OC知识点--完整的单例

完整的单例方法，保证init、alloc、copy、mutableCopy时，都能保证只创建唯一单例

//.h文件
#import <Foundation/Foundation.h>
@interface Singleton : NSObject
//单例方法
+(instancetype)sharedSingleton;
@end
//.m文件
#import "Singleton.h"
@implementation Singleton
//全局变量
static id _instance = nil;
//单例方法
+(instancetype)sharedSingleton{
    return [[self alloc] init];
}
////alloc会调用allocWithZone:
+(instancetype)allocWithZone:(struct _NSZone *)zone{
    //只进行一次
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}
//初始化方法
- (instancetype)init{
    // 只进行一次
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super init];
    });
    return _instance;
}
//copy在底层 会调用copyWithZone:
- (id)copyWithZone:(NSZone *)zone{
    return  _instance;
}
+ (id)copyWithZone:(struct _NSZone *)zone{
    return  _instance;
}
+ (id)mutableCopyWithZone:(struct _NSZone *)zone{
    return _instance;
}
- (id)mutableCopyWithZone:(NSZone *)zone{
    return _instance;
}