Do something when dictionary values change

Question

I'm looking for a way to run some code whenever the values of a dictionary changes. I'm still quite new to Swift, but this is what I have so far:

var objects: NSMutableDictionary {
    didChange(changeKind: keyValue, valuesAtIndexes: indexes, forKey: something){

    }
}

This however gives me a compiling error (Use of unresolved identifier something), and whatever I do, I can't seem to make it work. Any ideas?

Answer 1

What you're looking for is the didSet property observer. Usage is as follows:

var objects: NSMutableDictionary? {
    didSet {
        // stuff
    }
}

Note that willSet is another option if you want to be notified just before the property changes instead of just after. In the case of willSet, you're given a newValue variable representing the incoming value of the property and in the case of didSet, an oldValue property representing the outgoing value.

As @Paulw11 brought up below, this will not notify you when the contents of the dictionary change. Only when the variable is reassigned. If you want to be notified whenever the values inside the dictionary are updated, (AFAIK) you have to use an equivalent Swift Dictionary.

var swiftObjects: [NSObject: AnyObject]? {
    didSet {
        println(oldValue)
    }
}

Answer 2

How did you explain this:

class My {
    var dict = [String: String]() {
        didSet {
            print(oldValue)
        }
    }
}

let my = My()
my.dict["asd"] = "asd"
my.dict["bfg"] = "bfg"
my.dict = [String: String]()

Output:

[:]
["asd": "asd"]
["bfg": "bfg", "asd": "asd"]

So didSet called every time new value added.