iOS - 僅在修改時才下載文件（NSURL和NSData）

Question

我從服務器下載了一堆圖像文件，並且我希望確保只有在它們較新時才下載它們。 這種方法目前下載圖像就好了。但是，我不想浪費時間或精力在用戶每次登錄應用程序時重新下載圖像。相反，我只想下載任何文件A）不存在B）服務器上比在設備上更新

下面是我如何下載圖像： *圖像URL保存在覈心數據中與它關聯的視頻。 URL是使用一個簡單的換算方法，我建（generateThumbnailURL）

-(void)saveThumbnails{ 
    NSManagedObjectContext *context = [self managedObjectContextThumbnails]; 
    NSError *error; 
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; 
    NSEntityDescription *entity = [NSEntityDescription 
            entityForName:@"Videos" inManagedObjectContext:context]; 
    [fetchRequest setEntity:entity]; 
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error]; 
    NSLog(@"Videos: %i",fetchedObjects.count); 
    if (fetchedObjects.count!=0) { 
     for(Videos *currentVideo in fetchedObjects){ 
      // Get an image from the URL below 
      NSURL *thumbnailURL = [self generateThumbnailURL:[currentVideo.videoID intValue]]; 

      UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:thumbnailURL]]; 

      // Let's save the file into Document folder. 
      // You can also change this to your desktop for testing. (e.g. /Users/kiichi/Desktop/) 
      NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);//Find Application's Document Directory 
      NSString *documentsDirectory = [paths objectAtIndex:0]; 
      NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:@"DownloadedThumbnails"]; 
      //  NSString *dataPath = @"/Users/macminidemo/Desktop/gt";//DEBUG SAVING IMAGE BY SAVING TO DESKTOP FOLDER 

      //Check if Sub-directory exists, if not, try to create it 
      if (![[NSFileManager defaultManager] fileExistsAtPath:dataPath]){ 
       NSError* error; 
       if([[NSFileManager defaultManager] createDirectoryAtPath:dataPath withIntermediateDirectories:NO attributes:nil error:&error]){ 
        NSLog(@"New Folder Created!"); 
       } 
       else 
       { 
        NSLog(@"[%@] ERROR: attempting to write create new directory", [self class]); 
        NSAssert(FALSE, @"Failed to create directory maybe out of disk space?"); 
       } 
      } 
      NSArray *splitFilename = [[self generateThumbnailFilename:[currentVideo.videoID intValue]] componentsSeparatedByString:@"."];//Break Filename Extension Off (not always PNGs) 
      NSString *subString = [splitFilename objectAtIndex:0]; 
      NSString *formattedFilename = [NSString stringWithFormat:@"%@~ipad.png",subString]; 
      NSString *localFilePath = [dataPath stringByAppendingPathComponent:formattedFilename]; 
      NSData *imageData = [NSData dataWithData:UIImagePNGRepresentation(image)]; 
      [imageData writeToFile:localFilePath atomically:YES]; 
      NSLog(@"Image: %@ Saved!",formattedFilename); 
     } 
    } 
} 

Answer 1

我結束了使用這種方法來檢測該文件的修改日期： *上找到HERE

-(bool)isThumbnailModified:(NSURL *)thumbnailURL forFile:(NSString *)thumbnailFilePath{ 
    // create a HTTP request to get the file information from the web server 
    NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:thumbnailURL]; 
    [request setHTTPMethod:@"HEAD"]; 

    NSHTTPURLResponse* response; 
    [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:nil]; 

    // get the last modified info from the HTTP header 
    NSString* httpLastModified = nil; 
    if ([response respondsToSelector:@selector(allHeaderFields)]) 
    { 
     httpLastModified = [[response allHeaderFields] 
          objectForKey:@"Last-Modified"]; 
    } 

    // setup a date formatter to query the server file's modified date 
    // don't ask me about this part of the code ... it works, that's all I know :) 
    NSDateFormatter* df = [[NSDateFormatter alloc] init]; 
    df.dateFormat = @"EEE',' dd MMM yyyy HH':'mm':'ss 'GMT'"; 
    df.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"]; 
    df.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT"]; 

    // get the file attributes to retrieve the local file's modified date 
    NSFileManager *fileManager = [NSFileManager defaultManager]; 
    NSDictionary* fileAttributes = [fileManager attributesOfItemAtPath:thumbnailFilePath error:nil]; 

    // test if the server file's date is later than the local file's date 
    NSDate* serverFileDate = [df dateFromString:httpLastModified]; 
    NSDate* localFileDate = [fileAttributes fileModificationDate]; 

    NSLog(@"Local File Date: %@ Server File Date: %@",localFileDate,serverFileDate); 
    //If file doesn't exist, download it 
    if(localFileDate==nil){ 
     return YES; 
    } 
    return ([localFileDate laterDate:serverFileDate] == serverFileDate); 
} 

Answer 2

生成如果您的服務器支持HTTP緩存，您可以指定要緩存內容NSURLRequestReloadRevalidatingCacheData：

 
NSURLRequest* request = [NSURLRequest requestWithURL:thumbnailURL cachePolicy:NSURLRequestReloadRevalidatingCacheData timeoutInterval:20]; 
NSURLResponse* response; 
NSError* error; 
NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error]; 
UIImage* image = [UIImage imageWithData:data]; 

欲瞭解更多信息閱讀NSURLRequest documentation。