Question

I’d like to make a subclass of NSInputStream. Simply, I tried to code just like the following,

@interface SeekableInputStream : NSInputStream
{
    NSUInteger startOffset;
    NSUInteger totalReadLen;
}

- (NSInteger)read:(uint8_t *)buffer maxLength:(NSUInteger)len;
- (BOOL)getBuffer:(uint8_t **)buffer length:(NSUInteger *)len;
- (BOOL)hasBytesAvailable;
- (void)open:(NSUInteger)offset;

@end

and, I used the class like the following.

SeekableInputStream *stm = [[SeekableInputStream alloc] initWithURL:url];

Then, in runtime, I could meet the following error message.

-[SeekableInputStream initWithURL:]: unrecognized selector sent to instance 0x10018ff30

I didn’t override initWithURL for using parent’s method purposely.
Based on my knowledge, derived class can uses method of parent class, isn’t it?

Can not an extension method like initWithURL be inherited?

Is there anybody inform me how to do subclassing of NSInputStream?

Answer 1

From NSStream.h

// NSStream is an abstract class encapsulating the common API to NSInputStream and NSOutputStream.
// Subclassers of NSInputStream and NSOutputStream must also implement these methods.
@interface NSStream : NSObject
- (void)open;
- (void)close;

- (id <NSStreamDelegate>)delegate;
- (void)setDelegate:(id <NSStreamDelegate>)delegate;
 // By default, a stream is its own delegate, and subclassers of NSInputStream and NSOutputStream must maintain this contract. [someStream setDelegate:nil] must restore this behavior. As usual, delegates are not retained.

- (id)propertyForKey:(NSString *)key;
- (BOOL)setProperty:(id)property forKey:(NSString *)key;

- (void)scheduleInRunLoop:(NSRunLoop *)aRunLoop forMode:(NSString *)mode;
- (void)removeFromRunLoop:(NSRunLoop *)aRunLoop forMode:(NSString *)mode;

- (NSStreamStatus)streamStatus;
- (NSError *)streamError;
@end

// NSInputStream is an abstract class representing the base functionality of a read stream.
// Subclassers are required to implement these methods.
@interface NSInputStream : NSStream
- (NSInteger)read:(uint8_t *)buffer maxLength:(NSUInteger)len;
// reads up to length bytes into the supplied buffer, which must be at least of size len. Returns the actual number of bytes read.

- (BOOL)getBuffer:(uint8_t **)buffer length:(NSUInteger *)len;
// returns in O(1) a pointer to the buffer in 'buffer' and by reference in 'len' how many bytes are available. This buffer is only valid until the next stream operation. Subclassers may return NO for this if it is not appropriate for the stream type. This may return NO if the buffer is not available.

- (BOOL)hasBytesAvailable;
// returns YES if the stream has bytes available or if it impossible to tell without actually doing the read.
@end

As you can see, there are no initWithURL function. So, your super do not work, because it really don’t exist. Like MrTJ says, it is a category class. It is defined in:

// The NSInputStreamExtensions category contains additional initializers and convenience routines for dealing with NSInputStreams.
@interface NSInputStream (NSInputStreamExtensions)
- (id)initWithURL:(NSURL *)url NS_AVAILABLE(10_6, 4_0);

So, I think if you use it in your subclass, it can work.

#import <Foundation/NSStream.h>

You need to import the category. Remember you CAN’T subclass a category, just overwrite it and can’t call then (or if can, I don’t know how)