单例模式确保只创建一个类的实例。我如何在达特建立这个?
当前回答
这就是我如何在我的项目中实现单例
灵感来自flutter firebase => FirebaseFirestore.instance.collection('collectionName')
class FooAPI {
foo() {
// some async func to api
}
}
class SingletonService {
FooAPI _fooAPI;
static final SingletonService _instance = SingletonService._internal();
static SingletonService instance = SingletonService();
factory SingletonService() {
return _instance;
}
SingletonService._internal() {
// TODO: add init logic if needed
// FOR EXAMPLE API parameters
}
void foo() async {
await _fooAPI.foo();
}
}
void main(){
SingletonService.instance.foo();
}
来自我的项目的例子
class FirebaseLessonRepository implements LessonRepository {
FirebaseLessonRepository._internal();
static final _instance = FirebaseLessonRepository._internal();
static final instance = FirebaseLessonRepository();
factory FirebaseLessonRepository() => _instance;
var lessonsCollection = fb.firestore().collection('lessons');
// ... other code for crud etc ...
}
// then in my widgets
FirebaseLessonRepository.instance.someMethod(someParams);
其他回答
下面是一个组合了其他解决方案的简洁示例。可以通过以下方式访问单例对象:
使用指向实例的单例全局变量。 通用单例。模式实例。 使用默认构造函数,它是一个返回实例的工厂。
注意:您应该只实现三个选项中的一个,以便使用单例的代码保持一致。
Singleton get singleton => Singleton.instance;
ComplexSingleton get complexSingleton => ComplexSingleton._instance;
class Singleton {
static final Singleton instance = Singleton._private();
Singleton._private();
factory Singleton() => instance;
}
class ComplexSingleton {
static ComplexSingleton _instance;
static ComplexSingleton get instance => _instance;
static void init(arg) => _instance ??= ComplexSingleton._init(arg);
final property;
ComplexSingleton._init(this.property);
factory ComplexSingleton() => _instance;
}
如果需要进行复杂的初始化,则必须在稍后在程序中使用实例之前进行。
例子
void main() {
print(identical(singleton, Singleton.instance)); // true
print(identical(singleton, Singleton())); // true
print(complexSingleton == null); // true
ComplexSingleton.init(0);
print(complexSingleton == null); // false
print(identical(complexSingleton, ComplexSingleton())); // true
}
修改了@Seth Ladd关于谁更喜欢Swift风格的单例,比如。shared的答案:
class Auth {
// singleton
static final Auth _singleton = Auth._internal();
factory Auth() => _singleton;
Auth._internal();
static Auth get shared => _singleton;
// variables
String username;
String password;
}
示例:
Auth.shared.username = 'abc';
由const构造函数和工厂组成的Dart单例
class Singleton {
factory Singleton() =>
Singleton._internal_();
Singleton._internal_();
}
void main() {
print(new Singleton() == new Singleton());
print(identical(new Singleton() , new Singleton()));
}
你好,这样的东西怎么样?非常简单的实现,Injector本身是单例的,并且还添加了类。当然可以很容易地扩展。如果您正在寻找更复杂的东西,请检查这个包:https://pub.dartlang.org/packages/flutter_simple_dependency_injection
void main() {
Injector injector = Injector();
injector.add(() => Person('Filip'));
injector.add(() => City('New York'));
Person person = injector.get<Person>();
City city = injector.get<City>();
print(person.name);
print(city.name);
}
class Person {
String name;
Person(this.name);
}
class City {
String name;
City(this.name);
}
typedef T CreateInstanceFn<T>();
class Injector {
static final Injector _singleton = Injector._internal();
final _factories = Map<String, dynamic>();
factory Injector() {
return _singleton;
}
Injector._internal();
String _generateKey<T>(T type) {
return '${type.toString()}_instance';
}
void add<T>(CreateInstanceFn<T> createInstance) {
final typeKey = _generateKey(T);
_factories[typeKey] = createInstance();
}
T get<T>() {
final typeKey = _generateKey(T);
T instance = _factories[typeKey];
if (instance == null) {
print('Cannot find instance for type $typeKey');
}
return instance;
}
}
从Dart 2.13版本开始,就很容易用late关键字了。Late关键字允许我们延迟实例化对象。
作为一个例子,你可以看到:
class LazySingletonExample {
LazySingletonExample._() {
print('instance created.');
}
static late final LazySingletonExample instance = LazySingletonExample._();
}
注意:请记住,当你调用lazy instance field时,它只会被实例化一次。