typealias 探究

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typealias 是用来给已经存在的类型重新定义一个新的名字

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typealias Sample = UInt32
var num = Sample.min

typealias InputClosure = (String) -> Void
var closure: InputClosure?
closure!("A")

这个和Objective-C中的关键字typedef很相似

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# define CGFLOAT_TYPE double
typedef CGFLOAT_TYPE CGFloat

typedef void (^InputClosure)(NSString *sourceString);

typealias 在给类型重新定义一个新名字的时候，同时可以使得代码可读性更强，而且语法也变的简单。

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class ApiClient {
  // ....
  func getUsers(success: ((result: AnyObject, operation: AFHTTPRequestOperation) -> Void)? = nil,
                error: ((error: NSError, operation: AFHTTPRequestOperation) -> Bool)? = nil,
                finished: (() -> Void)? = nil) {
    // Do stuff 
  }
func getUser(user: User, success: ((result: AnyObject, operation: AFHTTPRequestOperation) -> Void)? = nil,
             error: ((error: NSError, operation: AFHTTPRequestOperation) -> Bool)? = nil,
             finished: (() -> Void)? = nil) {
    // Do stuff 
  }
func getInvitations(success: ((result: AnyObject, operation: AFHTTPRequestOperation) -> Void)? = nil,
                    error: ((error: NSError, operation: AFHTTPRequestOperation) -> Bool)? = nil,
                    finished: (() -> Void)? = nil) {
    // Do stuff 
  }
  // ....
}

上面的代码不仅读起来吃力，而且很多重复的，代码量也多
我们将typealias应用API client代码中

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class ApiClient {
  // ....
  typealias SuccessHandler = (result: AnyObject, operation: AFHTTPRequestOperation)
    -> Void
  typealias ErrorHandler = (error: NSError, operation: AFHTTPRequestOperation)
    -> Void
  typealias FinishedHandler = () -> Void
  func getUsers(success: (SuccessHandler)? = nil,
                error: (ErrorHandler)? = nil,
                finished: (FinishedHandler)? = nil) {
    // Do stuff
  }
  func getUsers(success: (SuccessHandler)? = nil,
                error: (ErrorHandler)? = nil,
                finished: (FinishedHandler)? = nil) {
    // Do stuff
  }
  func getUser(user: User, success: (SuccessHandler)? = nil,
               error: (ErrorHandler)? = nil,
               finished: (FinishedHandler)? = nil) {
    // Do stuff
  }
  // ....
}

这种改动更方便读写，而且最棒的是，在调用函数的时候根本不需要任何的改动

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var api = ApiClient()
api.getUsers({ (result, operation) -> Void in
	<#code#>
	}, error: { (error, operation) -> Bool in
		<#code#>
	}) { () -> Void in
		<#code#>
}

其他的例子也可以看 王巍的文章 TYPEALIAS 和泛型接口,其中讲到的更好理解更典型