Swift nil值行为

Question

Can you send messages to nil in Swift the same way you can in Objective-C without causing a crash?

I tried looking into the documentation and couldn't find anything relating to this.

Answer 1

Not exactly, you have to use Optional Chaining . In swift, an instance can only be nil if it is declared as an "optional" type. Normally this looks like this:

var optionalString : String?

Notice the ? after the String That is what makes it possible to be nil

You cannot call a method on that variable unless you first "unwrap" it, unless you use the aforementioned Optional Chaining.

With optional chaining you can call multiple methods deep, that all allow for a nil value to be returned:

var optionalResult = optionalString.method1()?.method2()?.method3()

optionalResult can also be nil. If any of the methods in the chain return nil, methods after it are not called, instead optionalResult immediately gets set to nil.

You cannot deal directly with an optional value until you explicitly handle the case that it is nil. You can do that in one of two ways:

Force it to unwrap blindly

println(optionalString!)

This will throw a runtime error if it is nil, so you should be very sure that it is not nil

Test if it is nil

You can do this by using a simple if statement:

if optionalString {
    println(optionalString!)
}
else {
    // it was nil
}

or you can assign it to a scoped variable so that you don't have to forcefully unwrap it:

if let nonoptionalString = optionalString {
    println(nonoptionalString)
}
else {
   // it was nil
}