单例

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface ImagePlay : NSObject<NSCopying>
+ (instancetype)sharedClass;
-(void)test;
@end

NS_ASSUME_NONNULL_END

#import "ImagePlay.h"
static ImagePlay *defaultImagePly = nil;
@implementation ImagePlay

+ (instancetype)sharedClass {

//第一步：静态实例，并初始化

? //??? static CDSharedClass *defaultShared = nil;
??? //第二部：只分配一次内存
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
??? defaultImagePly = [[[self class]alloc]init];
});

return defaultImagePly;
}

//第三部：重载allocWithZone方法
+ (instancetype)allocWithZone:(struct _NSZone *)zone {

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
??? defaultImagePly = [super allocWithZone:zone];
});

return defaultImagePly;
}

//第四部：重写copyWithZone
- (id)copyWithZone:(NSZone *)zone {
return defaultImagePly;
}

-(void)test{
??? NSLog(@"666");
}
@end