout* mainLayout = new QVBoxLayout;
    mainLayout->addWidget(new QLabel("Do you want to save changes?"));
    mainLayout->addLayout(buttonLayout);

    setLayout(mainLayout);
}

void SaveCancelDialog::saveButtonClicked() {
    setResult(QDialog::Accepted); // Set result code for Save
    close();
}

void SaveCancelDialog::cancelButtonClicked() {
    setResult(QDialog::Rejected); // Set result code for Cancel
    close();
}

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    SaveCancelDialog dialog;
    int result = dialog.exec();

    if (result == QDialog::Accepted) {
        // Save changes
        qDebug() << "Saving...";
    } else {
        // Discard changes (or handle Cancel)
        qDebug() << "Discarding changes.";
    }

    return app.exec();
}

QMessageBox with Custom Result Code

This code shows how to define a custom result code and use it with a QMessageBox:

#include <QtWidgets>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QMessageBox msgBox;
    msgBox.setText("Are you sure you want to exit?");
    msgBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
    msgBox.setDefaultButton(QMessageBox::No); // Set default button to No

    int customResult = QMessageBox::CustomCode; // Define custom result code

    // Add a custom button with the custom result code
    msgBox.addButton("Exit Anyway", QMessageBox::AcceptRole);
    QObject::connect(&msgBox, &QMessageBox::buttonClicked, [&msgBox, &customResult](QAbstractButton* button) {
        if (button->text() == "Exit Anyway") {
            msgBox.setResult(customResult);
        }
    });

    int result = msgBox.exec();

    if (result == QMessageBox::Yes) {
        qDebug() << "Exiting normally.";
    } else if (result == customResult) {
        qDebug() << "Exiting despite confirmation.";
    } else {
        qDebug() << "Cancelled exit.";
    }

    return app.exec();
}

---

Signals and Slots

Signals and slots are a powerful mechanism in Qt for inter-object communication. You can define custom signals in your dialog class to emit specific events based on user interaction (e.g., button clicks, form changes). The calling code can then connect to these signals and receive information about the dialog's state.

Example

#include <QtWidgets>

class MyDialog : public QDialog {
    Q_OBJECT

public:
    explicit MyDialog(QWidget *parent = nullptr);

signals:
    void optionSelected(const QString& option);

private slots:
    void onOptionButtonClicked(const QString& option);

    // ... other slots for other buttons
};

MyDialog::MyDialog(QWidget *parent) : QDialog(parent) {
    // ... create buttons and layout
    connect(button1, &QPushButton::clicked, this, &MyDialog::onOptionButtonClicked);
    connect(button2, &QPushButton::clicked, this, &MyDialog::onOptionButtonClicked);
    // ... connect other buttons
}

void MyDialog::onOptionButtonClicked(const QString& option) {
    emit optionSelected(option); // Emit signal with selected option
    close(); // Optionally, close the dialog after emitting
}

// In the calling code
void handleOptionSelection(const QString& option) {
    // Perform actions based on the selected option
}

int main() {
    QApplication app(argc, argv);

    MyDialog dialog;
    QObject::connect(&dialog, &MyDialog::optionSelected, nullptr, &handleOptionSelection);
    dialog.exec();

    return app.exec();
}

QObject Properties

You can create custom properties in your dialog class using QObject properties. The calling code can then set or get the value of these properties to exchange information with the dialog. This approach is suitable for simple data exchange but might become cumbersome for complex interactions.

Example

#include <Qt>

class MyDialog : public QDialog {
    Q_OBJECT

public:
    explicit MyDialog(QWidget *parent = nullptr);

    QString selectedOption() const { return m_selectedOption; }
    void setSelectedOption(const QString& option) { m_selectedOption = option; }

private:
    QString m_selectedOption;

    // ... other UI elements
};

MyDialog::MyDialog(QWidget *parent) : QDialog(parent) {
    // ... create buttons and layout
    // ... connect button clicks to update m_selectedOption
}

// In the calling code
int main() {
    QApplication app(argc, argv);

    MyDialog dialog;
    dialog.setSelectedOption("Option 2");
    dialog.exec();

    QString selectedOption = dialog.selectedOption();

    return app.exec();
}

Return Values from Dialog Methods

For specific user interactions within the dialog, you can define custom methods that return relevant data. This approach works well when the interaction is well-defined and the return value can encapsulate the desired information.

#include <QtWidgets>

class MyDialog : public QDialog {
    Q_OBJECT

public:
    explicit MyDialog(QWidget *parent = nullptr);

    QString getUserInput(); // Returns entered text from a line edit

private:
    // ... dialog UI elements
};

MyDialog::MyDialog(QWidget *parent) : QDialog(parent) {
    // ... create UI elements
}

QString MyDialog::getUserInput() {
    return lineEdit->text(); // Return text from the line edit
}

// In the calling code
int main() {
    QApplication app(argc, argv);

    MyDialog dialog;
    QString userInput = dialog.getUserInput();
    dialog.exec(); // Optionally, show the dialog after getting input

    return app.exec();
}