如何在Swift中将nil类型传递给参数？

Question

I am converting an iOS app from objective-c to swift. In the objective-c version of the app there was a method, AlertViewShow that gets called, and for one of its parameters nil was passed in like below.

AlertViewShow(APPNAME, INTERNET_NOT, nil);

The nil is passed in for a UIAlertViewDelegate type. Now when converting it to Swift I cannot do

AlertViewShow(APPNAME, INTERNET_NOT, nil) //error: cannot pass in nil into paramter

So, by looking at stack forums I tried this...

let alertViewNilObject: UIAlertViewDelegate? = nil
AlertViewShow(APPNAME, INTERNET_NOT, alertViewNilObject!) //compiler doesnt complain

So no errors pop up. But when I run the app it says: "fatal error: unwrapped a nil value".

How do I pass in nil into the parameter in Swift 2.0? Or is there another alternative besides passing in nil?

Answer 1

You need to make the argument itself take an optional value , rather than trying to force unwrap a nil optional (which will always crash).

func AlertViewShow(title:String, desc:String, delegate:AlertViewDelegate?) {

    if delegate != nil {
        // do some delegate-y things
    }

    ...
}

...

AlertViewShow(title: APPNAME, desc: INTERNET_NOT, delegate: nil)

If it's an obj-c method that you're not re-writing in Swift, you'll just have to add the nullable prefix to the argument.

-(void) AlertViewShow:(NSString*)title desc:(NSString*)description delegate:(nullable UIAlertViewDelegate*)delegate;

(although it's worth noting that method names should start with a lowercase, not an uppercase)

That should then get converted to an optional argument when bridging.