iOS13中present画面消除浮动效果，禁用与监测下拉手势

一、需知

iOS13更新后，通过 present 方式呈现出来的画面，默认呈现样式变更为UIModalPresentationStyle.automatic，表现为浮动效果，且可以通过下拉手势将画面dismiss。

需求一：希望iOS13以上`present`的画面样式跟以前一样显示为全屏效果

实现方式，设置modalPresentationStyle为fullScreen。
1.在创建实例时设置，代码如下：

let vc = ViewController()
vc.modalPresentationStyle = .fullScreen
present(vc, animated: true, completion: nil)

2.或者在初始化方法中设置，代码如下：

// 适用于纯代码构建的UIViewController
override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
    super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    modalPresentationStyle = .fullScreen
}

// 适用于xib或storyboard构建的UIViewController    
required init?(coder: NSCoder) {
    super.init(coder: coder)
    modalPresentationStyle = .fullScreen
}

需求二：希望iOS13以上保留浮动效果，禁用下拉手势`dismiss`画面

实现方式：设置isModalInPresentation属性为true。
代码跟上面的实现方式一样。

需求三：监测下拉手势，可以在用户下拉时作出反应

此实现方式只适用于present的画面放在UINavigationController里
实现方式：
1.设置代理

let vc = ViewController()
let navi = UINavigationController(rootViewController: vc)
navi.presentationController?.delegate = vc
present(vc, animated: true, completion: nil)

2.实现UIAdaptivePresentationControllerDelegate的代理方法func presentationControllerDidAttemptToDismiss(_ presentationController: UIPresentationController)，
此方法需同属性isModalInPresentation结合使用，当isModalInPresentation = true时，下拉试图dismiss画面时，才会调用此方法。

override func viewWillLayoutSubviews() {
    // Ensure textView.text is up to date
    textView.text = editedText
    
    // If our model has unsaved changes, prevent pull to dismiss and enable the save button
    let hasChanges = self.hasChanges
    isModalInPresentation = hasChanges
    saveButton.isEnabled = hasChanges
}
...
func presentationControllerDidAttemptToDismiss(_ presentationController: UIPresentationController) {
    // The user pulled down with unsaved changes
    // Clarify the user's intent by asking whether they intended to cancel or save
    print("isModalInPresentation=\(self.isModalInPresentation)")
    confirmCancel(showingSave: true)
}

上面的代码来自官方 Simple Code，请自行下载：
https://developer.apple.com/documentation/uikit/view_controllers/disabling_pulling_down_a_sheet