在PyQt图形用户界面上选择组合框选项时，在现有主窗口上显示图形

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QComboBox, QVBoxLayout, QWidget
from PyQt5.QtGui import QPainter, QPen, QColor
from PyQt5.QtCore import Qt

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("PyQt Graph")
        
        # 创建组合框控件
        self.combo_box = QComboBox()
        self.combo_box.addItem("Rectangle")
        self.combo_box.addItem("Circle")
        self.combo_box.addItem("Line")
        self.combo_box.currentIndexChanged.connect(self.updateGraph)
        
        # 创建绘图区域
        self.graph_widget = QWidget()
        self.graph_widget.setMinimumSize(400, 400)
        
        # 创建布局管理器
        layout = QVBoxLayout()
        layout.addWidget(self.combo_box)
        layout.addWidget(self.graph_widget)
        
        # 创建主窗口中心部件
        central_widget = QWidget()
        central_widget.setLayout(layout)
        self.setCentralWidget(central_widget)
        
    def updateGraph(self):
        # 获取选中的选项
        selected_option = self.combo_box.currentText()
        
        # 创建绘图对象
        painter = QPainter(self.graph_widget)
        painter.setRenderHint(QPainter.Antialiasing)
        
        # 清空绘图区域
        painter.fillRect(self.graph_widget.rect(), Qt.white)
        
        # 根据选项绘制图形
        if selected_option == "Rectangle":
            painter.setPen(QPen(Qt.black, 2))
            painter.drawRect(50, 50, 200, 100)
        elif selected_option == "Circle":
            painter.setPen(QPen(Qt.red, 2))
            painter.drawEllipse(100, 100, 150, 150)
        elif selected_option == "Line":
            painter.setPen(QPen(Qt.blue, 2))
            painter.drawLine(50, 200, 250, 200)
        
        # 刷新绘图区域
        self.graph_widget.update()

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