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
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。
其他回答
创建单例
class PermissionSettingService {
static PermissionSettingService _singleton = PermissionSettingService._internal();
factory PermissionSettingService() {
return _singleton;
}
PermissionSettingService._internal();
}
重置单例
// add this function inside the function
void reset() {
_singleton = PermissionSettingService._internal();
}
下面是在Dart中创建单例对象的几种不同方法的比较。
1. 工厂构造函数
class SingletonOne {
SingletonOne._privateConstructor();
static final SingletonOne _instance = SingletonOne._privateConstructor();
factory SingletonOne() {
return _instance;
}
}
2. 带有getter的静态字段
class SingletonTwo {
SingletonTwo._privateConstructor();
static final SingletonTwo _instance = SingletonTwo._privateConstructor();
static SingletonTwo get instance => _instance;
}
3.静态字段
class SingletonThree {
SingletonThree._privateConstructor();
static final SingletonThree instance = SingletonThree._privateConstructor();
}
如何实例化
上面的单例是这样实例化的:
SingletonOne one = SingletonOne();
SingletonTwo two = SingletonTwo.instance;
SingletonThree three = SingletonThree.instance;
注意:
我最初是把这个作为一个问题来问的,但发现上面所有的方法都是有效的,选择在很大程度上取决于个人喜好。
这是另一种可能的方式:
void main() {
var s1 = Singleton.instance;
s1.somedata = 123;
var s2 = Singleton.instance;
print(s2.somedata); // 123
print(identical(s1, s2)); // true
print(s1 == s2); // true
//var s3 = new Singleton(); //produces a warning re missing default constructor and breaks on execution
}
class Singleton {
static final Singleton _singleton = new Singleton._internal();
Singleton._internal();
static Singleton get instance => _singleton;
var somedata;
}
使用空安全操作符和工厂构造函数可以更好地创建单例对象。
class Singleton {
static Singleton? _instance;
Singleton._internal();
factory Singleton() => _instance ??= Singleton._internal();
void someMethod() {
print("someMethod Called");
}
}
用法:
void main() {
Singleton object = Singleton();
object.someMethod(); /// Output: someMethod Called
}
注意:? ?是一个Null感知操作符,如果左边值为Null,它将返回右边的值,这意味着在我们的例子中_instance ??Singleton._internal();, Singleton._internal()将在第一次调用object时返回,rest _instance将返回。
实例化后不能更改对象的单例
class User {
final int age;
final String name;
User({
this.name,
this.age
});
static User _instance;
static User getInstance({name, age}) {
if(_instance == null) {
_instance = User(name: name, age: age);
return _instance;
}
return _instance;
}
}
print(User.getInstance(name: "baidu", age: 24).age); //24
print(User.getInstance(name: "baidu 2").name); // is not changed //baidu
print(User.getInstance()); // {name: "baidu": age 24}