MFMailComposeViewController不可用时UIAlertController不显示的修复方案

问题场景 #

调用自定义Mail控制器时，当MFMailComposeViewController.canSendMail()返回true，邮件界面能正常显示；但返回false时，错误提示的UIAlertController完全弹不出来。

原代码实现 #

SomeViewController 代码 #

class SomeController: UIViewController {
    func contactWithDeveloper() {
        let detailController = Mail()
        detailController.messageTitle = "1"
        detailController.messageAddress = "1"
        detailController.messageText = "1"
        detailController.errorTitle = "1"
        detailController.errorMessage = "1"
        detailController.modalPresentationStyle = .overFullScreen
        self.present(detailController, animated: false, completion: nil)
    }
}

MailViewController 代码 #

class Mail: UIViewController, MFMailComposeViewControllerDelegate {
    
    var messageTitle = ""
    var messageAddress = ""
    var messageText = ""
    var errorTitle = ""
    var errorMessage = ""
    
    override func viewDidLoad() {
        super.viewDidLoad()
        send(title: messageTitle, address: [messageAddress], message: messageText, errorTitle: errorTitle, errorMessage: errorMessage)
    }
    
    func send(title: String, address: [String], message: String, errorTitle: String, errorMessage: String) {
        if MFMailComposeViewController.canSendMail() {
            let mail = MFMailComposeViewController()
            mail.mailComposeDelegate = self
            mail.setSubject(title)
            mail.setToRecipients(address)
            mail.setMessageBody(message, isHTML: false)
            present(mail, animated: true, completion: nil)
        } else {
            let alert = UIAlertController(title: errorTitle, message: errorMessage, preferredStyle: .alert)
            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: dismissHandler))
            self.present(alert, animated: true, completion: nil)
        }
    }
    
    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {
        controller.dismiss(animated: true, completion: nil)
        dismiss(animated: false, completion: nil)
    }
    
    func dismissHandler(aler: UIAlertAction) {
        dismiss(animated: false, completion: nil)
    }
}

问题原因 #

在viewDidLoad里直接调用send方法弹Alert时，当前的Mail控制器自身还没完成模态展示流程，视图层级根本没准备好，这时候调用present操作会被系统拒绝。而邮件界面能正常弹出，是因为系统对MFMailComposeViewController有特殊兼容处理，Alert没有这个待遇。

修复方案 #

把调用send方法的时机从viewDidLoad移到viewDidAppear(_:)，确保当前控制器完全展示、视图层级就绪后再弹Alert。同时修正几个细节问题：

1. 给Mail控制器添加空视图，避免视图层级异常
2. 修正dismissHandler的参数拼写错误
3. 用弱引用避免循环引用

修改后的MailViewController代码 #

class Mail: UIViewController, MFMailComposeViewControllerDelegate {
    
    var messageTitle = ""
    var messageAddress = ""
    var messageText = ""
    var errorTitle = ""
    var errorMessage = ""
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // 添加透明背景视图，确保控制器有合法的视图层级
        view.backgroundColor = .clear
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        // 视图完全展示后再执行弹窗操作
        send(title: messageTitle, address: [messageAddress], message: messageText, errorTitle: errorTitle, errorMessage: errorMessage)
    }
    
    func send(title: String, address: [String], message: String, errorTitle: String, errorMessage: String) {
        if MFMailComposeViewController.canSendMail() {
            let mail = MFMailComposeViewController()
            mail.mailComposeDelegate = self
            mail.setSubject(title)
            mail.setToRecipients(address)
            mail.setMessageBody(message, isHTML: false)
            present(mail, animated: true, completion: nil)
        } else {
            let alert = UIAlertController(title: errorTitle, message: errorMessage, preferredStyle: .alert)
            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: dismissHandler))
            present(alert, animated: true, completion: nil)
        }
    }
    
    func mailComposeController(_ controller: MFMailComposeViewController, didFinishWith result: MFMailComposeResult, error: Error?) {
        controller.dismiss(animated: true) { [weak self] in
            self?.dismiss(animated: false)
        }
    }
    
    func dismissHandler(alert: UIAlertAction) {
        dismiss(animated: false)
    }
}

内容的提问来源于stack exchange，提问作者Alex Smith