将数据保存到缓存目录时出错(iPhone)

- (void)writeData{

    if(cacheFileName==nil)
        return;
    if(cacheDirectoryPath==nil)
        return;


    if (![[NSFileManager defaultManager] fileExistsAtPath:[self filePath]]) {        

        NSData *imageData = UIImageJPEGRepresentation(self.image, 0.9);

        NSError *writeError= nil;

        BOOL didWrite =  [imageData writeToFile:[self filePath] options:NSAtomicWrite error:&writeError];

        if(writeError)
            NSLog([writeError localizedDescription]);

        if(didWrite)
            NSLog(@"image saved");
        else
            NSLog(@"image not saved");


    }
}





 - (NSString *)filePath{

    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES); 
    NSString *cacheDirectory = [paths objectAtIndex:0]; 
    NSString *filename = [cacheDirectory stringByAppendingPathComponent:cacheDirectoryPath]; 
    filename = [filename stringByAppendingPathComponent:cacheFileName]; 
    NSLog(filename);

    return filename;

}

2009-06-23 16:39:19.740 XXX[33454:20b] Operation could not be completed. (Cocoa error 4.)

 NSFileNoSuchFileError = 4,                 // Attempt to do a file system operation on a non-existent file

这没有意义,因为文件不应该存在。

- (BOOL)writeToFile:(NSString *)path atomically:(BOOL)flag