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
如何在飞镖扑动中创建类的单例实例
class ContactBook {
ContactBook._sharedInstance();
static final ContactBook _shared = ContactBook._sharedInstance();
factory ContactBook() => _shared;
}
其他回答
这是我做单例的方式,接受参数(你可以直接粘贴到https://dartpad.dev/):
void main() {
Logger x = Logger('asd');
Logger y = Logger('xyz');
x.display('Hello');
y.display('Hello There');
}
class Logger{
Logger._(this.message);
final String message;
static Logger _instance = Logger._('??!?*');
factory Logger(String message){
if(_instance.message=='??!?*'){
_instance = Logger._(message);
}
return _instance;
}
void display(String prefix){
print(prefix+' '+message);
}
}
输入:
Hello asd
Hello There asd
“? ? ! ?*'你看到的只是一个工作区,我做了初始化_instance变量暂时没有使它成为一个Logger?类型(空安全)。
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();
}
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}
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。
如何在飞镖扑动中创建类的单例实例
class ContactBook {
ContactBook._sharedInstance();
static final ContactBook _shared = ContactBook._sharedInstance();
factory ContactBook() => _shared;
}