如何修改/禁用sheetPresentationController全屏时presentingViewController的缩放后退行为？

当使用sheetPresentationController呈现全屏sheet时，底层的presenting VC默认会出现缩放、变暗的"推回"效果，你可以通过以下几种方式修改或完全禁用该行为：

方法1：取消底层视图变暗（简单快速） #

如果只需要取消变暗效果，保留缩放动画，直接设置sheetPresentationController的largestUndimmedDetentIdentifier为.full，这样当sheet切换到全屏detent时，底层视图会保持可见：

let sheetVC = YourSheetViewController()
sheetVC.modalPresentationStyle = .pageSheet

if let sheetPC = sheetVC.sheetPresentationController {
    sheetPC.detents = [.medium(), .full()]
    // 设置全屏时不暗化底层视图
    sheetPC.largestUndimmedDetentIdentifier = .full
}

present(sheetVC, animated: true)

方法2：完全禁用缩放+变暗效果（自定义转场） #

要彻底移除底层VC的缩放动画和变暗效果，需要自定义转场动画，替换系统默认的sheet转场逻辑：

步骤1：设置转场代理 #

在presenting VC中，为sheet VC指定自定义的transitioningDelegate：

let sheetVC = YourSheetViewController()
sheetVC.modalPresentationStyle = .pageSheet

if let sheetPC = sheetVC.sheetPresentationController {
    sheetPC.detents = [.medium(), .full()]
}
// 设置转场代理
sheetVC.transitioningDelegate = self
present(sheetVC, animated: true)

步骤2：实现转场代理方法 #

让presenting VC遵循UIViewControllerTransitioningDelegate，返回自定义的动画控制器：

extension YourPresentingViewController: UIViewControllerTransitioningDelegate {
    // 自定义present动画
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return CustomSheetPresentAnimator()
    }
    
    // 自定义dismiss动画
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return CustomSheetDismissAnimator()
    }
}

步骤3：编写自定义动画控制器 #

实现present和dismiss的动画逻辑，仅处理sheet视图的滑入滑出，不修改底层VC的状态：

// Present动画：sheet从底部滑入
class CustomSheetPresentAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.35 // 匹配系统默认动画时长
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let presentedView = transitionContext.view(forKey: .to) else {
            transitionContext.completeTransition(false)
            return
        }
        
        let container = transitionContext.containerView
        container.addSubview(presentedView)
        
        // 初始位置：屏幕底部
        presentedView.frame = CGRect(x: 0, y: container.bounds.height, width: container.bounds.width, height: container.bounds.height)
        
        UIView.animate(withDuration: transitionDuration(using: transitionContext), options: .curveEaseOut) {
            presentedView.frame = container.bounds
        } completion: { _ in
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    }
}

// Dismiss动画：sheet滑回底部
class CustomSheetDismissAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.35
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let dismissedView = transitionContext.view(forKey: .from) else {
            transitionContext.completeTransition(false)
            return
        }
        
        UIView.animate(withDuration: transitionDuration(using: transitionContext), options: .curveEaseIn) {
            dismissedView.frame = CGRect(x: 0, y: transitionContext.containerView.bounds.height, width: transitionContext.containerView.bounds.width, height: transitionContext.containerView.bounds.height)
        } completion: { _ in
            dismissedView.removeFromSuperview()
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    }
}

方法3：仅在全屏detent时禁用效果 #

如果需要在非全屏detent（比如medium）保留默认效果，仅全屏时禁用，可以通过UISheetPresentationControllerDelegate动态调整：

步骤1：设置代理 #

let sheetVC = YourSheetViewController()
sheetVC.modalPresentationStyle = .pageSheet

if let sheetPC = sheetVC.sheetPresentationController {
    sheetPC.detents = [.medium(), .full()]
    sheetPC.delegate = sheetVC // 让sheetVC自己做代理
}

present(sheetVC, animated: true)

步骤2：实现代理方法 #

让sheet VC遵循UISheetPresentationControllerDelegate，在detent切换时调整底层视图状态：

extension YourSheetViewController: UISheetPresentationControllerDelegate {
    func sheetPresentationController(_ sheetPresentationController: UISheetPresentationController, didChangeSelectedDetentIdentifier selectedDetentIdentifier: UISheetPresentationController.Detent.Identifier?) {
        UIView.animate(withDuration: 0.35) {
            if selectedDetentIdentifier == .full {
                // 全屏时恢复底层视图的原始大小，取消暗化
                self.presentingViewController?.view.transform = .identity
                sheetPresentationController.largestUndimmedDetentIdentifier = .full
            } else {
                // 非全屏时恢复系统默认的缩放和暗化
                self.presentingViewController?.view.transform = CGAffineTransform(scaleX: 0.9, y: 0.9)
                sheetPresentationController.largestUndimmedDetentIdentifier = nil
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Warpling