iOS显示ViewController的方式有两种：Push和Modal。当以Modal方式显示ViewController的时候，UIModalPresentationStyle属性决定了模态显示样式，定义如下：

typedef NS_ENUM(NSInteger, UIModalPresentationStyle) {
        UIModalPresentationFullScreen = 0,
        UIModalPresentationPageSheet,
        UIModalPresentationFormSheet,
        UIModalPresentationCurrentContext,
        UIModalPresentationCustom,
        UIModalPresentationOverFullScreen,
        UIModalPresentationOverCurrentContext,
        UIModalPresentationPopover,
        UIModalPresentationNone = -1,
};

以下内容将对上述各样式进行详细讲解：

UIModalPresentationFullScreen

覆盖全屏，并以RootViewController为Context进行显示。当显示完成后，会将属于PresentingViewController的所有视图暂时移出视图栈。

UIModalPresentationOverFullScreen

同上，但与UIModalPresentationFullScreen不同的是，当显示完成后，不会将属于PresentingViewController的视图移出视图栈。因此，如果该PresentedViewController的内容视图拥有透明度的话，可以透视到位于PresentingViewController的视图内容。比较以下两张视图层次结构图可以发现，当以Full Screen方式显示ViewController时，CurrentContextViewController及其所有子视图被移出了视图栈，但当以Over Full Screen方式显示ViewController时，CurrentContextViewController及其所有子视图并未被移出视图栈。

Full Screen Over Full Screen

UIModalPresentationCurrentContext

UIKit将自当前视图节点向上搜索视图控制器层次结构，并以第一个definesPresentationContext属性为true的ViewController为Context进行显示。当显示完成后，会将属于PresentingViewController的所有视图暂时移出视图栈。

UIModalPresentationOverCurrentContext

同上，但与UIModalPresentationCurrentContext不同的是，当显示完成后，不会将属于PresentingViewController的视图移出视图栈。

UIModalPresentationPageSheet

如果UIWindow的horizontalSizeClass为compact时，显示效果同UIModalPresentationFullScreen。
如果UIWindow的horizontalSizeClass为regular时，显示如下：

Page Sheet

UIModalPresentationFormSheet

如果UIWindow的horizontalSizeClass为compact时，显示效果同UIModalPresentationFullScreen。
如果UIWindow的horizontalSizeClass为regular时，显示如下：

Form Sheet

对于Form Sheet与Page Sheet，如何当UIWindow的horizontalSizeClass为compact时，仍然显示为Sheet样式而非Full Screen？只需实现UIAdaptivePresentationControllerDelegate代理，并设置被显示ViewController的presentationController.delegate即可。

extension CustomViewController: UIAdaptivePresentationControllerDelegate {
    func adaptivePresentationStyle(for controller: UIPresentationController) -> UIModalPresentationStyle {
        return .none
    }
}

UIModalPresentationCustom

在使用此样式呈现ViewController之前，需要实现UIViewControllerTransitioningDelegate协议，并将ViewController的transitioningDelegate属性设置为上述的自定义委托。

class CustomViewController: UIViewController {
    override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
        self.modalPresentationStyle = .custom
        self.transitioningDelegate = self
    }
}

extension CustomViewController: UIViewControllerTransitioningDelegate {
    func presentationController(forPresented presented: UIViewController, presenting: UIViewController?, source: UIViewController) -> UIPresentationController? {
        return CustomPresentationController(presentedViewController: presented, presenting: presenting)
    }
}

class CustomPresentationController: UIPresentationController {
    override var frameOfPresentedViewInContainerView: CGRect {
        let bounds = containerView!.bounds
        let size = CGSize(width: 300, height: 500)
        let origin = CGPoint(x: bounds.midX - size.width / 2, y: bounds.midY - size.height / 2)
        return CGRect(origin: origin, size: size)
    }

    override func containerViewWillLayoutSubviews() {
        presentedView?.frame = frameOfPresentedViewInContainerView
    }
}

UIModalPresentationPopover

如果UIWindow的horizontalSizeClass为compact时，显示效果同UIModalPresentationFullScreen。
如果UIWindow的horizontalSizeClass为regular时，显示如下：

Popover

class CustomViewController: UIViewController {
    @IBAction func presentPopover(_ sender: Any) {
        let vc = UIViewController()
        vc.view.backgroundColor = UIColor.red
        vc.modalPresentationStyle = .popover
        vc.preferredContentSize = CGSize(width: 200, height: 50)

        if let popoverPresentationController = vc.popoverPresentationController {
            let sourceView = sender as! UIButton
            popoverPresentationController.sourceView = sourceView
            popoverPresentationController.sourceRect = sourceView.bounds
            popoverPresentationController.delegate = self
        }

        self.present(vc, animated: true)
    }
}

extension CustomViewController: UIPopoverPresentationControllerDelegate {
    func adaptivePresentationStyle(for controller: UIPresentationController) -> UIModalPresentationStyle {
        // 如果当UIWindow的horizontalSizeClass为compact时，也想显示为Popover而非FullScreen，只需实现本方法并返回none即可。
        return .none
    }
}