Swift compile error, subclassing NSValue, using su

问题:

This code

class ID<T: AnyObject> : NSValue {
    init(baseObject: T) {
        super.init(nonretainedObject: baseObject)
    }
}

gives this compiler error:

error: must call a designated initializer of the superclass 'NSValue'
    super.init(nonretainedObject: baseObject)
    ^

How do I get rid of this?

Things I thought of

I thought the error might be because the NSValue initializer has an AnyObject? type (Note well: postfix ?). I tried various flavors of casting and [?!] postfixing in places, and it fixed nothing.

Also, presumably NSValue(nonretainedObject:) must call the designated initializer, right?

回答1:

NSValue(nonretainedObject:) isn't a designated initializer. The only initializer listed in the NSValue reference (and hence the designated initializer) is NSValue(value:CConstVoidPointer, withObjCType type:CString)

The other constructors are all convenience constructors derived from class helper methods.

You might try:

init(baseObject: T) {
    super.init(bytes:&baseObject, withObjCType:"^v")
}

"^v" is the type string returned by an NSValue created with valueWithNonRetained....

Unfortunately, I'm not coming up with an appropriate way to pass baseObject as a CConstVoidPointer.

Barring that, the best thing I can come up with is to wrap NSValue instead of subclassing it:

class ID<T:AnyObject> {
    let wrapped:NSValue

    init(baseObject:T) {
        wrapped = NSValue(nonretainedObject:baseObject)
    }
}

Finally got something to work, but it's a little bit ugly, basically add a convenience constructor that wraps the nonretainedObject constructor and then use that in your subclass:

extension NSValue {
    convenience init<T:AnyObject>(unretained:T) {
        self.init(nonretainedObject:unretained)
    }
}


class ID<T>:NSValue {
    convenience init<T:AnyObject>(unretained:T) {
        self.init(unretained:unretained)
    }
}

Depending on what you're actually trying to do the category alone might be sufficient?