利用 typealias 幫 function 型別取名

撰寫 Swift 程式時，我們有時會宣告 function 型別的變數或參數來儲存 function 或 closure，比方以下例子:

• 變數 buyBook2 是 function 型別

func buyBook(name: String, author: String) -> String {
    return "買了\(author)的\(name)"
}
var buyBook2: (String, String) -> String = buyBook

• 參數 sport 是 function 型別

func eatAndExercise(sport: () -> String) -> String {
    var message = "大吃特吃後"
    for _ in 1...10 {
        message += sport()
    }
    return message
}

func playTableTennis() -> String {
    "打桌球"
}

eatAndExercise(sport: playTableTennis)

不過 function 型別寫起來有點複雜，要寫 ( )，參數型別，->，回傳型別，因此如果想偷懶的話，我們可以用 typealias 幫 function 型別取名，方便之後使用。

接下來讓我們看看以下 Apple 官方使用 typealias 幫 function 型別取名的例子吧。

Fruta: Building a Feature-Rich App with SwiftUI

Apple Developer Documentation

利用 typealias 幫 function 型別取名 FetchCompletionHandler & PurchaseCompletionHandler。

typealias FetchCompletionHandler = (([SKProduct]) -> Void)
typealias PurchaseCompletionHandler = ((SKPaymentTransaction?) -> Void)
// MARK: - Store
class Store: NSObject, ObservableObject {

宣告 property fetchCompletionHandler & purchaseCompletionHandler，指定型別為 FetchCompletionHandler? & PurchaseCompletionHandler? 。

class Store: NSObject, ObservableObject {
   private var fetchCompletionHandler: FetchCompletionHandler?
   private var purchaseCompletionHandler: PurchaseCompletionHandler?

利用 FetchCompletionHandler & PurchaseCompletionHandler 指定參數的型別

extension Store {
    private func buy(_ product: SKProduct, completion: @escaping PurchaseCompletionHandler) {
        // Save our completion handler for later
        purchaseCompletionHandler = completion
        
        // Create the payment and add it to the queue
        let payment = SKPayment(product: product)
        SKPaymentQueue.default().add(payment)
    }
    
   
    private func fetchProducts(_ completion: @escaping FetchCompletionHandler) {
        guard self.productsRequest == nil else {
            return
        }
        // Store our completion handler for later
        fetchCompletionHandler = completion
        
        // Create and start this product request
        productsRequest = SKProductsRequest(productIdentifiers: allProductIdentifiers)
        productsRequest?.delegate = self
        productsRequest?.start()
    }

呼叫 fetchCompletionHandler & purchaseCompletionHandler 的程式碼片段。

• 呼叫 fetchCompletionHandler。

DispatchQueue.main.async {
    self.fetchCompletionHandler?(loadedProducts)  
    self.fetchCompletionHandler = nil
    self.productsRequest = nil
}

• 呼叫 purchaseCompletionHandler。

if shouldFinishTransaction {
                   
   SKPaymentQueue.default().finishTransaction(transaction)
   DispatchQueue.main.async {
       self.purchaseCompletionHandler?(transaction)
       self.purchaseCompletionHandler = nil
   }
}

其它例子: Today App，UIKit

Apple Developer Documentation