Swift相关

[jiangleligejiang]

基础部分

• 使用_代替忽略的部分

  let http404Error = (404, "Not Found")
  let (justTheStatusCode, _) = http404Error

• 元组访问对应的元素

  let http404Error = (404, "Not Found")
  print("The status is \(http404Error.0), message is \(http404Error.1)")
  let http200Status = (statusCode: 200, message: "OK")
  print("The status is \(http200Status.statusCode), message is \(http200Status.message)")

• 隐式解析可选类型

  
  let possibleString: String? = "An optional string."
  let forcedString: String = possibleString!

let assumedString: String! = "An implicitly unwrapped optional string." let implicitString: String = assumedString

- 断言
```swift
if age > 10 {
    print("You can ride the roller-coaster or the ferris wheel.")
} else if age > 0 {
    print("You can ride the ferris wheel.")
} else {
    assertionFailure("A person's age can't be less than zero.")
}

• assert和precondition的区别

  assert 和 assertionFailure 函数仅在优化未开启时有效。这对于检查那些耗性能的条件是很有用的，但通常情况下应尽量避免使用。precondition 和 preconditionFailure 函数在优化开启时也有效。

  precondition(index > 0, "Index must be greater than zero.")

[jiangleligejiang]

基本运算符

• 元组的比较

  如果两个元组的元素相同，且长度相同的话，元组就可以被比较。比较元组大小会按照从左到右、逐值比较的方式，直到发现有两个值不等时停止。如果所有的值都相等，那么这一对元组我们就称它们是相等的。

  (1, "zebra") < (2, "apple")   // true，因为 1 小于 2
  (3, "apple") < (3, "bird")    // true，因为 3 等于 3，但是 apple 小于 bird
  (4, "dog") == (4, "dog")      // true，因为 4 等于 4，dog 等于 dog

• 使用is来判断对象类型，相当于OC中的isKindofClass

  func fetchList() {
  guard let data = data else { return }
  
  switch data {
  case is Film:
  fetch(data.listItems, of: Starship.self)
  case is Starship:
  fetch(data.listItems, of: Film.self)
  default:
  print("Unknown type: ", String(describing: type(of: data)))
  }
  }

[jiangleligejiang]

字符串

• 多行字符串

  使用"""来包含多行的字符串

  let singleLineString = "These are the same."
  let multilineString = """
  These are the same.
  """

• 使用indices属性会创建一个包含全部索引的范围

  for index in greeting.indices {
  print("\(greeting[index]) ", terminator: "")
  }

• 使用区间来获取子字符串

  for index in greeting.indices {
  print("\(greeting[index]) ", terminator: "")
  }
  // 打印输出“G u t e n   T a g ! ”

[jiangleligejiang]

泛型

• 使用泛型来定义通用的数据结构，以便实现通用方法

  private func fetch<T: Decodable & Displayable>(_ list: [String], of: T.Type) {
  var items: [T] = []
  let fetchGroup = DispatchGroup()
  
  list.forEach { (url) in
  fetchGroup.enter()
  AF.request(url).validate().responseDecodable(of: T.self) { (response) in
    if let value = response.value {
      items.append(value)
    }
    fetchGroup.leave()
  }
  }
  
  fetchGroup.notify(queue: .main) {
  self.listData = items
  self.listTableView.reloadData()
  }
  }