swift – 我如何判断哪个警卫声明失败？

如果我有一堆链式守卫让我们发表声明,我怎样才能诊断出哪个条件失败了,不能将我的后卫分成多个陈述？

鉴于这个例子：

guard let keypath = dictionary["field"] as? String,let rule = dictionary["rule"] as? String,let comparator = FormFieldDisplayRuleComparator(rawValue: rule),let value = dictionary["value"]
    else
    {
        return nil
    }

如何判断4个let语句中哪个是失败的并调用了else块？

我能想到的最简单的事情是将语句分成4个连续的保护其他语句,但这感觉不对.

guard let keypath = dictionary["field"] as? String
    else
    {
        print("Keypath Failed to load.")

        self.init()
        return nil
    }

    guard let rule = dictionary["rule"] as? String else
    {
        print("Rule Failed to load.")

        self.init()
        return nil
    }

    guard let comparator = FormFieldDisplayRuleComparator(rawValue: rule) else
    {
        print("Comparator Failed to load for rawValue: \(rule)")

        self.init()
        return nil
    }

    guard let value = dictionary["value"] else
    {
        print("Value Failed to load.")

        self.init()
        return nil
    }

如果我想把它们全部放在一个警卫声明中,我可以想到另一种选择.检查guard语句中的nils可能有效：

guard let keypath = dictionary["field"] as? String,let value = dictionary["value"]
    else
    {

        if let keypath = keypath {} else {
           print("Keypath Failed to load.")
        }

        // ... Repeat for each let...
        return nil
    }

我甚至不知道是否会编译,但是我可能会使用一堆if语句或警卫开始.

什么是惯用的Swift方式？

@H_502_20@

func diagnose(file: String = #file,line: Int = #line) -> Bool { print("Testing \(file):\(line)") return true } // ... let dictionary: [String : AnyObject] = [ "one" : "one" "two" : "two" "three" : 3 ] guard // This line will print the file and line number let one = dictionary["one"] as? String where diagnose(),// This line will print the file and line number let two = dictionary["two"] as? String where diagnose(),// This line will NOT be printed. So it is the one that Failed. let three = dictionary["three"] as? String where diagnose() else { // ... }

swift – 我如何判断哪个警卫声明失败？

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