单例，确保程序运行中该类只有一个实例，常用于资源共享。例如：[UIApplication sharedApplication];
OC写法：
#import <Foundation/Foundation.h>
@interface Review_SingleInstance : NSObject
+(Review_SingleInstance *)shareInstance;
@end
#import "Review_SingleInstance.h"
@implementation Review_SingleInstance
static Review_SingleInstance *_instance = nil;
+(Review_SingleInstance *)shareInstance{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [[[self class] alloc] init];
});
return _instance;
}
//为了防止使用alloc、new重复创建对象，需要添加下面方法
+(instancetype)allocWithZone:(struct _NSZone *)zone{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [super allocWithZone:zone];
});
return _instance;
}
调用：
Review_SingleInstance *instance1 = [Review_SingleInstance shareInstance];
NSLog(@"%@",instance1);
Swift写法：
import UIKit
class RISDesignPattern:NSObject{
// MARK: - 单例
static let instance : RISDesignPattern = RISDesignPattern()
private override init() {
}
}
调用：
// MARK: - 实例化设计模式 - 单例
let designPattern = RISDesignPattern.instance
关于单例类属性的问题
单例类中属性赋值 - 需要创建获取空间的属性，要放在shareInstance中，才不会重复创建，因为init方法会被重新执行
