3.swift

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
		
	// 取出 toView, 在上面的示意圖中代表的就是 A 畫面
        guard let toView = transitionContext.viewController(forKey: .to)?.view else {
                return
        }
        
	// 取出 container view
        let containerView = transitionContext.containerView

	// 建立我們的 mask view，並坐初步設置
        let maskView = UIView()
        maskView.frame.size = CGSize(width: 1, height: 1)
        maskView.center = startPoint
        maskView.backgroundColor = .black
        maskView.layer.cornerRadius = 0.5
        toView.mask = maskView
        containerView.addSubview(toView)
        
	// 因為最終 mask view 的圓，需要覆蓋到整個畫面，所以計算出 mask view需要的大小
        let containerFrame = containerView.frame
        let maxY = max(containerFrame.height - startPoint.y, startPoint.y)
        let maxX = max(containerFrame.width - startPoint.x, startPoint.x)
        let maxSize = max(maxY, maxX) * 2.1

	// 最後我們使用 UIView.animation 顯式動畫，將我們的 mask view 擴散到整個畫面
        UIView.animate(
            	withDuration: durationTime,
            	delay: 0,
            	options: [.curveEaseOut],
            	animations: {
                		maskView.frame.size = CGSize(width: maxSize, height: maxSize)
                		maskView.layer.cornerRadius = maxSize / 2.0
                		maskView.center = self.startPoint
        }) { (flag) in
            	transitionContext.completeTransition(flag)
        }
    }