Question

I am working on displaying several Images in a scrollview loaded from a MapServer which returns an image of showing a map.
So what I did is I have created 4 UIImageViews, put them in a NSMutableDictionary.
Then when scrolling to the desirable Image it will initiate to load the data from the url asynchronous.
so first I display an UIActivityIndicatorView, then it will load the data and at the end hide the UIActivityIndicatorView and display the UIImageView.

everything is working more or less fine, apart that it takes sooo long until the image displays, allthough the image is not that big and I have a log text indicating that the end of the function arrived… this log message appears immediately but the image still does not show…
If I call the URL by the Webbrowser the image is shown immediately as well.

below you see my piece of code.

- (void) loadSRGImage:(int) page {

UIImageView *currentSRGMap   = (UIImageView *)[srgMaps objectForKey:[NSString stringWithFormat:@"image_%i", page]];
UIActivityIndicatorView *currentLoading  = (UIActivityIndicatorView *)[srgMaps objectForKey:[NSString stringWithFormat:@"loading_%i", page]];


// if the image has been loaded already, do not load again
if ( currentSRGMap.image != nil ) return;

if ( page > 1 ) {

    MKCoordinateSpan currentSpan;
    currentSpan.latitudeDelta   = [[[srgMaps objectForKey:[NSString stringWithFormat:@"span_%i", page]] objectForKey:@"lat"] floatValue];
    currentSpan.longitudeDelta  = [[[srgMaps objectForKey:[NSString stringWithFormat:@"span_%i", page]] objectForKey:@"lon"] floatValue];

    region.span         = currentSpan;
    region.center       = mapV.region.center;
    [mapV setRegion:region animated:TRUE];
    //[mapV regionThatFits:region];
}
srgLegende.hidden = NO;

currentSRGMap.hidden = YES;
currentLoading.hidden = NO;
[currentLoading startAnimating];

NSOperationQueue *queue = [NSOperationQueue new];
NSInvocationOperation *operation = [[NSInvocationOperation alloc] 
                                    initWithTarget:self
                                    selector:@selector(loadImage:) 
                                    object:[NSString stringWithFormat:@"%i", page]];

[queue addOperation:operation]; 
[operation release];

}

- (void) loadImage:(NSInvocationOperation *) operation {

NSString *imgStr = [@"image_" stringByAppendingString:(NSString *)operation];
NSString *loadStr = [@"loading_" stringByAppendingString:(NSString *)operation];

WGS84ToCH1903 *converter = [[WGS84ToCH1903 alloc] init];

CLLocationCoordinate2D coord1   = [mapV convertPoint:mapV.bounds.origin toCoordinateFromView:mapV];
CLLocationCoordinate2D coord2   = [mapV convertPoint:CGPointMake(mapV.bounds.size.width, mapV.bounds.size.height) toCoordinateFromView:mapV];
int x1 = [converter WGStoCHx:coord1.longitude withLat:coord1.latitude];
int y1 = [converter WGStoCHy:coord1.longitude withLat:coord1.latitude];
int x2 = [converter WGStoCHx:coord2.longitude withLat:coord2.latitude];
int y2 = [converter WGStoCHy:coord2.longitude withLat:coord2.latitude];

NSString *URL = [NSString stringWithFormat:@"http://map.ssatr.ch/mapserv?mode=map&map=import/dab/maps/dab_online.map&mapext=%i+%i+%i+%i&mapsize=320+372&layers=DAB_Radio_Top_Two", y1, x1, y2, x2];

NSData* imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:URL]];
UIImage* image = [[UIImage alloc] initWithData:imageData];
[imageData release];

UIImageView *currentSRGMap   = (UIImageView *)[srgMaps objectForKey:imgStr];
UIActivityIndicatorView *currentLoading  = (UIActivityIndicatorView *)[srgMaps objectForKey:loadStr];

currentSRGMap.hidden = NO;
currentLoading.hidden = YES;
[currentLoading stopAnimating];

[currentSRGMap setImage:image]; //UIImageView
[image release];

NSLog(@"finished loading image: %@", URL);

}

Answer 1

I had a similar thing in my app, and i used the SDWebImage from https://github.com/rs/SDWebImage. This has a category written over UIImageView. You need to mention a placeholder image and a url to the UIImageView. It will display the image once it is downloaded and also maintains a cache of the downloaded images, hence avoiding multiple server calls.