NSData的文件路径

Question

I have NSData that I'm loading from a filePath. The data is then getting passed around and I need the filePath for where the data came from.

What's the easiest way to do this short of passing the filePath around as an extra parameter?

Answer 1

Wrap both in a container object that represents the file, eg

@interface File : NSObject

@property (strong) NSData *fileContents;
@property (copy) NSString *filePath;

// perhaps, even a nice constructor to fill these properties
- (id)initWithFilePath:(NSString *)filePath;

@end

Answer 2

While you should probably create a separate struct or class that contains the data you need, you can also add an association to NSData.

You can even do it as a category on NSData for convenience... assuming you are using a string... use NSURL if using a URL... that way, none of your other code needs to change... you can still use your NSData as before, with the newly added property.

NOTE: There are lots of reasons to use or not use categories. I assume you will make the choice best for your code situation, and defer the wars over good/bad use of categories to others.

Try something like this...

NSData+AssociatedFilePath.h

#import <Foundation/Foundation.h>

@interface NSData (AssociatedFilePath)
@property (nonatomic, strong) NSString *filePath;
@end

NSData+AssociatedFilePath.m

#import <objc/runtime.h>

static char kFilePath;

@implementation NSData (AssociatedFilePath)

- (void)setFilePath:(NSString*)filePath
{
    objc_setAssociatedObject(self, &kFilePath, filePath, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSString*)filePath
{
  return objc_getAssociatedObject(self, &kFilePath);
}

@end

Now, in your code, you can do this...

NSData *data = // whatever you do to create the data object..
data.filePath = someFilePath;

Whenever you want the file path of a NSData object...

NSString *filePath = myNSDataObject.filePath;

Since it is a category, you can use this on any NSData object, and if a file path was never set, it will just return nil.

This is a general solution for any time you need to add behavior to an existing class (of course, following appropriate practices for when to best use categories).