iPhone：在UITableView中平滑滚动

Question

I have a UITableView, and custom cells on it. On cell I have a UILabel, but before I set text to UILabel I did really hard work on text...like find the text in another text, highlight some words on it, and only then I set it to label. So when I scroll my list, it has delay because of this hard work. Any idea how to improve performance ? Maybe to do all hard work in another thread ??

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:
        (NSIndexPath *)indexPath {
static NSString *customCellIdentifier =
    @"CellIdentifier";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:
    customCellIdentifier];
if (cell == nil) {
    NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"CustomTableRow"
        owner:self options:nil];
    if (nib.count > 0) {
        cell = self.customTableRow;
    }
}

   self.myLabel.text = [self giveMeTheTextThatINeed];

return cell;
}

[self giveMeTheTextThatINeed] - did a hard work on text that takes some time.

Answer 1

Make a new thread for every cell. this thread calls [self giveMeTheTextThatINeed:indexPath], and resets the label(s) in the cell. I'm assuming you can't get your data any faster, so you want to maintain the scrolling in the table and spin the hard work out to the thread. When the thread is finished, update the cell. You see this a lot in cells with a thumbnail image where the thumbnail only gets uploaded after a while, and is blank or has a placeholder there first.

Answer 2

Any way for you to precompute the values you'll need? In other words, start doing your "hard work" (in another thread) when the app starts, and store it somewhere so that, if it's ready, you can just grab it when the table view asks for it. It's hard to answer without more detail about what the hard work is and how much data we're talking about.

I don't know about just doing the hard work on another thread as you suggested, since you still have to give something to tableView:cellForRowAtIndexPath: . I suppose you could return some kind of template cell at first, and then update it with reloadRowsAtIndexPaths:withRowAnimation: when the hard work is done.

Answer 3

I believe the method giveMeTheTextThatINeed needs to take the current cell as one of the parameters (or another parameter dependent on the cell content), eg: [self giveMeTheTextThatINeed:indexPath] . Otherwise you could store the text as an instance variable and set it in all cells from that variable.

So, with that in mind, the easiest way is to store the result of the computation in an additional dictionary, where indexPath (or the other parameter) would be the key:

self.myLabel.text = [self->myDictionary objectForKey:indexPath];

Now, you could either pre-populate that dictionary before the cells are drawn (eg in viewWillAppear), or cache them once they are calculated so that they are not recalculated when the cells are scrolled, eg:

NSString* calculatedText = [self->myDictionary objectForKey:indexPath];
if(calculatedText == nil)
{
    calculatedText = [self giveMeTheTextThatINeed:indexPath];
    [self->myDicationary setValue:calculatedText forKey:indexPath];
}
self.myLabel.text = calculatedText;