昨天，我們在 Xcode console 上一直看到這個 warning

ForEach<Array<String>, String, Text>: the ID q occurs multiple times within the collection, this will give undefined results!

在 List 那一段，我們使用 logs 本身當成 ID，而 logs 是型態為 [String]，如果這個 array 裡面已經有 q，那再加上一個 q，就會有數個 q 了。而 ID 本身的意思就是「唯一值」，這時候值不唯一，就會噴出警告。(不過在 UI 上，仍然能正常顯示複數個 q，看來 List 有做其他的事情在 Array 更新的時候)

List(logs, id: \.self) { log in
        let eachLog = "打出的字為: \(log)"
        Text(eachLog)
      }

看來 String 在這裡不是個很好的資料結構，我們設計一個 TypingLog 來放進 logs 吧。這個 TypingLog 為了要能被 List 呈現，他需要 conform Identifiable 和 Hashable 這兩個 protocol。

struct TypingLog: Identifiable, Hashable {
    
    let id: UUID = .init()
    
    let typedString: String
}

然後修改一下 logs 的型別

@State private var logs: [TypingLog] = []

再修改一下 list

List(logs, id: \.self) { log in
        let eachLog = "打出的字為: \(log.typedString)"
        Text(eachLog)
      }

再改一下 timer

/// 猴子開始打字囉
  private func askMonkeysTyping() {
    
    typingTimer = Timer.publish(every: 0.1, on: .main, in: .common)
      .autoconnect()
      .sink { _ in
        let typedCharacter = createRandomString()
        print("發動產生文字: \(typedCharacter)")
        // 如果想和 console 一樣，最新的在最下面，就用 append()
        let typingLog = TypingLog(typedString: typedCharacter)
        logs.insert(typingLog, at: 0)
      }
  }

然後，再 Build 起來讓猴子打個字，現在再也不會看到剛剛那個 warning 了。

整個 view 的程式碼如下

import SwiftUI
import Combine

struct InfiniteMonkeyTypingContentView: View {
  
  @State private var targetText = ""
  
  @State private var monkeyTyperCount = 1
  
  @State private var logText = ""
  
  @State private var textStyle = UIFont.TextStyle.body
  /// 發動「猴子」打字的 timer
  @State private var typingTimer: AnyCancellable?
  
  @State private var logs: [TypingLog] = []
  
  private var targetHint: String {
    if targetText.isEmpty {
      return "目前沒目標，請輸入目標文字在輸入框"
    }
    return "你的目標為: \(targetText)"
  }
  
  var body: some View {
    
    VStack {
      Text("無限猴子打字機")
        .font(.largeTitle)
        .padding(.top, 20)
      
      Text(targetHint)
        .lineLimit(1)
        .padding()
      
      TextField("請輸入目標", text: $targetText)
        .autocapitalization(.none)
        .padding()
        .textFieldStyle(.roundedBorder)
      
      monkeyTyperStepper
      
      monkeyActionButtons
      
      monkeyLogsAndClearLogs
      
      List(logs, id: \.self) { log in
        let eachLog = "打出的字為: \(log.typedString)"
        Text(eachLog)
      }
      Spacer()
    }
  }
  
  private var monkeyTyperStepper: some View {
    HStack {
      Stepper("猴子數: \(monkeyTyperCount)") {
        stepperIncrease()
      } onDecrement: {
        stepperDecrease()
      }
    }
    .padding()
  }
  
  private var monkeyActionButtons: some View {
    HStack {
      Button("猴子停手") {
        stopMonkeysTyping()
      }
      
      Button("叫猴子開始打字囉") {
        askMonkeysTyping()
      }
    }
    .buttonStyle(.bordered)
    .padding()
  }
  
  private var monkeyLogsAndClearLogs: some View {
    HStack {
      Spacer()
      Text("猴子的打字紀錄")
      Button {
        // TODO: - 清掉打字
      } label: {
        Text("清除打字紀錄")
      }
      .padding(.leading, 20)
      .buttonStyle(.bordered)
      Spacer()
    }
  }
  
  private func stepperIncrease() {
    monkeyTyperCount += 1
  }
  
  private func stepperDecrease() {
    if monkeyTyperCount > 1 {
      monkeyTyperCount -= 1
    }
  }
  /// 猴子開始打字囉
  private func askMonkeysTyping() {
    
    typingTimer = Timer.publish(every: 0.1, on: .main, in: .common)
      .autoconnect()
      .sink { _ in
        let typedCharacter = createRandomString()
        print("發動產生文字: \(typedCharacter)")
        // 如果想和 console 一樣，最新的在最下面，就用 append()
        let typingLog = TypingLog(typedString: typedCharacter)
        logs.insert(typingLog, at: 0)
      }
  }
  /// 叫猴子停手
  private func stopMonkeysTyping() {
    
    typingTimer?.cancel()
  }
}

extension InfiniteMonkeyTypingContentView {
  
  private var alphabet: [String] {
    let characters = "abcdefghijklmnopqrstuvwxyz"
    var chars: [String] = []
    for char in characters {
      chars.append(String(char))
    }
    return chars
  }
  
  private func createRandomString() -> String {
    
    var resultString = ""
    
    for _ in 0..<monkeyTyperCount {
      resultString += alphabet.randomElement() ?? ""
    }
    return resultString
  }
}