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Initialization Closureとglobal定数を組み合わせて、computedライクなglobal定数を作る

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Initialization Closureとは?

Initialization closureは、以下の特徴を持ちます

  • closure内でpropertyに操作を加えられる
  • stored propertyである(値が割り当てられるタイミングはclassからinstanceが生成された時)

If a stored property’s default value requires some customization or setup, you can >use a closure or global function to provide a customized default value for that >property. Whenever a new instance of the type that the property belongs to is >initialized, the closure or function is called, and its return value is assigned >as the property’s default value.

公式ドキュメント

主にUIをコードで記述する際に用いられます。
参考記事

Globalな定数(変数)の特徴

Globalな定数(変数)は以下の特徴を持つ

  • lazyである(割り当てられるタイミングは値が呼ばれた時)

Global constants and variables are always computed lazily, in a similar manner to >Lazy Stored Properties. Unlike lazy stored properties, global constants and >variables don’t need to be marked with the lazy modifier.
Local constants and variables are never computed lazily.

それぞれの特徴を組み合わせると、computedライクなglobal定数を作ることができます。

ViewController.swift
let somePattern: FetchedValue = {
   let defalutValue = "hoge"
   return isFetchedCompleted ? fetchedValue : defaultValue
}()

これは、外部サービスを使って、複数画面でABテストを行いたい/アプリの挙動を変えたい場合に以下の性質を持つので便利です。

  • lazyなので、アプリ起動後にfetchが間に合わない場合でも、対象画面に遷移するまで時間を稼げる(=テストパターンに割り振られるユーザー数を増やせる)
  • globalなので、各画面でfetchを行わなくて良い
  • 各画面でfetchを行わないので、画面毎に挙動が異なることがない
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