如何子类化NSMutableData

Question

I am trying to subclass NSMutableData to add the ability to subdata without copying. Here is code

@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

But the problem is when I try to instantiate myMutableData, I got this error

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

Why? So inheritance does not work? Thanks

Answer 1

NSData and NSMutableData are part of a class cluster. That means you need to do more work when subclassing to ensure that your subclass is fully valid.

In other words, don't subclass...

It's much easier for you to do what you want using a category, a wrapper or a helper / utility class. The best option is probably a wrapper which can return either the internal data directly or a specified range of the data.

Answer 2

This calls for a category. However, a category cannot by default have properties and instance variables. Hence you need to #import <objc/runtime.h> and use associated objects to get and set value of parent .

@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