Nineth Day of Swift learning: Optional types and application scenarios

An introduction to the optional types

• Attention:
  • A knowledge point that is more understood in Swift when an optional type is available
  • Learn more about using Xcode tips to get started
  • Slowly understand the principles and benefits as you learn.
• Concept:
  • In OC Development, if a variable is paused, it can be assigned a value of 0 (base attribute type) or an assignment of Null (object type)
  • Nil is also a special type in swift development. It is not possible to assign a value because it does not match the true type (Swift is a strongly typed language)
  • However, in the development of the value of nil, it is inevitable. Therefore, optional types are available
• Values for the optional type:
  • Null value
  • has a value

Define an optional type

• There are two ways to define an optional type
  • The most basic wording
  • Grammar sugar (common)

// 错误写法// let string : String = nil// 正确写法:// 注意:name的类型是一个可选类型,但是该可选类型中可以存放字符串.// 写法一:定义可选类型let name : Optional<String> = nil// 写法二:定义可选类型,语法糖(常用)let name : String? = nil

Use of optional types

Walkthrough One: Assigning a value to an optional typeDefine an optional typevar string:optional<String> =NilAssigning a value to an optional typeError notation: Therefore, the optional type can only hold strings string =123Correct wording: string ="Hello World"Print resultsprint (string) //Result: Optional ("Hello World") \n//because the print out is optional type, all will take Optional//Walkthrough Two: Remove the value of the optional type / /Remove the actual value of the optional type (unpacking) print (string!) //Result: Hello world\n//Note: If the optional type is nil, forcing the fetch of the value (unpacking), the error string = nilprint (string!) //error //correct wording: if string! = nil {print (string!)} //simple notation: To make it easy to use String//optional bindings in an if statement if let str = string {print (str)}< /span>                 

Import UIKit//all properties in the class must have an initialization value when the object is initializedclassPerson:nsobject {var name:string?//has a value or is a null valuevar view:uiview?}//1. Define an optional type//1> How to define optional types in general//var name:optional<string>//2> Grammar Sugarvar name:string?//2. Assigning a value to an optional typeName =" Why"//3. Take a value from an optional type//Optional ("why")print (name)//to take a value from an optional type: Optional type!--> forced unpacking//print (name!)//4. Note: If there is no value in the optional type, forcing the unpacking program crashes//forcing unpacking is a very risky operation: it is recommended to determine if there are any values in the optional type before unpacking.ifName! =Nil {print (name!) Print (name!) Print (name!)}//5. Optional Bindings//1> Determines if the name has a value and does not execute {} If there is no value.//2> If there is a value, the optional type is unpacked and the value after the unpacking is assigned to the preceding constant//if let Tempname = name {//print (tempname)//}ifLet name =Name { print (name) print (name) print}

Two: Optional type of application scenario

Import UIKit/*1: When the type of two objects is not the same, can not be directly assigned, Nsurl (string:urlstr) Create a URL to get the type is Nsurl? Optional type, can also not label Nsurl?, with type Deduction 2: Optional type of unpacking: 1:! To be forced to unpack, you must first determine whether it is nil, otherwise the forced unpacking of nil will occur when the crash 2: You can also use optional binding*/Let urlstr="www.baidu.com"Let url:nsurl? = Nsurl (string: URLSTR)//if URL! = Nil {//Let request = Nsurlrequest (url:url! as URL)//}//Optional BindingsifLet URL =URL {Let request= Nsurlrequest (Url:url asURL)} 

Nineth Day of Swift learning: Optional types and application scenarios