Swift Constructors and convenience constructors

Constructor Basics

构造函数is a special function that is used primarily to initialize an object when it is created, to set an initial value for an object, the 成员变量 constructor in OC is Initwithxxx, and in Swift, because of the support for function overloading , all constructors areinit

function of the constructor

• Allocate spacealloc
• Set the initial valueinit

Required Properties

• Custom Person Objects

class Person: NSObject {    /// 姓名 var name: String /// 年龄 var age: Int}

Prompt Error- Class ‘Person‘ has no initializers‘Person‘ 类没有实例化器s

Cause: If a required attribute is defined in a class, you must allocate space for these required properties through the constructor and set the initial value

• 重写The constructor of the parent class

/// `重写`父类的构造函数override init() {}

Prompt Error- Property ‘self.name‘ not initialized at implicitly generated super.init call属性 ‘self.name‘ 没有在隐式生成的 super.init 调用前被初始化

• Adding super.init() calls Manually

/// `重写`父类的构造函数override init() {    super.init()}

Prompt Error- Property ‘self.name‘ not initialized at super.init call属性 ‘self.name‘ 没有在 super.init 调用前被初始化

• Set the initial value for the required property

/// `重写`父类的构造函数override init() {    name = "张三"    age = 18 super.init()}

Summary

• Non-Optional property, you must set the initial value in the constructor to ensure that the property is correctly initialized when the object is instantiated.
• Before calling the parent class constructor, you must ensure that the properties of this class have been initialized
• The constructor in Swift does not have to be writtenfunc

Constructors for subclasses

• When customizing a subclass, you need to set the initial values for the properties defined in this class first in the constructor
• Then call the parent class's constructor to initialize the properties defined in the parent class.

/// 学生类class Student: Person { /// 学号 var no: String override init() { no = "001" super.init() }}

Summary

• The constructors of this class are called first to initialize the properties of this class
• Then call the parent class's constructor to initialize the properties of the parent class
• After Xcode 7 Beta 5, the constructor of the parent class is automatically called, strongly recommended super.init() , and the readability of the code execution thread is maintained
• super.init()Must be placed after the initialization of this class of properties to ensure all initialization of this class of properties is complete

OptionalProperty

• Set the object property type toOptional

class Person: NSObject {    /// 姓名 var name: String? /// 年龄 var age: Int?}

• 可选属性Does not need to set the initial value, the default initial value is nil
• 可选属性Allocates space when setting values, is deferred allocation space, more in line with the principle of delayed creation in mobile development

Overloaded constructors

• Support for function overloading in Swift, same function name, different parameter types

/// `重载`构造函数////// - parameter name: 姓名/// - parameter age:  年龄////// - returns: Person 对象init(name: String, age: Int) { self.name = name self.age = age super.init()}

Precautions

• If the constructor is overloaded, but the default constructor is not implemented init() , the system no longer provides a default constructor
• Cause, when instantiating an object, you must allocate space and set an initial value through the constructor for the object property, and for the class that has the required parameters, the default init() cannot complete the allocation space and set the initial value of the work

To adjust the constructor of a subclass

• 重写The constructor of the parent class

/// `重写`父类构造函数////// - parameter name: 姓名/// - parameter age:  年龄////// - returns: Student 对象override init(name: String, age: Int) { no = "002" super.init(name: name, age: age)}

• 重载constructor function

/// `重载`构造函数////// - parameter name: 姓名/// - parameter age:  年龄/// - parameter no: 学号////// - returns: Student 对象init(name: String, age: Int, no: String) { self.no = no super.init(name: name, age: age)}

Note: If you are overloading a constructor, you must super work with the initialization of the parent class property to complete

重载And 重写

• 重载, function name is the same, parameter name/parameter number is different
  • Overloaded functions are not just limited to构造函数
  • Function overloading is an important symbol of face object programming language
  • Function overloading simplifies programmer's memory
  • OC does not support function overloading, which is an alternative to OCwithXXX...
• 重写, subclasses need to be extended on the basis of the parent class's own methods, requiring the override keyword

KVC Dictionary to model constructors

/// `重写`构造函数////// - parameter dict: 字典////// - returns: Person 对象init(dict: [String: AnyObject]) { setValuesForKeysWithDictionary(dict)}

• The code above will be error-compiled!

• Reason:

  • KVC is specific to OC, KVC is essentially a runtime that dynamically sends   to an object; Setvalue:forkey:   ; method to set the value of the object's properties
  • Therefore, before using the KVC method, you need to ensure that the object has been properly instantiated

• Add   super.init ()   also error

• Cause:

  • Required Properties must complete initialization assignment before calling the parent class constructor

• To modify the required parameters to optional parameters, the adjusted code is as follows:

 ///personal model  Class person: nsobject {///name var Name: string? ///ages var Age: int? ///' override ' constructor //////-parameter dict: Dictionary //////-Returns:person object init (dict: [ Span class= "Hljs-type" >string: anyobject]) {super.init () setvaluesforkeyswithdictionary (Dict)}}        

Run the test and still get an error

Error message: this class is not key value coding-compliant for the key age.这个类的键值 age 与 键值编码不兼容

• Reason:
  • In Swift, if the property is optional, no space is allocated for the property at initialization time
  • The basic data type in OC is 可选 the concept of saving a value that does not exist
• WORKAROUND: Set the initial value for the base data type
• The modified code is as follows:

/// 姓名var name: String?/// 年龄var age: Int = 0/// `重写`构造函数////// - parameter dict: 字典////// - returns: Person 对象init(dict: [String: AnyObject]) { super.init() setValuesForKeysWithDictionary(dict)}

Tip: When defining a class, you must set the initial value for the basic data Type property, otherwise you will not be able to set the value correctly using KVC

KVC function Call Order

Init (dict: [String:Anyobject]) {Super.Init () setvaluesforkeyswithdictionary (Dict)}override func setvalue (Value:anyobject, Forkey key:string) {print (" key \ (Key) \ ( Value) super.setvalue (value, Forkey:key)}//' NSObject ' By default, when a key value is found that is not defined, a ' nsundefinedkeyexception ' exception is thrown override func setvalue (Value:anyobject?, Forundefinedkey key:string) {print ( "UndefinedKey \ (key) \ (Value) "}        

• setValuesForKeysWithDictionaryThe function is called in the dictionary by key repeating the setValue:forKey
• If the function is not implemented forUndefinedKey , the program crashes directly
  • NSObject default throws an exception when a key value is found that is not defined NSUndefinedKeyException
• If implemented forUndefinedKey , it is guaranteed setValuesForKeysWithDictionary to continue traversing the subsequentkey
• If the parent class forUndefinedKey is implemented, the subclass can no longer implement this function

KVC functions for subclasses

/// 学生类class Student: Person { /// 学号 var no: String?}

• If the parent class has implemented related methods in the parent class, no more related methods are implemented in the subclass

Convenience convenient constructor function

• By default, all constructor methods are specified constructorsDesignated
• convenienceThe constructor of the keyword decoration is to facilitate the constructor function
• Convenience constructors have the following characteristics:
  • Can returnnil
  • Only the convenience constructor can call theself.init()
  • The convenience constructor cannot be 重写 orsuper

/// `便利构造函数`////// - parameter name: 姓名/// - parameter age:  年龄////// - returns: Person 对象，如果年龄过小或者过大，返回 nilconvenience init?(name: String, age: Int) { if age < 20 || age > 100 { return nil } self.init(dict: ["name": name, "age": age])}

Note: In Xcode, self.init No smart hints are entered

/// 学生类class Student: Person { /// 学号 var no: String? convenience init?(name: String, age: Int, no: String) { self.init(name: name, age: age) self.no = no }}

Convenient constructor Application scenarios

• Determines whether an object is created based on a given parameter, rather than having to instantiate an object as specified by the constructor
• In practical development, the constructors of existing classes can be extended to simplify the creation of objects with convenient constructors

Summary of constructor functions

• Specifies that the constructor must call the specified constructor of its immediate parent class (unless there is no parent class)
• Convenience constructors must call other 指定构造函数 or used methods defined in the same class self.父类的便利构造函数
• The convenience constructor can returnnil
• Convenience constructors cannot be overridden

Swift Constructors and convenience constructors