[Flutter] dart: class； abstract class； factory； Class, abstract class, factory constructor

One 、 origin

JJ Recently in use Flutter Before refactoring Native Project time , For Network Library dio When packaging, I found Dio In the instance, it is Dio Object time , I found out by accident that Dioclass Abstract class , But it can be used Dio() Constructor instantiation , Why is that ？ see Flutter Source code collection objects List, Map It is defined as abstract class shock !!! why?

From the factory Factory constructor .

Two 、 About dart in class Statement of

1. Take up a

Example adoption Official documents And document description , Due to documents and blog A number of , No more details here .

class Logger {
  final String name;
  bool mute = false;
  static final Map<String, Logger> _cache = <String, Logger>{};

  factory Logger(String name) {
    return _cache.putIfAbsent(name, () => Logger._internal(name));
  }

  factory Logger.fromJson(Map<String, Object> json) {
    return Logger(json['name'].toString());
  }

  Logger._internal(this.name);

  void log(String msg) {
    if (!mute) print(msg);
  }
}

The above code appears factory It will be described below

3、 ... and 、abstract class abstract class

3.1 dart Abstract classes in cannot be instantiated ？

The answer is yes , Like other languages, abstract classes cannot be instantiated .

How to explain the following code ？

 Map<String, dynamic> map = Map();

Track down Map The definition of

// from: DartPackage/sky_engine/core/main.dart
...
/// Creates an empty [LinkedHashMap].
abstract class Map<K, V> {
  external factory Map();
  ...
}

• Map The modifier defined is abstract, Why can we instantiate through constructor ？
• and Map Similar to that List, Here to Map For example
• Here external and facotry What do you mean , Is it their influence ？ Follow this clue to find out .

3.2 warren

3.2.1 external

9.4 External Functions externalFunctions
An external function is a function whose body is provided separately from its
declaration. An external function may be a top-level function (19), a method
(10.2, 10.8), a getter (10.3), a setter (10.4), or a non-redirecting constructor
(10.7.1, 10.7.2). External functions are introduced via the built-in identifier
external (17.38) followed by the function signature.
Document index

JJ The understanding of the :external: Compile identity , The main function of keywords is ： Separate declaration from implementation .external Function may be a top-level function , Or the method , Or a getter, One setter Or, Non redirected constructor . Balabala so much , Indicates that the keyword Is used to define Statement Of . Later, it mainly explains , It's for solving dart.vm It is introduced in the different implementation of browser, client and server platforms . This is nonsense again , Um. , Let's follow this clue to find this Map Of body( Realization )！

3.2.1 body

According to the clue , We know ,Map() Created a /// Creates an empty [LinkedHashMap]. And then we start with
LinkedHashMap Starting with , There are also documents about dart.vm To the , So I can only go and have a look sdk 了 .IDE open sdk Global search .

Fortunately, the search results are not many , Just look at it one by one , Go to the first file first . Limited space , Only snippets of code are intercepted ,

@patch
class LinkedHashMap<K, V> {
  @patch
  factory LinkedHashMap(
     ...
    if (isValidKey == null) {
      if (hashCode == null) {
        if (equals == null) {
          return new _InternalLinkedHashMap<K, V>();
        }
        hashCode = _defaultHashCode;
      } else {
        if (identical(identityHashCode, hashCode) &&
            identical(identical, equals)) {
          return new _CompactLinkedIdentityHashMap<K, V>();
        }
        ...

• Here we know ,LinkedHashMap It's back _InternalLinkedHashMap and _CompactLinkedIdentityHashMap object .

3.2.2 @Path What is it ？

path The main function of is and external Associated annotations .

3.2.3 The final search scope is narrowed down to the above two objects

The catalogue is ur flutter sdk path/bin/cache/dart-sdk/lib/_internal/vm/bin/compact_hash.dart

// VM-internalized implementation of a default-constructed LinkedHashMap.
@pragma("vm:entry-point")
class _InternalLinkedHashMap<K, V> extends _HashVMBase
    with
        MapMixin<K, V>,
        _HashBase,
        _OperatorEqualsAndHashCode,
        _LinkedHashMapMixin<K, V>
    implements LinkedHashMap<K, V> {
  _InternalLinkedHashMap() {
    _index = _uninitializedIndex;
    _hashMask = _HashBase._UNINITIALIZED_HASH_MASK;
    _data = _uninitializedData;
    _usedData = 0;
    _deletedKeys = 0;
  }

• It can be explained that ： abstract In an abstract class factory The constructor returns instance example , It comes from Implementation class ( For the time being abstract Class as interface,dart It doesn't contain interface keyword ).

3.2.4 _CompactLinkedIdentityHashMap

The process of exploration is the same .

3.3 summary

3.3.1 Reason for instantiation of abstract class real constructor

Because of abstract classes factory The role of factory constructors , Returns an instance of the implementation class instance .

Official documents describe ：factory Constructor returns instance Not all instances are created from this implementation class , It may come from the cache , That is, what is shown in Chapter 2 ; It may also come from subclasses .

3.3.2 Scenarios to be studied

• factory For the creation of singleton mode .

class Person {
	  
  factory Person() => _instance;

  Person._internal();
  
  static final Person _instance = Person._internal();

}

• Dio The idea in

dio Library code Dio Design idea .

1. There are two instance object types of the factory constructor here DioForBrowser,DioForNative
2. in order to Mobile and Web side The support of uses a more ingenious Entrance judgment Please refer to the notes section for details
3. The instantiation of multi terminal support comes from this dart-clang-issue;

...
//	 Entry judgment code 
import 'entry_stub.dart'
    if (dart.library.html) 'entry/dio_for_browser.dart'
    if (dart.library.io) 'entry/dio_for_native.dart';
abstract class Dio {
  factory Dio([BaseOptions? options]) => createDio(options);
  ....
}

logs

• jeversonjee 22.07.02 3.3.4.