**Course Title:** PySide6 Application Development **Section Title:** Multithreading and Asynchronous Operations **Topic:** Building an app that performs background tasks while keeping the UI responsive.(Lab topic) In the previous topics, we covered the basics of multithreading and asynchronous operations in PySide6. We learned how to use QThread and QRunnable to perform background tasks, and how to handle long-running tasks without freezing the UI using Qt's signal-slot mechanism. In this lab topic, we'll put our knowledge to the test by building a simple app that performs background tasks while keeping the UI responsive. We'll use the concepts we learned earlier to create a real-world application that demonstrates the power of multithreading and asynchronous operations in PySide6. **App Requirements** Our app will be a simple file downloader that allows users to download files from the internet while keeping the UI responsive. The app will have the following features: * A text input field where users can enter the URL of the file they want to download * A "Download" button that starts the download process * A progress bar that shows the progress of the download * A label that displays the downloaded file size * A "Cancel" button that stops the download process **App Implementation** Here's a step-by-step guide to implementing our file downloader app: **Step 1: Create a new PySide6 project** Create a new PySide6 project in your preferred IDE. Name the project `file_downloader`. **Step 2: Create the UI** Create a new `main.ui` file in your project directory. Design the UI as follows: ```markdown * Text input field ( QLineEdit ) * Download button ( QPushButton ) * Progress bar ( QProgressBar ) * Label ( QLabel ) * Cancel button ( QPushButton ) ``` **Step 3: Implement the UI logic** Create a new `main.py` file in your project directory. Import the necessary modules and create a new class that inherits from `QMainWindow`: ```python import sys import os from PySide6.QtCore import * from PySide6.QtGui import * from PySide6.QtWidgets import * from PySide6.QtNetwork import * class MainWindow(QMainWindow): def __init__(self): super(MainWindow, self).__init__() # Load the UI file self.ui = QFormLayout() self.ui.addRow(QLabel("URL")) self.url_input = QLineEdit() self.ui.addRow(self.url_input) self.download_button = QPushButton("Download") self.download_button.clicked.connect(self.download_file) self.ui.addRow(self.download_button) self.progress_bar = QProgressBar() self.ui.addRow(self.progress_bar) self.file_size_label = QLabel() self.ui.addRow(self.file_size_label) self.cancel_button = QPushButton("Cancel") self.cancel_button.clicked.connect(self.cancel_download) self.ui.addRow(self.cancel_button) self.setCentralWidget(QWidget()) self.centralWidget().setLayout(self.ui) def download_file(self): # Implement the download logic here pass def cancel_download(self): # Implement the cancel logic here pass ``` **Step 4: Implement the download logic** We'll use the `QThread` class to perform the download in a separate thread. This will keep the UI responsive while the download is in progress. Create a new `download_thread.py` file in your project directory: ```python from PySide6.QtCore import * from PySide6.QtNetwork import * class DownloadThread(QThread): def __init__(self, url): super(DownloadThread, self).__init__() self.url = url self.cancelled = False def run(self): manager = QNetworkAccessManager() request = QNetworkRequest(QUrl(self.url)) reply = manager.get(request) loop = QEventLoop() reply.readyRead.connect(self.handleReadyRead) reply.downloadProgress.connect(self.handleProgress) reply.finished.connect(loop.quit) loop.exec_() self.file = reply.readAll() def handleReadyRead(self): if self.cancelled: return # Process the downloaded data here print(self.file) def handleProgress(self, bytes_received, bytes_total): if self.cancelled: return print(f"Download progress: {bytes_received}/{bytes_total}") def cancel(self): self.cancelled = True ``` **Step 5: Implement the cancel logic** In the `cancel_download` method, we'll call the `cancel` method of the `DownloadThread` class to stop the download: ```python def cancel_download(self): self.download_thread.cancel() ``` **Step 6: Connect the UI elements** In the `MainWindow` class, connect the UI elements to the corresponding methods: ```python self.download_button.clicked.connect(self.download_file) self.cancel_button.clicked.connect(self.cancel_download) ``` **Step 7: Implement the download method** In the `download_file` method, create a new instance of the `DownloadThread` class and start the thread: ```python def download_file(self): self.download_thread = DownloadThread(self.url_input.text()) self.download_thread.start() ``` **Conclusion** In this lab topic, we built a simple file downloader app that performs background tasks while keeping the UI responsive. We used the `QThread` class to perform the download in a separate thread, which allowed us to keep the UI responsive while the download was in progress. **References** * [Qt Documentation: QThread](https://doc.qt.io/qt-6/qthread.html) * [Qt Documentation: QRunnable](https://doc.qt.io/qt-6/qrunnable.html) * [Qt Documentation: QNetworkAccessManager](https://doc.qt.io/qt-6/qnetworkaccessmanager.html) **Leave a comment/ask for help** If you have any questions or need help with any part of this lab topic, please leave a comment below. **What's next?** In the next topic, we'll cover "Introduction to QGraphicsView and QGraphicsScene" from the section "Working with Graphics and Animations".