iphone - Is slow for UILabel to setText? - iphone

Question

My code is simple. I have a UIScrollView and below it there are five UILabel. What I set in the UIScrollView delegate scrollViewDidScroll is that Every 150 pixel that the scrollView moved, I update the five UILabel with NSStrings in an NSArray.
Here is the codes:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
int offsetY = scrollView.contentOffset.Y;
BOOL canUpdate = (offsetY % 150 == 0)?YES:NO;
if (canUpdate) {
int index = offsetY / 150;
for (int i = 0;i < 5; i++) {
UILabel *label = [labelArray objectAtIndex:i];
label.text = [stringArray objectAtIndex:index];
}
}
}
It is quite slow. I mean, if I scroll quite quickly (let's say it may need update the labels several times before it stops), I can see the scroll lags, just like something is blocking its moving.
Is UILabel setText really that slow??
thanks

Answer 1

No, its not slow. You should try setting your label in scrollViewDidScroll delegate method.
UPDATE:
So you mean it still doesnt work !? As far as i can guess, you are trying to check if your scroll view has scrolled 150px and then update some label.
In your code, you are basically checking for exact 150 px value, as you have used modulo (%) operation.
It will not be always that you have scrolled and the scroll view offset is exactly at 150, 300, 450, etc.

Answer 2

No, absolutely. If it was slow, you wouldn't be able to efficiently use the contact list on the iPhone.
If your code is slow, you should look for other pieces of code that are slowing the scroll process. Switching to the scrollViewDidScroll: method, as suggested by #pratikshabhisikar, could also be of help.
Try printing a log in your scrollViewDidEndDecelerating: to see how many times it's been called.

Answer 3

pratikshabhisikar has it right.
You are only updating one of your labels when your contentOffset is exactly a multiple of 150 (as an integer).
(offsetY % 150)
returns true only if offsetY is divisible by 150 and leaves no remainder. So if you are scrolling too fast, that offset could skip the 'sweet spot' (multiples of 150) and you won't get an update.
try:
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
int index = floor(scrollView.contentOffset.y / 150.0f);
for (int i = 0;i < 5; i++) {
UILabel *label = [labelArray objectAtIndex:i];
label.text = [stringArray objectAtIndex:index];
}
}
The 'floor' function ensures you will get the rounded down version as an int after the division - so anywhere from 0 - 150 will return 0 etc.
You might like to cache the last index loaded too (since you will be reloading the text for any amount of scrolling at present (i.e only enter the for loop if index != mCurrentIndex.