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;
}



Question: What happens if ~3.3v is connected to a GPIO set as output?
Answer: Possibly.

If the gpio is at logic 1 it is already at 3V3 so there will be no difference.

If the gpio is at logic 0 it is at ground and connecting 3V3 will in effect be a short-circuit.  That can kill the gpio and or bank of gpios and or the Pi (unless an appropriate current limiting resistor is in place).