Difference between KVO and KVC and Delegate

Ans : There is no way to find any differences between KVC and KVO. Both are different things.

1. KVC - Key Value Coding

We can get and set value of class property using string.

Code for example :

import UIKit

class Employee : NSObject {

    @objc var name = String()

    @objc var age = 0

    @objc var assets = ["ID Card", "Macbook"]

}

We should make sure that Employee inherits from NSObject because it confirms protocol named NSKeyValueCodiing.

We also make sure that @objc should be added as it is objective c runtime for making those properties available for coding. emp.setValue("Manna", forKeyPath: #keyPath(<#T##@objc property sequence#>))

Using KVC,

let emp = Employee()

emp.setValue("Manan", forKey: "name")

Here, we set value of name property using string "name". Here there is chance to misspell property name.

Another way,

emp.setValue("Manna", forKeyPath: #keyPath(Employee.name))

Benefit of this way,  There is no any chance to misspell as it only accepts valid key path other wise it gives compile time error.

Another way,

emp.setValuesForKeys([

                        "name" : "Manan",

                        "age"  : 29

                ])

What will happened, if we have some private properties in class. They are not accessible directly using their value. We have to make extension of class to use their values.
or

@objc private var name = String()

emp.setValue("Manan", forKey: "name")

emp.value(forKey: "name")

We can access private properties as above using KVC. emp.name will give compile time error as name is private,  but using KVC it is possible to access.

We can access array and add item in this array,

let mutableArray = emp.mutableArrayValue(forKeyPath: #keyPath(Employee.assets))

mutableArray.add("Laptop Bag")

2. KVO - Key value observer

When we want to do something when property values changes, we can use KVO concept. We can observer property and on value changed we can take action.

For that,
A special method named observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) should be implemented to the observing class.


self.child1.addObserver(self, forKeyPath: "name",  optional: [.new, .old], context: child1context]


There are some parameters :

• addObserver:  This is the observing class, usually the self object. 
• forKeyPath: I guess you can understand what’s this for. It is the string you used as a key or a key path and matches to the property you want to observe. Note that you specify here either a single key, or a key path. 
• options: an array of NSKeyValueObservingOptions values. 
• context: This is a pointer that can be used as a unique identifier for the change of the property we observe. Usually this is set to nil or NULL. We’ll see more about this later.

We have to implement  following observerValue method and it is mandatory to adopt KVO concept.

Sometimes, we don't want notification when some  property value changed. Then we do following :

Credit : HackerMoon

Know more about KVO : Click here

Difference between Delegate, Notification and KVO

Use a delegate if you want to talk to only one object. For example, a tableView has a delegate - only one object should be responsible for dealing with it.

Use notifications if you want to tell everyone that something has happened. For example in low memory situations, a notification is sent telling your app that there has been a memory warning. Because lots of objects in your app might want to lower their memory usage it's a notification.

I don't think KVO is a good idea at all and try not to use it but, if you want to find out if a property has changed you can listen for changes.