**Course Title:** Qt 6 Application Development with C++ **Section Title:** Multithreading and Asynchronous Operations **Topic:** Building an application with background tasks and maintaining a responsive UI.(Lab topic) In this lab, we will explore how to build an application that performs background tasks and maintains a responsive UI using Qt 6. We have already discussed multithreading in Qt using QThread, QRunnable, and QThreadPool. Now, let's apply these concepts to a real-world example. **Lab: Building a Background Task Application** In this lab, we will create an application that performs a long-running operation in the background and maintains a responsive UI. Our goal is to display the progress of the operation in a QProgressBar while performing the operation in a separate thread. **Step 1: Create a new Qt project** Create a new Qt 6 project in Qt Creator. Select "Application" under the Projects tab and choose "Qt Widgets Application" as the project type. **Step 2: Create a new class for the background task** Create a new C++ class called `BackgroundTask` that inherits from `QObject`. This class will perform our long-running operation in a separate thread. ```cpp // backgroundtask.h #ifndef BACKGROUNDTASK_H #define BACKGROUNDTASK_H #include <QObject> #include <QThread> class BackgroundTask : public QObject { Q_OBJECT public: explicit BackgroundTask(QObject *parent = nullptr); void startTask(); signals: void taskProgress(int progress); void taskFinished(); private: void runTask(); }; #endif // BACKGROUNDTASK_H ``` ```cpp // backgroundtask.cpp #include "backgroundtask.h" #include <QDebug> BackgroundTask::BackgroundTask(QObject *parent) : QObject(parent) { } void BackgroundTask::startTask() { // Run the task in a new thread QThread *thread = new QThread; this->moveToThread(thread); connect(thread, &QThread::started, this, &BackgroundTask::runTask); connect(this, &BackgroundTask::taskFinished, thread, &QThread::quit); thread->start(); } void BackgroundTask::runTask() { for (int i = 0; i <= 100; ++i) { // Simulate long-running operation QThread::msleep(100); // Emit signal to update progress bar emit taskProgress(i); } emit taskFinished(); } ``` **Step 3: Create a UI for the progress bar** Create a new UI for the progress bar by dragging a `QProgressBar` onto the main window in Qt Designer. ```cpp // mainwindow.h #ifndef MAINWINDOW_H #define MAINWINDOW_H #include <QMainWindow> #include <QProgressBar> #include "backgroundtask.h" namespace Ui { class MainWindow; } class MainWindow : public QMainWindow { Q_OBJECT public: explicit MainWindow(QWidget *parent = nullptr); ~MainWindow(); private slots: void on_pushButton_clicked(); private: Ui::MainWindow *ui; BackgroundTask *task; QProgressBar *progressBar; }; #endif // MAINWINDOW_H ``` ```cpp // mainwindow.cpp #include "mainwindow.h" #include "ui_mainwindow.h" MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow) { ui->setupUi(this); progressBar = ui->progressBar; task = new BackgroundTask; connect(task, &BackgroundTask::taskProgress, progressBar, &QProgressBar::setValue); connect(task, &BackgroundTask::taskFinished, progressBar, &QProgressBar::reset); } MainWindow::~MainWindow() { delete ui; } void MainWindow::on_pushButton_clicked() { task->startTask(); } ``` **Step 4: Run the application** Run the application and click the push button to start the background task. The progress bar should update as the task progress. **Key Concepts:** * Use QThread to move objects to separate threads * Use signals and slots to communicate between threads * Use QProgressBar to display progress of background tasks * Use QThread::msleep to simulate long-running operations **Practical Takeaways:** * Always perform long-running operations in a separate thread to maintain a responsive UI * Use signals and slots to communicate between threads and UI components * Use QProgressBar to display progress of background tasks **External Resources:** * [Qt Documentation: QThread](https://doc.qt.io/qt-6/qthread.html) * [Qt Documentation: QProgressBar](https://doc.qt.io/qt-6/qprogressbar.html) * [Qt Documentation: Signal-Slot](https://doc.qt.io/qt-6/signalsandslots.html) We hope this lab has helped you understand how to build an application with background tasks and maintain a responsive UI using Qt 6. Please leave a comment if you have any questions or need further clarification on any of the concepts. We would be happy to help. In the next topic, we will explore the QGraphicsView and QGraphicsScene framework in Qt.