GCD on UITableView where NSURL is implemented - ios

Question

i been trying to implement a GCD on my project which shows the fetch data from an XML, even though its my first time doing it i've succesfully(maybe) implemented in on nameLabel and detailLabel which are both string, but the imageLabel(commented part of the code) doesn't give anything when i try to implement the same GCD as both strings, i dont know whats happening but when i run the project it gives an unknown exception, i would like to know if how to implement a GCD on the commented part of the code so i will be able to show the image in the imageView.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
static NSString *CellIdentifier = #"CustomCell";
dataFileHolder *currentData = [[xmlParser listPopulated] objectAtIndex:indexPath.row];
CustomCellXMLClass *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if(cell == nil) {
cell = [[CustomCellXMLClass alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:#"cell"];
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:#"CustomCellXMLSample" owner:self options:nil];
cell = [nib objectAtIndex:0];
}
myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(myQueue, ^{
NSString *nameLabel = [currentData nameOfCat];
NSString *dataToCacheLabel = [myCache objectForKey:nameLabel];
if(nameLabel != nil){
dataToCacheLabel = [NSString stringWithString:nameLabel];
if (dataToCacheLabel != nil) {
[myCache setObject:dataToCacheLabel forKey:nameLabel];
dispatch_async(dispatch_get_main_queue(), ^{
[cell.nameLabel setText:dataToCacheLabel];
});
}
}
NSString *detailLabel = [currentData descriptionOfCat];
NSString *stringToCache = [myCache objectForKey:detailLabel];
if (detailLabel != nil) {
stringToCache = [NSString stringWithString:detailLabel];
if (stringToCache != nil) {
[myCache setObject:stringToCache forKey:detailLabel];
dispatch_async(dispatch_get_main_queue(), ^{
[cell.detailLabel setText:stringToCache];
});
}
}
// NSString *imageURL = [currentData imageLink];
// NSData *dataToCache;
// if (imageURL != nil) {
//
// dataToCache = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageURL]];
// if (dataToCache != nil) {
//
// [myCache setObject:dataToCache forKey:imageURL];
// [cell.imageShow setImage:[UIImage imageWithData:dataToCache]];
//
// }
// else {
//
// NSURL *imageURL = [NSURL URLWithString:#"http://i178.photobucket.com/albums/w255/ace003_album/190579604m.jpg"];
// dataToCache = [NSData dataWithContentsOfURL:imageURL];
// [myCache setObject:dataToCache forKey:imageURL];
// [cell.imageShow setImage:[UIImage imageWithData:dataToCache]];
// }
//
// }
[self.activityIndicator performSelectorOnMainThread:#selector(stopAnimating) withObject:nil waitUntilDone:YES];
});
return cell;
}

Answer 1

You need to dispatch the parts where you update the cell back to the main thread like you are doing for the first two. This part: dispatch_async(dispatch_get_main_queue(),...) It'll probably look something like this:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"CustomCell";
static NSInteger usageCount = 0;
dataFileHolder *currentData = [[xmlParser listPopulated] objectAtIndex:indexPath.row];
CustomCellXMLClass *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if(cell == nil) {
cell = [[CustomCellXMLClass alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:#"cell"];
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:#"CustomCellXMLSample" owner:self options:nil];
cell = [nib objectAtIndex:0];
}
// Give it a new tag every time, so we can tell if this is "our" use of the cell
cell.tag = ++usageCount;
myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(myQueue, ^{
NSString *nameLabel = [currentData nameOfCat];
NSString *dataToCacheLabel = [myCache objectForKey:nameLabel];
if(nameLabel != nil){
dataToCacheLabel = [NSString stringWithString:nameLabel];
if (dataToCacheLabel != nil) {
[myCache setObject:dataToCacheLabel forKey:nameLabel];
dispatch_async(dispatch_get_main_queue(), ^{
if (cell.superview && usageCount == cell.tag)
{
[cell.nameLabel setText:dataToCacheLabel];
}
});
}
}
NSString *detailLabel = [currentData descriptionOfCat];
NSString *stringToCache = [myCache objectForKey:detailLabel];
if (detailLabel != nil) {
stringToCache = [NSString stringWithString:detailLabel];
if (stringToCache != nil) {
[myCache setObject:stringToCache forKey:detailLabel];
dispatch_async(dispatch_get_main_queue(), ^{
if (cell.superview && usageCount == cell.tag)
{
[cell.detailLabel setText:stringToCache];
}
});
}
}
NSURL *fallbackImageURL = [NSURL URLWithString:#"http://i178.photobucket.com/albums/w255/ace003_album/190579604m.jpg"];
NSString *imageURL = [currentData imageLink];
NSArray* urls = imageURL ? #[ imageURL, fallbackImageURL ] : #[ fallbackImageURL ];
for (NSURL* url in urls)
{
UIImage* cachedImage = [myCache objectForKey: url];
if (!cachedImage)
{
NSData* imageData = [NSData dataWithContentsOfURL:url];
cachedImage = [UIImage imageWithData:dataToCache];
[myCache setObject:cachedImage forKey:url];
}
if (cachedImage)
{
dispatch_async(dispatch_get_main_queue(), ^{
if (cell.superview && usageCount == cell.tag)
{
[cell.imageShow setImage: uiImage];
}
});
break;
}
}
});
return cell;
}
In the long term, you should probably also consider any of a number of asynchronous loading approaches instead of dataWithContentsOfURL: which will block the background thread waiting for the data to be fully received. But that's a second-order issue here, and not what you asked about.
EDIT: Edited for #Rob's comments. Check the cache for a pre-cached image before fetching, and guard against writing values into reused or invisible cells (assuming you're not using tag for something else.) But really, just use an asynchronous image download manager.