Timer
-------

``QTimer`` provides a timer that emits periodic signals.

.. code-block:: python
    :caption: scripts/PYQT4/05_timer/main.py

    # -*- coding: utf-8 -*-

    import sys
    from PyQt5.QtWidgets import QApplication, QMainWindow, QTextEdit
    from PyQt5.QtCore import QTimer

    class TimerTextEdit(QTextEdit):
        def __init__(self, parent=None):
            super(TimerTextEdit, self).__init__(parent)

            self.timer = QTimer(self)
            self.timer.timeout.connect(self.append_message)
            self.timer.start(5000)  # 5000ms = 5s

        def append_message(self):
            self.append("5 seconds have passed!")

    class TimerApp(QMainWindow):
        def __init__(self):
            super(TimerApp, self).__init__()

            self.text_edit = TimerTextEdit(self)
            self.setCentralWidget(self.text_edit)

            self.init_ui()

        def init_ui(self):
            self.setWindowTitle('QTimer Example')
            self.setGeometry(100, 100, 400, 300)

    if __name__ == '__main__':
        app = QApplication(sys.argv)
        window = TimerApp()
        window.show()
        sys.exit(app.exec_())

This script implements a simple timer application using PyQt5. It provides the user with a ``QTextEdit`` widget. Inside this widget, a ``QTimer`` is set up to display a message every 5 seconds.

- ``TimerTextEdit``: A subclass of ``QTextEdit``. It uses the ``QTimer`` to append a message every 5 seconds.
  
- ``TimerApp``: The main window class. It uses ``TimerTextEdit`` as its central widget.