[fluent] dart generic (generic class | generic method | generic with specific type constraints)

One 、Dart Generic classes and generic methods

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Generic action : by class , Interface , Method Provide reusability , Support data types with uncertain types ;

Generic classes : Improve code reuse ;

Generic methods : Parameters or return values have generic type constraints , The parameter or return value type must conform to the corresponding generic type , When using generic types, type checking constraints will be carried out , If the wrong type is set , Compile times error ;

Generic class examples :

///  Generic action  :  by   class  ,  Interface  ,  Method   Provide reusability  ,  Support data types with uncertain types //////  Generic classes  :  Improve code reuse ///  This class is a cache class  ,  The data type of cache is  T  Generic  ,  This type can be any type class Cache<T>{ 
      ///  Cache data is stored in this  Map  Collection   Map<String, Object> _map = Map();  ///  Set generic cache data  ,  The method is generic   ///  Here will be  T  Type of data is stored in  map  Collection   void setCacheItem(String key, T value){ 
        _map[key] = value;  }  ///  Fetch generic cache data  ,  The method is generic   T getCachedItem(String key){ 
        return _map[key];  }}

Test the above generic classes :

///  Generic test class class Generic{ 
      ///  This method tests generic classes and generic methods   void test(){ 
        //  Create generic class objects  ,  The generic type is set to  String  type     Cache<String> cache = Cache();    //  When calling generic methods  ,  The passed in parameter must conform to the corresponding generic type     //  Generic constraint  :  When using generic types, type checking constraints will be carried out  ,  If the wrong type is set  ,  Compile times error     cache.setCacheItem("name", "Tom");    //  Get cached content     String value = cache.getCachedItem("name");    print(" Generic testing ,  Type string ,  The obtained cache content is  ${value}");    //  Create generic class objects  ,  The generic type is set to  int  type     Cache<int> cache2 = Cache();    //  When calling generic methods  ,  The passed in parameter must conform to the corresponding generic type     //  Generic constraint  :  When using generic types, type checking constraints will be carried out  ,  If the wrong type is set  ,  Compile times error     cache2.setCacheItem("age", 18);    //  Get cached content     int value2 = cache2.getCachedItem("age");    print(" Generic testing ,  Type integer ,  The obtained cache content is  ${value2}");  }}

Print the results :

I/flutter (24673):  Generic testing ,  Type string ,  The obtained cache content is  TomI/flutter (24673):  Generic testing ,  Type integer ,  The obtained cache content is  18

Two 、Dart Specific type constraints in generics

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Generics can also have specific type constraints , If you specify that the generic type must be a subclass of a class , Use <T extends Person> Constraining that the generic must be a subclass of a class ;

Generic class sample code :

///  Specific type constraints in generics ///  Constrain generics to subclasses of a type class Member<T extends Person>{ 
      T _person;  ///  Set... In the constructor  T _person  Members of the value of the   Member(this._person);  ///  obtain  _person  Name   String getName(){ 
        return _person.name;  }}

The two classes mentioned above are in 【Flutter】Dart object-oriented ( Name the construction method | Factory construction method | Name factory construction method ) In the definition of ;

Test the above generic classes :

    ///  Generic class testing     ///  Class generic requirements are  T extends Person ,  The generic type must be  Person  Subclasses of     /// Student  yes  Person  Subclasses of     Member<Student> member = Member(Student(6, "Tom", 18));    String name = member.getName();    print(" Generic class testing ,  Acquired  T extends Person  generic  name  Field is  ${name}");

Test print results :

I/flutter (24673):  Generic class testing ,  Acquired  T extends Person  generic  name  Field is  Tom

3、 ... and 、Dart Built in generics

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stay Flutter Of main.dart Medium State It's a generic class ;

class _MyHomePageState extends State<MyHomePage> { 
    }

State Class requires a generic T , This generic type must inherit StatefulWidget class ;

abstract class State<T extends StatefulWidget> extends Diagnosticable { 
    }

Here MyHomePage Namely The generic type , yes StatefulWidget Subclasses of classes , accord with Generic requirements ;

Four 、 Complete code example

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Generic classes , Generic methods , Generic test related code :

import 'package:flutterapphello/Dart_OOP.dart';///  Generic test class class Generic{ 
      ///  This method tests generic classes and generic methods   void test(){ 
        //  Create generic class objects  ,  The generic type is set to  String  type     Cache<String> cache = Cache();    //  When calling generic methods  ,  The passed in parameter must conform to the corresponding generic type     //  Generic constraint  :  When using generic types, type checking constraints will be carried out  ,  If the wrong type is set  ,  Compile times error     cache.setCacheItem("name", "Tom");    //  Get cached content     String value = cache.getCachedItem("name");    print(" Generic testing ,  Type string ,  The obtained cache content is  ${value}");    //  Create generic class objects  ,  The generic type is set to  int  type     Cache<int> cache2 = Cache();    //  When calling generic methods  ,  The passed in parameter must conform to the corresponding generic type     //  Generic constraint  :  When using generic types, type checking constraints will be carried out  ,  If the wrong type is set  ,  Compile times error     cache2.setCacheItem("age", 18);    //  Get cached content     int value2 = cache2.getCachedItem("age");    print(" Generic testing ,  Type integer ,  The obtained cache content is  ${value2}");            ///  Generic class testing     ///  Class generic requirements are  T extends Person ,  The generic type must be  Person  Subclasses of     /// Student  yes  Person  Subclasses of     Member<Student> member = Member(Student(6, "Tom", 18));    String name = member.getName();    print(" Generic class testing ,  Acquired  T extends Person  generic  name  Field is  ${name}");  }}///  Generic action  :  by   class  ,  Interface  ,  Method   Provide reusability  ,  Support data types with uncertain types //////  Generic classes  :  Improve code reuse ///  This class is a cache class  ,  The data type of cache is  T  Generic  ,  This type can be any type class Cache<T>{ 
      ///  Cache data is stored in this  Map  Collection   Map<String, Object> _map = Map();  ///  Set generic cache data  ,  The method is generic   ///  Here will be  T  Type of data is stored in  map  Collection   void setCacheItem(String key, T value){ 
        _map[key] = value;  }  ///  Fetch generic cache data  ,  The method is generic   T getCachedItem(String key){ 
        return _map[key];  }}///  Specific type constraints in generics ///  Constrain generics to subclasses of a type class Member<T extends Person>{ 
      T _person;  ///  Set... In the constructor  T _person  Members of the value of the   Member(this._person);  ///  obtain  _person  Name   String getName(){ 
        return _person.name;  }}

It involves Person and Student class :

///  Definition  Dart  class ///  And  Java  Language is similar to ,  All classes inherit by default  Object  class class Person{ 
      ///  Defining variables   String name;  int age;  ///  Private field   int _achievement;  ///  Standard construction method ,  The following method is a common construction method   Person(this.name, this.age);  /// get  Method  :  Set private fields  achievement  Of  get  Method ,  ///  Let the outside world have access to  Person  Object's  _achievement  Private member   int get achievement => _achievement;  /// set  Method  :  Set private fields  achievement  Of  set  Method ,  ///  Let the outside world set  Person  Object's  _achievement  Private member value   set achievement(int achievement){ 
        _achievement = achievement;  }  ///  Static methods  ,  Called by the class name   static log(){ 
        print("log");  }  ///  Overrides the method of the parent class   @override  String toString() { 
        return "$name : $age";  }}///  Inherit class Student extends Person{ 
      ///  Private variables ,  Variables starting with an underscore are private variables   int _grade;  String school;  String city;  String address;  ///  Parent constructor call  :  If the parent class has a constructor with non empty parameters ,  Subclasses must implement constructors with the same parameters   ///  If the class has a parent  ,  Then call the constructor of the parent class first  ,  Complete the initialization of the parent class   ///  Then you can complete your initialization   ///  /// this.school  Specify own parameters   /// {this.school}  Is an optional parameter ,  The optional parameter must be the last in the constructor parameter list   ///  ///  Default parameters  :  Among the optional parameters, if the user does not initialize the optional parameter  ,  Then specify a default value   /// {this.city = " Beijing "}  Specifies if the user does not initialize  city  Variable ,  Then initialize it  " Beijing "  A string value   ///  ///  Initialization list  :  The content after the colon is the initialization list   ///  The parent constructor is also an initialization list   ///  In addition to the parent class constructor  ,  You can also initialize sample variables before subclass constructor body   ///  Different initialization instance variables are separated by commas   ///  ///  Parent class constructor  :  If the parent class has no default constructor  ( Nonparametric construction method ) ,  ///  The parent constructor must be called in the initialization list  , super(name, age) ;  ///  ///  Construct method body  :  It can be omitted  ;  Student(this._grade, String name, int age,         { 
    this.school, this.city = " Beijing "})         : address = " Haidian District, Beijing " ,         super(name, age);  //  Name the construction method   //  Define format  :  Class name . Method name ()  //  Parent constructor  :  If the parent class does not have a default constructor ,  The subclass must call the constructor of the parent class   Student.cover(Student student):super(student.name, student.age);  //  A named constructor can also have a method body   Student.init(Student student):super(student.name, student.age){ 
        print(" Name the construction method  : name : ${student.name}, age : ${student.age}");  }  //  Name factory construction method  : factory  Class name . Method name   //  The named factory constructor can have a return value   //  If there is  final  Decorated member  ,  It must be initialized in the named constructor   //  But in the naming factory construction method  ,  It can be uninitialized  final  Type members   //  The named factory constructor can have a return value   factory Student.init2(){ 
        return Student(1, "Tom", 18);  }}

Test code entry : stay main.dart Medium _MyHomePageState Class build In the method ;

///  Omit other source code class _MyHomePageState extends State<MyHomePage> { 
      @override  Widget build(BuildContext context) { 
        //  Test generics     Generic generic = Generic();    generic.test();  }}

Print the results :

I/flutter (24673):  Generic testing ,  Type string ,  The obtained cache content is  TomI/flutter (24673):  Generic testing ,  Type integer ,  The obtained cache content is  18I/flutter (24673):  Generic class testing ,  Acquired  T extends Person  generic  name  Field is  Tom

5、 ... and 、 Related resources

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Reference material :

• Flutter Official website : https://flutter.dev/
• Flutter Developing documents : https://flutter.cn/docs ( Strongly recommend )
• official GitHub Address : https://github.com/flutter
• Flutter The Chinese community : https://flutter.cn/
• Flutter Practical tutorial : https://flutter.cn/docs/cookbook
• Flutter CodeLab : https://codelabs.flutter-io.cn/
• Dart Chinese document : https://dart.cn/
• Dart Developer website : https://api.dart.dev/
• Flutter Chinese net ( unofficial , The translation is very good ) : https://flutterchina.club/ , http://flutter.axuer.com/docs/
• Flutter Related issues : https://flutterchina.club/faq/ ( It is recommended to watch it at the introductory stage )

Blog source download :

• GitHub Address : https://github.com/han1202012/flutter_app_hello ( Keep updating with the progress of the blog , There may not be the source code of this blog )

• Blog source snapshot : https://download.csdn.net/download/han1202012/15463304( The source code snapshot of this blog , You can find the source code of this blog )