如何在QPlainTextEdit中检测滚动条下移

from PyQt5.QtWidgets import QApplication, QPlainTextEdit
from PyQt5.QtCore import Qt

class MyPlainTextEdit(QPlainTextEdit):
    def __init__(self, parent=None):
        super().__init__(parent)
    
    def scrollContentsBy(self, dx, dy):
        # 调用父类的滚动事件处理
        super().scrollContentsBy(dx, dy)
        
        # 判断滚动条的位置
        if dy > 0:
            # 滚动条下移的操作
            print("滚动条下移了！")
            # 其他操作...
            
if __name__ == '__main__':
    app = QApplication([])
    plainTextEdit = MyPlainTextEdit()
    plainTextEdit.show()
    app.exec_()