// 使用特殊值作為“無”可能產(chǎn)生歧義//var errorCode = 404//errorCode = 0// 使用可選型var errorCode:Int? = 404print( errorCode )errorCode = nilprint( errorCode )var color: UIColor? = nil// Int? 和 Int 不是一種類型let imInt = 405errorCode = imInt//imInt = errorCode// 必須顯示地聲明可選型的類型//var imOptional = nilvar imOptional: String? = nil

var errorCode:String? = "404"print( errorCode )// 可選型不可以直接使用//"The errorCode is" + errorCode// 強(qiáng)制解包"The errorCode is " + errorCode!// 強(qiáng)制解包是危險(xiǎn)的errorCode = nil//"The errorCode is" + errorCode!// 判斷不是nilif errorCode != nil{"The errorCode is " + errorCode!}else{"No error"}// if let 解包if let unwrappedErrorCode = errorCode{"The errorCode is " + unwrappedErrorCode}else{"No error"}// 可以使用相同的變量名if let errorCode = errorCode{"The errorCode is " + errorCode}else{"No error"}// 使用if-let同時(shí)解包多個(gè)變量var errorMessage:String? = "Not found"if let errorCode = errorCode{if let errorMessage = errorMessage{"The errorCode is " + errorCode + "/nThe errorMessage is " + errorMessage}}

//多層解包（swift 2.0使用）if let errorCode = errorCode , errorMessage = errorMessage{"The errorCode is " + errorCode + "/nThe errorMessage is " + errorMessage}//以上多層解包方法在swift3.0中應(yīng)寫為if let errorCode = errorCode , let errorMessage = errorMessage{"The errorCode is " + errorCode + "/nThe errorMessage is " + errorMessage}// where（swift 2.0使用）if let errorCode = errorCode , errorMessage = errorMessage where errorCode == "404"{print("Page not found")}//在swift3.0中應(yīng)寫為 // whereif let errorCode = errorCode , let errorMessage = errorMessage , errorCode == "404"{print("Page not found")}

var errorMessage: String? = "Not Found"if let errorMessage = errorMessage{errorMessage.uppercased()}//使用 Optional chainingerrorMessage?.uppercased()var uppercaseErrorMessage = errorMessage?.uppercased()//uppercaseErrorMessage為可選型if let errorMessage = errorMessage?.uppercased(){errorMessage}// Optional chaining 也可以使用!let uppercaseErrorMessage2 = errorMessage!.uppercased()//此時(shí) uppercaseErrorMessage2 為String類型。此時(shí)為強(qiáng)制解包，容易出現(xiàn)錯(cuò)誤

// Nil-Coalescingvar errorMessage: String? = nillet message: Stringif let errorMessage = errorMessage{message = errorMessage}else{message = "No error"}// 使用三目運(yùn)算符let message2 = errorMessage == nil ? "No error" : errorMessage!// 注意: 此時(shí)使用errorMessage需要強(qiáng)制解包.以保證message2是一個(gè)String，而非String?// 三目運(yùn)算符?:的實(shí)質(zhì)就是一個(gè)if else// 使用 ??let message3 = errorMessage ?? "No error"// 注意: 此時(shí)使用errorMessage不需要強(qiáng)制解包. // ??符號(hào)已經(jīng)保證了訪問到errorMessage時(shí), errorMessage不是nil// Swift將為我們自動(dòng)解包, 保證message2永遠(yuǎn)是一個(gè)String, 而不是String?//這種用法叫做 Nil-Coalescing