Property observers willSet and didSet; Property getters and setters

Question

What is the difference between willSet - didSet, and get - set, when working with this inside a property?

From my point of view, both of them can set a value for a property. When, and why, should I use willSet - didSet, and when get - set?

I know that for willSet and didSet, the structure looks like this:

var variable1 : Int = 0 {     didSet {         println (variable1)     }     willSet(newValue) {     ..     } }  var variable2: Int {     get {         return variable2     }     set (newValue){     } } 

Answer 1

When and why should I use willSet/didSet

• willSet is called just before the value is stored.
• didSet is called immediately after the new value is stored.

Consider your example with outputs:

---

var variable1 : Int = 0 {         didSet{             print("didSet called")         }         willSet(newValue){             print("willSet called")         }     }      print("we are going to add 3")       variable1 = 3      print("we added 3") 

Output:

we are going to add 3 willSet called didSet called we added 3 

it works like pre/post -condition

On the other hand, you can use get if you want to add, for example, a read-only property:

var value : Int {  get {     return 34  } }  print(value)  value = 2 // error: cannot assign to a get-only property 'value' 

Answer 2

@Maxim's answer is for the 1st part of your question.

As for when to use get and set: when you want a computed property. This:

var x: Int 

creates a stored property, which is automatically backed up by a variable (not directly accessible though). Setting a value to that property is translated in setting the value in the property, and similarly for getting.

Instead:

var y = {     get { return x + 5 }     set { x = newValue - 5} } 

will create a computed property, which is not backed up by a variable - instead you have to provide the implementation of the getter and/or setter, usually reading and writing values from another property and more generally as a result of a computation (hence the computed property name)

Suggested reading: Properties

Note: your code:

var variable2: Int {     get{         return variable2     }     set (newValue){     } } 

is wrong because in the get you are trying to return itself, which means calling get recursively. And in fact the compiler will warn you with a message like Attempting to access 'variable2' within its own getter.