从if语句全局设置变量

Question

While encoding JSON, I´m unwrapping stuff with an if let statement, but I'd like to make a variable globally available

do {
  if
    let json = try JSONSerialization.jsonObject(with: data) as? [String: String], 
    let jsonIsExistant = json["isExistant"] 
  {
    // Here I would like to make jsonIsExistant globally available
  }

Is this even possible? If it isn't, I could make an if statement inside of this one, but I don't think that would be clever or even possible.

Answer 1

delclare jsonIsExistant at the place you want it. If you are making an iOS App, than above viewDidLoad() create the variable

var jsonIsExistant: String?

then at this point use it

do {
    if let json = try JSONSerialization.jsonObject(with: data) as? [String: String], 
    let tempJsonIsExistant = json["isExistant"] {
        jsonIsExistant = tempJsonIsExistant
    }
}

This could be rewritten like so though

do {
    if let json = try JSONSerialization.jsonObject(with: data) as? [String: String] { 
        jsonIsExistant = json["isExistant"]
    }
} catch {
    //handle error
}

If handled the second way, then you have to check if jsonIsExistant is nil before use, or you could unwrap it immediately with a ! if you are sure it will always have a field "isExistant" every time that it succeeds at becoming json.

Answer 2

It doesn't make sense to expose a variable to the outside of an if let statement:

if let json = ... {
    //This code will only run if json is non-nil.
    //That means json is guaranteed to be non-nil here.
}
//This code will run whether or not json is nil.
//There is  a guarantee json is non-nil.

You have a few other options, depending on what you want to do:

---

You can put the rest of the code that needs json inside of the if . You said you didn't know if nested if statements are "clever or even possible." They're possible, and programmers use them quite often. You also could extract it into another function:

func doStuff(json: String) {
    //do stuff with json
}

//...
if let json = ... {
    doStuff(json: json)
}

---

If you know that JSON shouldn't ever be nil , you can force-unwrap it with ! :

let json = ...!

---

You can make the variable global using a guard statement. The code inside of the guard will only run if json is nil . The body of a guard statement must exit the enclosing scope, for example by throwing an error, by returning from the function, or with a labeled break:

//throw an error
do {
    guard let json = ... else {
        throw SomeError
    }
    //do stuff with json -- it's guaranteed to be non-nil here.
}



//return from the function 
guard let json = ... else {
    return
}
//do stuff with json -- it's guaranteed to be non-nil here.



//labeled break
doStuff: do {
    guard let json = ... else {
        break doStuff
    }
    //do stuff with json -- it's guaranteed to be non-nil here.
}