単例の完全な書き方

#import 

@interface SUNTool : NSObject
+(instancetype)shareTool;
@end

#import "SUNTool.h"
@implementation SUNTool

//0.      
static SUNTool *_instance;

//1.alloc-->allocWithZone
+(instancetype)allocWithZone:(struct _NSZone *)zone
{
    //               
//    @synchronized(self) {
//        if (_instance == nil) {
//            _instance = [super allocWithZone:zone];
//        }
//    }
    
    //         
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}

//2.     
+(instancetype)shareTool
{
    return [[self alloc]init];
}

//3.  
-(id)copyWithZone:(NSZone *)zone
{
    return _instance;
}

-(id)mutableCopyWithZone:(NSZone *)zone
{
    return _instance;
}