了解有关解析最新SDK / Swift 1.2的语法问题

Question

Why does the 2nd snippet work and the first not work?

This code does not work :

func logIn() {
    PFUser.logInWithUsernameInBackground(tv_username.text, password:tv_password.text) {
        (user: PFUser!, error: NSError!) -> Void in
        if user != nil {
            // Yes, User Exists
            //self.loginInitialLabel.text = "User Exists"
        } else {
            // No, User Doesn't Exist
        }
    }
}

This code works :

func logIn() {
    PFUser.logInWithUsernameInBackground(tv_username.text, password:tv_password.text) {
        (user, error) -> Void in
        if user != nil {
            // Yes, User Exists
            //self.loginInitialLabel.text = "User Exists"
        } else {
            // No, User Doesn't Exist
        }
    }
}

Below is the error message. I am looking for a clear explanation of why some online docs have hte first example but only the 2nd one works. Did Parse change their SDK without changing documentation or is this some artifact of Swift 1.2 change? I am using XCode 6.3 and Swift 1.2.

Zoom of the error message:

Answer 1

• In the first example, you specify the types of user and error explicitly ( PFUser! and NSError! ) respectively.

• In the second example, you permit the type of user and error to be supplied implicitly.

Thus, the fact that the first example gives a compile error must mean that your explicit types are no longer correct. It could be the exclamation marks; try removing them.

The real way to figure out what types they are, though, is to use the second example, compile it, and then to put the cursor inside user and then inside error and read off the types from Quick Help on the right side of the Xcode window, as I do here:

That little trick has solved many Swift type mysteries for me!