iOS单例写法

#import <Foundation/Foundation.h>

@interface Singleton : NSObject<NSCopying,NSMutableCopying>
+ (instancetype)shareInstance;
@end

#import "Singleton.h"

@implementation Singleton

static Singleton *instance = nil;

/*
//不使用GCD的写法
+ (instancetype)shareInstance {
    
    if (instance == nil) {
        instance = [[super allocWithZone:NULL] init];
        NSLog(@"执行一次");
    }
    
    return instance;
}
*/


//使用GCD
+ (instancetype)shareInstance {
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[super allocWithZone:NULL] init];
    });
    
    return instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    return [Singleton shareInstance];
}

- (instancetype)copyWithZone:(nullable NSZone *)zone {
    return [Singleton shareInstance];
}

- (instancetype)mutableCopyWithZone:(nullable NSZone *)zone {
    return [Singleton shareInstance];
}
@end

参考链接:IOS单例的写法