[swift-evolution] [Review] SE-0018 Flexible Memberwise Initialization

[Matthew Johnson]

> On Jan 7, 2016, at 12:51 PM, Joe Groff <jgroff at apple.com> wrote:
> 
> 
>> On Jan 7, 2016, at 10:49 AM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> 
>>> 
>>> On Jan 7, 2016, at 12:37 PM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>>> 
>>> 
>>>> On Jan 7, 2016, at 10:32 AM, David Owens II <david at owensd.io <mailto:david at owensd.io>> wrote:
>>>> 
>>>> And this is more clear than this?
>>>> 
>>>> class Foo {
>>>>   var x,y,z: Int
>>>>   init(x: Int, y: Int, z: Int) {
>>>>     self.x = x
>>>>     self.y = y
>>>>     self.z = z
>>>>   }
>>>> }
>>> 
>>> No, it isn't, but Matthew asked… I'm personally not too motivated to support anything more than all-or-nothing memberwise initialization, and tend to agree that anything more specialized deserves an explicit implementation.
>> 
>> Maybe you would feel differently if you were an app developer.  Different kinds of code have different needs.  The most important use cases I have in mind are related to UI code, which is often the majority of the code in an app.
> 
> Do you have any concrete examples in mind?
> 
> -Joe
> 

Here is an example where partial memberwise initialization would apply.  It is similar to something in a real project:

public class FontPicker: UIControl {
  public let fonts: [UIFont]

  public var fontSize: CGFloat = 22
  public var foregroundColor: UIColor = UIColor.darkGrayColor()
  public var backgroundColor: UIColor = UIColor.whiteColor()
  // A bunch of other appearance attributes here

  private let collectionView: UICollectionView
  private let layout: UICollectionViewLayout
  // other internal state required by the implementation

  public memberwise init(...) {
    // configure the collection view and add it as a subview
  }
}

A couple points are relevant here:

1. Memberwise initialization is very valuable for the appearance attributes, but is useless if it exposes our implementation details.

2. In many custom UI widgets the appearance attributes don’t really need to be mutable post-initialization.  At the same time, it is necessary to allow, but not require a value to be specified.  It would be ideal if they were `let` properties with a default value, but still able to participate in memberwise initialization.  Without that capability we are forced to choose between the advantages of using a `let` property and the advantages of memberwise initialization.

UI widgets are a great example.  View controllers often have a similar divide between state provided by the user and state related to internal implementation details.

Matthew

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