单例

#import <Foundation/Foundation.h>
@class SingleClass;

static SingleClass *instance = nil;

@interface SingleClass : NSObject <NSCopying>
+ (instancetype)shareInstance;
@end

@implementation SingleClass
//类方法，获取单例对象
+ (instancetype)shareInstance{
    if (!instance) {
        instance = [[self alloc] init];
    }
    return instance;
}
//类方法，调用对象的alloc方法时返回已存在的单例
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
    if (!instance) {
        instance = [super allocWithZone:zone];
    }
    return instance;
}
//copy时，返回已存在的单例
- (id)copyWithZone:(NSZone *)zone{
    return instance;
}
//copy时，返回已存在的单例
- (id)copy{
    return instance;
}
@end