ios - 如何子类化NSMutableData

Question

我正在尝试对NSMutableData进行子类化，以在不复制的情况下为子数据添加功能。这是代码

@interface myMutableData : NSMutableData

- (NSData *)subdataWithNoCopyingAtRange:(NSRange)range;

@end

@interface myMutableData()

@property (nonatomic, strong) NSData *parent;

@end

@implementation myMutableData

- (NSData *)subdataWithNoCopyingAtRange:(NSRange)range
{
    unsigned char *dataPtr = (unsigned char *)[self bytes] + range.location;

    myMutableData *data = [[myMutableData alloc]     initWithBytesNoCopy:dataPtr length:range.length freeWhenDone:NO];

    data.parent = self;

    return data;
}

@end

但是问题是当我尝试实例化myMutableData时，出现了此错误

"-initWithCapacity: only defined for abstract class.  Define -[myMutableData initWithCapacity:]!'"

为什么？那么继承不起作用？谢谢

Answer 1

这需要一个类别。但是，类别默认情况下不能具有属性和实例变量。因此，您需要#import <objc/runtime.h>并使用关联的对象来获取和设置parent的值。

@interface NSMutableData(myMutableData)

- (NSData *)subdataWithNoCopyingAtRange:(NSRange)range;

@property (nonatomic, strong) NSData *parent;

@end

@implementation NSMutableData(myMutableData)

- (NSData *)subdataWithNoCopyingAtRange:(NSRange)range
{
    unsigned char *dataPtr = (unsigned char *)[self bytes] + range.location;

    NSMutableData *data = [[NSMutableData alloc]     initWithBytesNoCopy:dataPtr length:range.length freeWhenDone:NO];
    data.parent = self;
    return data;
}

-(NSData*)parent
{
    return objc_getAssociatedObject(self, @selector(parent));
}

-(void)setParent:(NSData *)parent
{
    objc_setAssociatedObject(self, @selector(parent), parent, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end