Swift Variables Initialization

问题

I have a question about variables initialization in swift.

I have two ways to initialize a variable (as "property" of a class in Objective-C).

Which of them is the most correct?

class Class {

  var label: UILabel!

  init() { ... label = UILabel() ... }

}

class Class {

  var label = UILabel()

  init() { … }

}

回答1:

Actually you have 5 ways to initialize properties.

There is no correct way, the way depends on the needs.
Basically declare objects like UILabel always – if possible – as constant (let).

The 5 ways are:

Initialization in the declaration line
```
let label = UILabel(frame:...
```
Initialization in the init method, you don't have to declare the property as implicit unwrapped optional.
```
let label: UILabel
init() { ... label = UILabel(frame:...) ... }
```

The first two ways are practically identical.

Initialization in a method like viewDidLoad, in this case you have to declare the property as (implicit unwrapped) optional and also as var
```
var label: UILabel!

on viewDidLoad()
 ...
 label = UILabel(frame:...)
}
```
Initialization using a closure to assign a default (computed) value. The closure is called once when the class is initialized and it is not possible to use other properties of the class in the closure.
```
let label: UILabel = {
   let lbl = UILabel(frame:...)
   lbl.text = "Foo"
   return lbl
}()
```
Lazy initialization using a closure. The closure is called (once) when the property is accessed the first time and you can use other properties of the class.
The property must be declared as var
```
let labelText = "Bar"

lazy var label: UILabel = {
   let lbl = UILabel(frame:...)
   lbl.text = "Foo" + self.labelText
   return lbl
}()
```

回答2:

Both ways are correct, but sometimes you should use the initialization in init() method. For example, here the target for barButton will be not set, cause the self does not exist yet.

class Foo {
    var barButton = UIBarButtonItem(title: "add", style: .Plain, target: self, action: #selector(self.someMethod))

    init(){
        //init here
    }
}

The correct way for this case is :

class Foo {
    var barButton : UIBarButton? 

    init(){
        barButton = UIBarButtonItem(title: "add", style: .Plain, target: self, action: #selector(self.someMethod))
    }
}

To sum up, both ways are correct, but you have to consider when to use each one. More information about it on Apple documentation