UIAlert无法将类型'（_）throws->（）'的值转换为预期的参数类型'（（（UIAlertAction）-> Void）？

Question

I've been looking for solutions to an issue I'm having with some code in a UIAlertAction . I want to update a value in an SQLite database from true to false for a given number. I've managed to get the if/else loops working so I know I'm getting into the right place. But when I try and run the database update query from the UIAlertAction, I get the above error.

Code below:

} else {
    print ("Badge in School - check out")
    let checkOut = UIAlertController(title: "Sign Out?", message: "Click on the 'Sign Out' button below to end your visit", preferredStyle: .alert)

    let signOut = UIAlertAction(title: "Sign Out", style: .destructive) { (UIAlertAction) in
        print ("Set Badge State to false")

        let badgeOut = self.badgeTable.filter(self.badgeNumber == badgeNumberInt!)
        let badgeOutUpdate = badgeOut.update(self.badgeIn <- false)

        try self.myData.run(badgeOutUpdate)
    }

    let cancel = UIAlertAction(title: "Cancel", style: .cancel)

    checkOut.addAction(signOut)
    checkOut.addAction(cancel)

    present (checkOut, animated: true, completion: nil)
}

If I change:

try self.myData.run(badgeOutUpdate)

to

try! self.myData.run(badgeOutUpdate)

the code runs and does exactly what I'd expect.

Any explanations or pointers to documents that can explain it to an idiot like me would be gratefully received!

Answer 1

The run method you are calling can throw an exception, but the alert action closure does not expect an exception to be thrown. You must handle the exception because the alert action is not setup to allow the exception to bubble up. When you add the ! to the try, you're saying "do this and if an exception happens, crash the app." Another alternative is to wrap it in a do-catch block:

do {
    try self.myData.run(badgeOutUpdate)
} catch {
    // Handle the exception here
}

If an exception is encountered, it will execute the catch block. If not, the catch will be ignored. Another option is to use the following:

try? self.myData.run(badgeOutUpdate)

This will perform the call that can throw. If that call does throw an exception, it fails silently and your program continues on. This may be fine in some instances, or it may mean that nothing else will work in your program.

Each situation is different, choose which works best for you: crash on exception, handle the exception, fail silently.

The Swift documentation covers this pretty well: https://docs.swift.org/swift-book/LanguageGuide/ErrorHandling.html

Answer 2

You need do-catch block

do {
      try self.myData.run(badgeOutUpdate)
  }
catch {
       print(error)
}