How to build my cells inside a thread from a delegate? - ios

Question

I have this tableView and I want to build the cells in a custom way.
The way I do it is by adding subviews to the cell.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *cellIdentifier = #"cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
if(cell == nil)
{
cell=[[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];
}
else
{
UIImageView* img=[UIImage imageNamed:#"abc.png"];
[cell addSubview:img];
//processing the final cell takes time!
}
return cell;
}
But, since some jittery frame rate is affecting the responsiveness of the table, I'd like to pass all this creation of the cell into a thread. so I want to put an image of some sort, for the meantime, and once finished, the thread would update cell to be the final cell.
Is this a normal approach?
If so, how do I update the cell from the thread?
Do I need to define all variables as __block? before going into the thread?

Answer 1

since some jittery frame rate is affecting the responsiveness of the table, I'd like to pass all this creation of the cell into a thread.
The vast majority of UIKit, which includes UITableViewCell, isn't thread safe. From Apple's documentation:
For the most part, UIKit classes should be used only from an application’s main thread. This is particularly true for classes derived from UIResponder or that involve manipulating your application’s user interface in any way.
...only a few drawing and graphics context methods in UIKit are thread safe. It's a really bad idea to create your cells on a secondary thread. You don't want to be doing this.
There's a lot of material out there are ways to efficiently create and draw cells into table views - as someone has pointed out in the comments, you seem to be adding a UIImageView not at cell creation, but on re-use. This means your cells are going to have many, many image views being constantly added to them. This is obviously not a good idea.
A better idea is to create a UITableViewCell subclass that already contains the views you're going to need, or to add them at creation and access them via view tags.