Cannot reference a local function with capture from another local function (swift) - ios

Question

var first_name = ""
func problemFunc() {
FBRequestConnection.startForMeWithCompletionHandler { (connection: FBRequestConnection!, result: AnyObject!, error: NSError!) -> Void in
if let fbGraphUserDict = result as? Dictionary<String, AnyObject>{
first_name = fbGraphUserDict["first_name"] as NSString
println(first_name)
}
}
}
PFFacebookUtils.logInWithPermissions(permissions, {
(user: PFUser!, error: NSError!) -> Void in
if user == nil {
NSLog("Uh oh. The user cancelled the Facebook login.")
} else if user.isNew {
NSLog("User signed up and logged in through Facebook!")
} else {
NSLog("User logged in through Facebook!")
problemFunc() // error is here
}
})
This code is inside an #Ibaction button. I cannot build because the call to problemFunc() triggers the error message in the title of this post. If I move the first_name var definition inside the problemFunc it will work ok. But I need it out, because another function will need to access its value.
I'm really not sure at what causes this problem, if you have a clue, please help.

Answer 1

Use a closure instead of a function:
var first_name = ""
let problemFunc = { () -> () in
FBRequestConnection.startForMeWithCompletionHandler { (connection: FBRequestConnection!, result: AnyObject!, error: NSError!) -> Void in
if let fbGraphUserDict = result as? Dictionary<String, AnyObject>{
first_name = fbGraphUserDict["first_name"] as NSString
println(first_name)
}
}
}
PFFacebookUtils.logInWithPermissions(permissions, {
(user: PFUser!, error: NSError!) -> Void in
if user == nil {
NSLog("Uh oh. The user cancelled the Facebook login.")
} else if user.isNew {
NSLog("User signed up and logged in through Facebook!")
} else {
NSLog("User logged in through Facebook!")
problemFunc() // error is here
}
})

Answer 2

Here are the basic principles in play: (from Apple's docs: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID103)
"Global and nested functions, as introduced in Functions, are actually special cases of closures. Closures take one of three forms:
Global functions are closures that have a name and do not capture any values.
Nested functions are closures that have a name and can capture values from their enclosing function.
Closure expressions are unnamed closures written in a lightweight syntax that can capture values from their surrounding context."
ie this is ok
func someFunc() {
func nestFunc() {}
}
but this is not
func someFunc() {
func nestFunc() {
func nestedFunc2() { }
}
}
If you look at this in Xcode the third function (func nestedFunc2) will give you the error "Cannot reference a local function with capture from another local function"
The top function (func someFunc) is a global scope function and those work like regular functions/methods.
The second function (func nestFunc) is a nested function which is a named closure one level deep that can capture the scope of its parent global function.
Nested functions, can capture the scope of a global function but not the scope of another nested function.
That's why we need a closure i.e.
func someFunc() {
func nestFunc() {
let strictClosure = { () -> () in
//this is where you write the code
}
}
}

Answer 3

#fluidsonic answer should solve the problem. However note that you're doing some spaghetti code, because you are modifying a variable captured by a closure, and executed in the context of another function. That's hard to track if you need to debug, and more generally hard to follow when and how that variable is modified.
A more linear and better readable flow is to define problemFunc as a function taking a function as parameter, and calling that function rather than directly setting the value in the first_name variable:
let problemFunc = { (callback: (String -> Void) -> ()) in
FBRequestConnection.startForMeWithCompletionHandler { (connection: FBRequestConnection!, result: AnyObject!, error: NSError!) -> Void in
if let fbGraphUserDict = result as? Dictionary<String, AnyObject>{
let first_name = fbGraphUserDict["first_name"] as NSString
callback(first_name) // << here you call the callback passing the `first_name` local variable
println(first_name)
}
}
}
and do the actual assignment to first_name in a closure you define when calling problemFunc:
PFFacebookUtils.logInWithPermissions(permissions, {
(user: PFUser!, error: NSError!) -> Void in
if user == nil {
NSLog("Uh oh. The user cancelled the Facebook login.")
} else if user.isNew {
NSLog("User signed up and logged in through Facebook!")
} else {
NSLog("User logged in through Facebook!")
problemFunc { (name: String) -> Void in
first_name = name
}
}
})