单例

#import "Tool.h"
static Tool * _instance = nil;
@implementation Tool
/*
 单例设计模式一般用于工具类，让整个应用只存在一个对象无论怎么创建都是一个
 一般情况下单例用share开头或者default开头
 */
+(instancetype)shareInstance
{   //alloc的底层会调用allocWithZone方法,所以写单例的时候重写allocWithZone方法即可
    Tool * t = [[self alloc]init];
    return t;
}

+(instancetype)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken,^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}
-(id)copyWithZone:(NSZone *)zone
{
    return _instance;
}
-(id)mutableCopyWithZone:(NSZone *)zone
{
    return _instance;
}
@end