WHY THE BACKGROUND CHANGE ITS COLOR AUTOMATIC?

override func viewDidLoad() { super.viewDidLoad() view.backgroundColor = .gray view.insetsLayoutMarginsFromSafeArea = false

    setupCanvasView()
    setupToolPicker()
}

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    
    // 确保画布成为第一响应者以显示工具栏
    canvasView.becomeFirstResponder()
}

func setupCanvasView() {
    canvasView.frame = view.bounds
    canvasView.delegate = self
    canvasView.insetsLayoutMarginsFromSafeArea = false
    
    // 设置画布属性以支持绘画
    canvasView.alwaysBounceVertical = true
    canvasView.alwaysBounceHorizontal = true
    canvasView.isScrollEnabled = true
    canvasView.showsVerticalScrollIndicator = true
    canvasView.showsHorizontalScrollIndicator = true
    canvasView.contentSize = CGSize(width: 500,height: 500)
    
    // 启用绘画功能
    canvasView.drawingPolicy = .anyInput // 支持手指和 Apple Pencil
    view.addSubview(canvasView)
}

AS THE CODE ABOVE,WHEN I DRAW ON THE CANVAS,THE BACKGROUND CHANGE ITS COLOR TO WHITE.WHY? !