What is the difference between the following 3 declarations? - swift

Question

var title: UILabel {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}
let title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
lazy var title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
If I put 'let' in the first one, compiler will complain that 'computed property do not allow let'. Ok, kind of makes sense. The only difference between the first one and second is '=' and '()'. So, does it mean that it is not a computed property anymore?

Answer 1

1.
var title: UILabel {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}
It is a read only computed property. Computed properties cannot be let. These are calculated using other stored/computed properties. So they don't have any backing store of their own. Hence, computed properties are always declared as var.
2.
let title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
It is a stored property. This is assigned a closure that returns a UILabel object. This closure is executed during the instantiation process of the object and the returned UILabel object is assigned to title.
3.
lazy var title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
It is a lazy stored property. It is also assigned a closure that returns a UILabel object. But this closure is not executed during the instantiation process. It is executed whenever this property is first used. After the execution of the closure, the UILabel object returned is assigned to title.

Answer 2

This is computed get-only property, it calculates every time when you try to get it value:
var title: UILabel {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}
This is regular property initialized immidiatly by in-place invoked closure (which playing default value role):
let title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
This is lazy property, which will be initialized only on first access by in-place invoked closure:
lazy var title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()

Answer 3

In addition to all #PGDev has said, I would like to point out another way to write your second/third declaration:
Instead of:
let title: UILabel = {
let label = UILabel()
textLabel.font = .systemFontOfSize(13)
return label
}()
you can write:
let title: UILabel = {
$0.font = .systemFontOfSize(13)
return $0
}(UILabel())
It does exactly the same as the above, just the code is written differently :)