What is the nature of a class property when declared with “= {}()” in Swift?





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I have a Swift class from some sample code, and within it there is a property captureSession declared like so:



private lazy var captureSession: AVCaptureSession = {

    let session = AVCaptureSession()



    guard

        let backCamera = AVCaptureDevice.default(for: .video),

        let input = try? AVCaptureDeviceInput(device: backCamera)

        else { return session }

    session.addInput(input)

    return session

}()


I don't think captureSession is a computed property, neither is it a closure. Then what is it?






ios swift avcapturesession







share|improve this question































    1















    I have a Swift class from some sample code, and within it there is a property captureSession declared like so:



    private lazy var captureSession: AVCaptureSession = {
    
    let session = AVCaptureSession()
    

    
    guard
    
        let backCamera = AVCaptureDevice.default(for: .video),
    
        let input = try? AVCaptureDeviceInput(device: backCamera)
    
        else { return session }
    
    session.addInput(input)
    
    return session
    
}()


    I don't think captureSession is a computed property, neither is it a closure. Then what is it?






    ios swift avcapturesession







    share|improve this question



























      1












      1








      1








      I have a Swift class from some sample code, and within it there is a property captureSession declared like so:



      private lazy var captureSession: AVCaptureSession = {
      
    let session = AVCaptureSession()
      

      
    guard
      
        let backCamera = AVCaptureDevice.default(for: .video),
      
        let input = try? AVCaptureDeviceInput(device: backCamera)
      
        else { return session }
      
    session.addInput(input)
      
    return session
      
}()


      I don't think captureSession is a computed property, neither is it a closure. Then what is it?






      ios swift avcapturesession







      share|improve this question
















      I have a Swift class from some sample code, and within it there is a property captureSession declared like so:



      private lazy var captureSession: AVCaptureSession = {
      
    let session = AVCaptureSession()
      

      
    guard
      
        let backCamera = AVCaptureDevice.default(for: .video),
      
        let input = try? AVCaptureDeviceInput(device: backCamera)
      
        else { return session }
      
    session.addInput(input)
      
    return session
      
}()


      I don't think captureSession is a computed property, neither is it a closure. Then what is it?






      ios swift avcapturesession




      ios swift avcapturesession






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Nov 22 '18 at 8:28









      Shruti Thombre

      8292824




      8292824










      asked Nov 22 '18 at 7:28









      Gang FangGang Fang

      12518




      12518
























          3 Answers
          3






          active

          oldest

          votes


















          5














          captureSession is lazy property but ={}() is not regarding lazy initialization.
          It is Setting a Default Property Value with a Closure or Function. This is an example.



          let titleLabel: UILabel = {
          
    let label = UILabel()
          
    label.numberOfLines = 0
          
    label.textColor = UIColor.textColor
          
    label.font = UIFont(name: "HelveticaNeue-Medium", size: 14)
          
    return label
          
}()


          You can find more information at the end of this document.






          share|improve this answer


























          • In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

            – Gang Fang
            Nov 22 '18 at 11:07



















          3














          This is Lazy initialisation. It is often used when initial value is relatively expensive to create. So you create value when you sure you need it. So captureSession will be created when you access it first time and then stored in captureSession variable.



          The syntax ={}() depicts a closure (anonymous function) which is called when you access your property. This closure return Type is AVCaptureSession.



          Other words: Using lazy var a: SomeType = { ... }() you postpone object a creation until you really need it. When you access it first time, the variable a will take a result of the closure.




          I like the explanation written here, Official documentation can also be helpful.






          share|improve this answer





















          • 1





            The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

            – Sulthan
            Nov 22 '18 at 9:05











          • @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

            – fewlinesofcode
            Nov 22 '18 at 9:06





















          1















          Lazy properties allow you to create certain parts of a Swift type when needed, rather than doing it as part of its initialization process. This can be useful in order to avoid optionals, or to improve performance when certain properties might be expensive to create. It can also help with keeping initializers more clean, since you can defer some of the setup of your types until later in their lifecycle.




          In simple words captureSession instance won't be created until you access it the first time, and after instantiation will return the same instance everytime.



          The advantage of this approach is that you can keep both the property declaration and its setup in one place.



          Here's a nice article on lazy initialization: Using lazy properties in Swift






          share|improve this answer


























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            3 Answers
            3






            active

            oldest

            votes








            3 Answers
            3






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            5














            captureSession is lazy property but ={}() is not regarding lazy initialization.
            It is Setting a Default Property Value with a Closure or Function. This is an example.



            let titleLabel: UILabel = {
            
    let label = UILabel()
            
    label.numberOfLines = 0
            
    label.textColor = UIColor.textColor
            
    label.font = UIFont(name: "HelveticaNeue-Medium", size: 14)
            
    return label
            
}()


            You can find more information at the end of this document.






            share|improve this answer


























            • In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

              – Gang Fang
              Nov 22 '18 at 11:07
















            5














            captureSession is lazy property but ={}() is not regarding lazy initialization.
            It is Setting a Default Property Value with a Closure or Function. This is an example.



            let titleLabel: UILabel = {
            
    let label = UILabel()
            
    label.numberOfLines = 0
            
    label.textColor = UIColor.textColor
            
    label.font = UIFont(name: "HelveticaNeue-Medium", size: 14)
            
    return label
            
}()


            You can find more information at the end of this document.






            share|improve this answer


























            • In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

              – Gang Fang
              Nov 22 '18 at 11:07














            5












            5








            5







            captureSession is lazy property but ={}() is not regarding lazy initialization.
            It is Setting a Default Property Value with a Closure or Function. This is an example.



            let titleLabel: UILabel = {
            
    let label = UILabel()
            
    label.numberOfLines = 0
            
    label.textColor = UIColor.textColor
            
    label.font = UIFont(name: "HelveticaNeue-Medium", size: 14)
            
    return label
            
}()


            You can find more information at the end of this document.






            share|improve this answer















            captureSession is lazy property but ={}() is not regarding lazy initialization.
            It is Setting a Default Property Value with a Closure or Function. This is an example.



            let titleLabel: UILabel = {
            
    let label = UILabel()
            
    label.numberOfLines = 0
            
    label.textColor = UIColor.textColor
            
    label.font = UIFont(name: "HelveticaNeue-Medium", size: 14)
            
    return label
            
}()


            You can find more information at the end of this document.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Nov 22 '18 at 11:17









            savepopulation

            7,94743753




            7,94743753










            answered Nov 22 '18 at 8:02









            Emre ÖzdilEmre Özdil

            328510




            328510













            • In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

              – Gang Fang
              Nov 22 '18 at 11:07



















            • In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

              – Gang Fang
              Nov 22 '18 at 11:07

















            In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

            – Gang Fang
            Nov 22 '18 at 11:07





            In the doc it says: "Note that the closure’s end curly brace is followed by an empty pair of parentheses. This tells Swift to execute the closure immediately.", that is what I missed!

            – Gang Fang
            Nov 22 '18 at 11:07













            3














            This is Lazy initialisation. It is often used when initial value is relatively expensive to create. So you create value when you sure you need it. So captureSession will be created when you access it first time and then stored in captureSession variable.



            The syntax ={}() depicts a closure (anonymous function) which is called when you access your property. This closure return Type is AVCaptureSession.



            Other words: Using lazy var a: SomeType = { ... }() you postpone object a creation until you really need it. When you access it first time, the variable a will take a result of the closure.




            I like the explanation written here, Official documentation can also be helpful.






            share|improve this answer





















            • 1





              The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

              – Sulthan
              Nov 22 '18 at 9:05











            • @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

              – fewlinesofcode
              Nov 22 '18 at 9:06


















            3














            This is Lazy initialisation. It is often used when initial value is relatively expensive to create. So you create value when you sure you need it. So captureSession will be created when you access it first time and then stored in captureSession variable.



            The syntax ={}() depicts a closure (anonymous function) which is called when you access your property. This closure return Type is AVCaptureSession.



            Other words: Using lazy var a: SomeType = { ... }() you postpone object a creation until you really need it. When you access it first time, the variable a will take a result of the closure.




            I like the explanation written here, Official documentation can also be helpful.






            share|improve this answer





















            • 1





              The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

              – Sulthan
              Nov 22 '18 at 9:05











            • @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

              – fewlinesofcode
              Nov 22 '18 at 9:06
















            3












            3








            3







            This is Lazy initialisation. It is often used when initial value is relatively expensive to create. So you create value when you sure you need it. So captureSession will be created when you access it first time and then stored in captureSession variable.



            The syntax ={}() depicts a closure (anonymous function) which is called when you access your property. This closure return Type is AVCaptureSession.



            Other words: Using lazy var a: SomeType = { ... }() you postpone object a creation until you really need it. When you access it first time, the variable a will take a result of the closure.




            I like the explanation written here, Official documentation can also be helpful.






            share|improve this answer















            This is Lazy initialisation. It is often used when initial value is relatively expensive to create. So you create value when you sure you need it. So captureSession will be created when you access it first time and then stored in captureSession variable.



            The syntax ={}() depicts a closure (anonymous function) which is called when you access your property. This closure return Type is AVCaptureSession.



            Other words: Using lazy var a: SomeType = { ... }() you postpone object a creation until you really need it. When you access it first time, the variable a will take a result of the closure.




            I like the explanation written here, Official documentation can also be helpful.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Nov 22 '18 at 7:41

























            answered Nov 22 '18 at 7:35









            fewlinesofcodefewlinesofcode

            2,1371820




            2,1371820








            • 1





              The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

              – Sulthan
              Nov 22 '18 at 9:05











            • @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

              – fewlinesofcode
              Nov 22 '18 at 9:06
















            • 1





              The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

              – Sulthan
              Nov 22 '18 at 9:05











            • @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

              – fewlinesofcode
              Nov 22 '18 at 9:06










            1




            1





            The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

            – Sulthan
            Nov 22 '18 at 9:05





            The = { ... }() syntax is not part of the lazy initialization in any way though. You can use it even for normal variables and on the other hand, you don't need it for lazy variables.

            – Sulthan
            Nov 22 '18 at 9:05













            @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

            – fewlinesofcode
            Nov 22 '18 at 9:06







            @Sulthan, the = {...}() is just a way to write closure result assigned to variable. I was answering in the context of the question.

            – fewlinesofcode
            Nov 22 '18 at 9:06













            1















            Lazy properties allow you to create certain parts of a Swift type when needed, rather than doing it as part of its initialization process. This can be useful in order to avoid optionals, or to improve performance when certain properties might be expensive to create. It can also help with keeping initializers more clean, since you can defer some of the setup of your types until later in their lifecycle.




            In simple words captureSession instance won't be created until you access it the first time, and after instantiation will return the same instance everytime.



            The advantage of this approach is that you can keep both the property declaration and its setup in one place.



            Here's a nice article on lazy initialization: Using lazy properties in Swift






            share|improve this answer






























              1















              Lazy properties allow you to create certain parts of a Swift type when needed, rather than doing it as part of its initialization process. This can be useful in order to avoid optionals, or to improve performance when certain properties might be expensive to create. It can also help with keeping initializers more clean, since you can defer some of the setup of your types until later in their lifecycle.




              In simple words captureSession instance won't be created until you access it the first time, and after instantiation will return the same instance everytime.



              The advantage of this approach is that you can keep both the property declaration and its setup in one place.



              Here's a nice article on lazy initialization: Using lazy properties in Swift






              share|improve this answer




























                1












                1








                1








                Lazy properties allow you to create certain parts of a Swift type when needed, rather than doing it as part of its initialization process. This can be useful in order to avoid optionals, or to improve performance when certain properties might be expensive to create. It can also help with keeping initializers more clean, since you can defer some of the setup of your types until later in their lifecycle.




                In simple words captureSession instance won't be created until you access it the first time, and after instantiation will return the same instance everytime.



                The advantage of this approach is that you can keep both the property declaration and its setup in one place.



                Here's a nice article on lazy initialization: Using lazy properties in Swift






                share|improve this answer
















                Lazy properties allow you to create certain parts of a Swift type when needed, rather than doing it as part of its initialization process. This can be useful in order to avoid optionals, or to improve performance when certain properties might be expensive to create. It can also help with keeping initializers more clean, since you can defer some of the setup of your types until later in their lifecycle.




                In simple words captureSession instance won't be created until you access it the first time, and after instantiation will return the same instance everytime.



                The advantage of this approach is that you can keep both the property declaration and its setup in one place.



                Here's a nice article on lazy initialization: Using lazy properties in Swift







                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited Nov 22 '18 at 7:43

























                answered Nov 22 '18 at 7:37









                Daniyal RazaDaniyal Raza

                27423




                27423






























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