swift - 展开可选值 : Swift - FMDB - executeQuery 时意外发现 nil

Question

我是 Swift、FMDB 和一般开发的新手，我和我在运行时遇到 fatal error :unexpectedly found nil while unwrapping an Optional Value. 错误发生在 executeQuery 行

初步尝试及相关代码:

        var rightAnswer:FMResultSet?
        var memberDatabase:FMDatabase?
    ....
override func viewDidLoad() {
let path = NSBundle.mainBundle().pathForResource("1_celebs", ofType: "sqlite3")
        memberDatabase = FMDatabase(path: path)
        if memberDatabase!.open(){
            print("database is ready")//it works if I comment out the stuff in loadquestion() 
        }
        else{
            print("error finding database")
        }
...
    func loadQuestion(){
        let querySQL = "SELECT Quote, Answer, answerNumber, Celeb1, Celeb2, Celeb3, img1, img2, img3, feedbackImg, Context FROM celebs WHERE answeredRight = 'no' ORDER BY RANDOM() LIMIT 1"

        rightAnswer = memberDatabase!.executeQuery(querySQL, withArgumentsInArray: queryParameters)
        rightAnswer!.next()

然后我在 func loadQuestion()

中尝试了这个

let results:FMResultSet? = memberDatabase!.executeQuery(querySQL, withArgumentsInArray: nil)
        while(results?.next() == true)
        {...}

然后我试了一下:

do{
   rightAnswer = try memberDatabase!.executeQuery(querySQL, withArgumentsInArray: queryParameters)
   while rightAnswer!.next(){...}
 } catch let error as NSError {
    print("failed: \(error.localizedDescription)")
 }

然后，这个:

   do{
   let rs = try memberDatabase!.executeQuery(querySQL, values: nil)
    while rs.next(){...}
} catch let error as NSError {
  print("failed: \(error.localizedDescription)")
}

我每次都在 executeQuery 行上遇到同样的错误!如果我试图摆脱感叹号和问号，那么我会在控制台上收到一条错误消息，指出无法打开数据库。

Answer 1

问题是 memberDatabase 是 nil。

您应该确保您正在填充该变量。

另一个建议，你应该避免使用 force unwrap 运算符。它确实绕过了编译器对 nil(或可能的 nil)值的检查。

Swift 确实提供了更好的解决方案，例如

条件展开

if let memberDatabase = memberDatabase {
    try memberDatabase.executeQuery(querySQL, values: nil)
}

后卫

guard let memberDatabase = memberDatabase else { return }
try memberDatabase.executeQuery(querySQL, values: nil)