CODEWITHSUNDEEP
iosswiftswift2
Alexander Khitev
Alexander Khitev

Reputation: 6861

How to observe change of value and show it? In Swift

I have a variable that takes the number of objects in the array in a different class, how can I keep track of the change of this variable in the current class? I did a lot of different attempts but failed.

var digitIndex: Int! {
        set {
            self.digitIndex = newValue
        }
        get {
            return firstClass.indexesOfSelectedNodes().count
        }
    }

    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if context == &digitIndex {
            if let newValue = change?[NSKeyValueChangeNewKey] {
                print("newValue")
                infoLabel.text = "\(newValue)"
            }
        }
    }

Upvotes: 0

Views: 2902

Answers (3)

dfrib
dfrib

Reputation: 73186

You can make use of a delegate handling the communication between your two classes.

An example follows below, using a custom delegate MyDelegate (protocol). A delegate instance is initialized in MyOtherClass (e.g., a view controller class), which makes a delegate callback whenever the array myArr is updated in this class. This, in turn, updates the value of digitIndex in MyCurrentClass (e.g., some custom control), which conforms to MyDelegate by implementing the blueprinted arrUpdated(..) method. Finally, the didSet property observer on digitIndex in MyCurrentClass tells us via console print-out that its value has been updated.

protocol MyDelegate: class {
    func arrUpdated(arr: [Int])
}

class MyDifferentClass {
    private var myArr : [Int] = [] {
        didSet {

            // Call delegate.
            delegate?.arrUpdated(myArr)
        }
    }

    weak var delegate: MyDelegate?
}

class MyCurrentClass: MyDelegate {
    var myDifferentClass : MyDifferentClass
    var digitIndex: Int = 0 {
        didSet {
            print("digitIndex updated: \(digitIndex)")
        }
    }

    init(a: MyDifferentClass) {
        myDifferentClass = a
        myDifferentClass.delegate = self
    }

    // MyDelegate
    func arrUpdated(arr: [Int]) {
        digitIndex = arr.count
    }
}

Test:

var a = MyDifferentClass()
var b = MyCurrentClass(a: a)

a.myArr.append(1) // prints 'digitIndex updated: 1'

Upvotes: 0

user3441734
user3441734

Reputation: 17544

1) in your code

var digitIndex: Int! {
        set {
            self.digitIndex = newValue
        }
        get {
            return firstClass.indexesOfSelectedNodes().count
        }
    }

digitIndex is computed property! You are not able to set self.digitIndex within its own setter!

this code, even though compiled, will run forever :-)

var i: Int {
    get {
        return 10
    }
    set {
        i = newValue
    }
}

i = 100
print(i)

2) How to use willSet and didSet (for STORED properties)?

class C1 {}
class C2 {}

class C {
    var objects: [AnyObject] = [C1(),C1(),C2()] {
        willSet {
            print(objects, objects.count, newValue)
        }
        didSet {
            print(objects, objects.count)
        }
    }
    func add(object: AnyObject) {
        objects.append(object)
    }
}

let c = C()
c.add(C1())
/*
[C1, C1, C2] 3 [C1, C1, C2, C1]
[C1, C1, C2, C1] 4
*/

var i: Int = 0 {
willSet {
    print(newValue, "will replace", i)
}
didSet {
    print(oldValue, "was replaced by", i)
}
}

i = 100
/*
100 will replace 0
0 was replaced by 100
*/

you could combine the computed and stored properties for your advantage

// 'private' storage
var _j:Int = 0
var j: Int {
get {
    return _j
}
set {
    print(newValue)
    if newValue < 300 {
    _j = newValue
    } else {
        print(newValue, "refused")
    }
}
}

print(j) // prints 0
j = 200  // prints 200
print(j) // prints 200
j = 500  // prints 500 refused
print(j) // prints 200

Upvotes: 3

Hitendra Solanki
Hitendra Solanki

Reputation: 4941

Try this : 

var digitIndex: Int! {
        set {
            self.digitIndex = newValue
        }
        get {
            return firstClass.indexesOfSelectedNodes().count
        }
        didSet {
            //you will get new value here
        }
    }

No need to add observer for our class properties, you just required to add observer for the properties provided by super class.

Upvotes: 0

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