使用Cocoa Touch如何为视图控制器实现自定义展示和过渡效果

要为视图控制器实现自定义展示和过渡效果，可以使用UIViewControllerTransitioningDelegate协议和UIViewControllerAnimatedTransitioning协议来实现。以下是一个简单的示例：

1. 创建一个自定义的转场动画类CustomTransitionAnimator，实现UIViewControllerAnimatedTransitioning协议：

import UIKit

class CustomTransitionAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.5
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let fromVC = transitionContext.viewController(forKey: .from),
              let toVC = transitionContext.viewController(forKey: .to),
              let containerView = transitionContext.containerView else {
            return
        }
        
        containerView.addSubview(toVC.view)
        toVC.view.alpha = 0.0
        
        UIView.animate(withDuration: transitionDuration(using: transitionContext), animations: {
            toVC.view.alpha = 1.0
        }) { _ in
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    }
    
}

2. 在视图控制器中设置transitioningDelegate，并实现UIViewControllerTransitioningDelegate协议方法：

class ViewController: UIViewController, UIViewControllerTransitioningDelegate {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        transitioningDelegate = self
    }
    
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return CustomTransitionAnimator()
    }
    
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return CustomTransitionAnimator()
    }
    
}

现在，当你展示或dismiss这个视图控制器时，将会使用你自定义的转场动画效果。