How to get an object index from an array in swift

Question

I am creating a class in swift work as a container of a set of protocols. Below is the source code. The KeyValueObserverDelegate protocol is added in KeyValueObserverService class by addObserver() method. The problem happens on removeObserver() method, the line is index = array.indexOf($0 == observer).

I got an error:

anonymous closure argument not contained in a closure.

I don't know what wrong with my class. How can I get the index of an object from an array?

class KeyValueObserverService{

    private var observerList:Dictionary<String, [KeyValueObserverDelegate]> = Dictionary()

    func addObserver(key:String, observer:KeyValueObserverDelegate){
        var array:Array<KeyValueObserverDelegate>?
        if observerList.keys.contains(key){
            array = observerList[key]
        } else {
            array = Array<KeyValueObserverDelegate>()
            self.observerList[key] = array
        }
        array?.append(observer)
    }

    func updateValueForKey(key:String, value:AnyObject?){
        let array = self.observerList[key];
        if array == nil{
            return
        }
        for  element in array!{
            element.valueChanged(value)
        }
    }

    func removeObserver(key:String, observer:KeyValueObserverDelegate){
        if self.observerList.keys.contains(key) == false{
            return
        }
        var array:[KeyValueObserverDelegate] = self.observerList[key]!;

        let index:Int?


        index = array.indexOf($0 == observer)

        array.removeAtIndex(index!)
    }
}

protocol KeyValueObserverDelegate :class{
    func valueChanged(value:AnyObject?)
}

Answer 1

Read the error message carefully

...not contained in a closure

By definition a closure is enclosed in a pair of curly braces

array.indexOf({$0 == observer})

or with trailing closure syntax

array.indexOf{$0 == observer}

Edit:

Since a protocol does not conform to Equatable by default use the identity operator

array.indexOf{$0 === observer}