Lazy Loading UIScrollView - how to make scrolling smoother - ios

Question

I have accomplished lazy loading in a scrollView like this:
-(void)scrollViewDidScroll:(UIScrollView *)myScrollView {
int currentPage = (1 + myScrollView.contentOffset.x / kXItemSpacingIphone);
for (ItemView* itemView in [self.itemRow subviews]){
if (itemView.tag >= currentPage-2 && itemView.tag <= currentPage+2)
{
//keep it visible
if (!itemView.isLoaded) {
[itemView layoutWithData:[self.items objectAtIndex:itemView.tag-1]];
}
}
else
{
//hide it
if (itemView.isLoaded) {
[itemView unloadData];
}
}
}
}
Basically loading the view if it's +/- 2 "pages" from being on screen. This greatly reduces the amount of memory I'm using (instead of loading 20+ ItemViews at once), which is good. However, all the loading/unloading does make the scrolling a bit choppy especially on slower devices. Here is what is actually happening on the ItemView loading:
- (void)layoutWithData:(Item*)_data {
self.data = _data;
//grab the image from the bundle
UIImage *img;
NSString *filePath = [[NSBundle mainBundle] pathForResource:_data.image ofType:#"jpg"];
if(filePath.length > 0 && filePath != (id)[NSNull null]) {
img = [UIImage imageWithContentsOfFile:filePath];
}
UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
[btn setImage:img forState:UIControlStateNormal];
[btn addTarget:self action:#selector(tapDetected:) forControlEvents:UIControlEventTouchUpInside];
btn.frame = CGRectMake(0, 0, kItemPosterWidthIphone, kItemPosterHeightIphone);
[self addSubview:btn];
self.isLoaded = YES;
}
And the ItemView unloading:
- (void)unloadData{
for(UIView *subview in [self subviews]) {
[subview removeFromSuperview];
}
self.data = nil;
self.isLoaded = NO;
}
Again, what can I do to make the loading/unloading faster and therefore the UIScrollView more smooth?
Attempting async:
- (void)layoutWithData:(Item*)_data {
self.data = _data;
self.isLoaded = YES;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
UIImage *img;
NSString *filePath = [[NSBundle mainBundle] pathForResource:_data.image ofType:#"jpg"];
if(filePath.length > 0 && filePath != (id)[NSNull null]) {
img = [UIImage imageWithContentsOfFile:filePath];
}
dispatch_async(dispatch_get_main_queue(), ^{
UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
[btn setImage:img forState:UIControlStateNormal];
[btn addTarget:self action:#selector(tapDetected:) forControlEvents:UIControlEventTouchUpInside];
btn.frame = CGRectMake(0, 0, kItemPosterWidthIphone, kItemPosterHeightIphone);
self.imageView = btn;
[self addSubview:btn];
});
});
}

Answer 1

I ran into this problem a while back and I moved loading images from disk to a background thread. Try it and see if it's faster.
See here: loading images from disk in iPhone app is slow
Edit: image loading lags could also be caused by delayed processing of UIImages
See here: Setting image property of UIImageView causes major lag