Swift 2 if let with do-try-catch

Question

在 Swift 1.2 中我有这个:

if let filePath = NSBundle.mainBundle().pathForResource("some", ofType: "txt"),
       data = String(contentsOfFile: filePath, encoding: NSUTF8StringEncoding) {
    for line in data.componentsSeparatedByCharactersInSet(NSCharacterSet.newlineCharacterSet()) {
        // Do something
    }      
} else {
    println("some.txt is missing")
}

在 Swift 2 中，我不能再这样做了，因为 pathForResource 和 contentsOfFile 都可以抛出，也可以返回可选值。我可以修复它，但它现在看起来非常冗长:

do {
    if let filePath = try NSBundle.... {
        do {
            if let data = try String.... {
                for line in data..... {
                    // Do something
                }
            } else {
                print("Nil data")
            }
        } catch {
            print("contentsOfFile threw")
        }
    } else {
        print("Nil pathForResource")
    }
} catch {
    print("pathForResource threw")
}      

我想我错过了一些东西 - 感谢您的帮助。

Answer 1

使用 guard 语法代替 if-let。

这是一个示例:

do {

    guard let filePath = try NSBundle .... else {
        // if there is exception or there is no value
        throw SomeError
    }
    guard let data = try String .... else {
    }

} catch {

}

if-let 和 guard 的区别在于展开值的范围。如果您使用 if-let filePath 值仅在 if-let block 内可用。如果您使用保护文件路径值可用于保护调用的范围。

这是swift book中的相关部分

A guard statement, like an if statement, executes statements depending on the Boolean value of an expression. You use a guard statement to require that a condition must be true in order for the code after the guard statement to be executed. Unlike an if statement, a guard statement always has an else clause—the code inside the else` clause is executed if the condition is not true.