asynchronously loaded images in uitableview disappear on scrolling up - ios

Question

i am able to fetch images asynchronously on to uitableview.i am fetching these images are from a url.on scrolling up uitableview these images disappear and they take time to load again and sometimes they dont load at all.i dont want to use any 3rd party libraries.i dont want to go with synchronous approach.please suggest any correct approach to improve performance.thanks for help in advance.my code is below:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
static NSString *identifier=#"cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:identifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:#"identifier"];
}
UIImageView *imgVw=[[UIImageView alloc]initWithFrame:CGRectMake(250, 10, 40, 30)];
[cell.contentView addSubview:imgVw];
Attributes *att = [listOfObjects objectAtIndex:indexPath.row];
strImgUrl=#"http:image url";
strImgName=att.classifiedImg;
if (strImgName == nil) {
UIImage *myImg=[UIImage imageNamed:#"user_circle.png"];
imgVw.image=myImg;
}
else{
strImg=[strImgUrl stringByAppendingString:strImgName];
}
dispatch_async(dispatch_get_global_queue(0,0), ^{
NSData *data = [[NSData alloc] initWithContentsOfURL: [NSURL URLWithString:strImg]];
if ( data == nil )
return;
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *img=[UIImage imageWithData: data];
imgVw.image=img;
});
});
return cell;
}

Answer 1

UITableViews are designed to reuse cells. When you scroll up for example the first cell might get reused to show the 5th cell since the 1st one is now off screen. When you scroll back up cellForRowAtIndexPath is called again, and you are async downloading the same image again. If you want to have it load instantly you will need to cache the images after downloading them the first time so the next time that image is needed you can directly pull it from the cache.
Many third party libraries do this (AFNetworking), but if you don't want to use them, you will have to cache the images manually.

Answer 2

It seems like your reuse code contains bug, which causes everytime to create new cells. The following code uses reuseIdentifier as #"identifier"
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:#"identifier"];
}
Change it to:
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:identifier];
}
EDIT:
On reusing tableView cells, you dont need to create and add subviews to cell each time. Instead, just create the subview when creating cell and then if cell is reusing, just get the subview using tag and update your content from datasource.
Your code can be modified like below:
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:identifier];
UIImageView *imgVw=[[UIImageView alloc]initWithFrame:CGRectMake(250, 10, 40, 30)];
imgVw.tag = indexPath.row;
[cell.contentView addSubview:imgVw];
}
UIImageView imgVw = (UIImageView*)[cell.contentView viewWithTag:indexPath.row];
//Rest is same as you posted.
Hope this will fix the issue.