在JavaFX canvas中访问GraphicsContext内的元素

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.stage.Stage;

public class CanvasExample extends Application {
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Canvas Example");
        
        // 创建Canvas对象
        Canvas canvas = new Canvas(400, 300);
        
        // 获取GraphicsContext对象
        GraphicsContext gc = canvas.getGraphicsContext2D();
        
        // 在GraphicsContext内进行绘制操作
        gc.strokeRect(50, 50, 300, 200);
        
        Group root = new Group();
        root.getChildren().add(canvas);
        
        Scene scene = new Scene(root, 400, 300);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}