Mutate subviews async - ios

Question

I have a problem with adding and removing views in background.
I tried [self performSelectorInBackground:#selector(loadDataInScrollView) withObject:nil];
And
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
//Background Thread
dispatch_async(dispatch_get_main_queue(), ^(void){
//Run UI Updates
});
});
And everytime i get Collection <CALayerArray: 0x165a7e50> was mutated while being enumerated.'
Code:
- (void)loadDataInScrollView {
int thresholdStart = currentPage * numberOfElementsPerLine - (numberOfPreloadedPages / 2) * numberOfElementsPerLine;
int start = thresholdStart > 0 ? thresholdStart : 0;
int thresholdEnd = currentPage * numberOfElementsPerLine + numberOfVisibleElements + (numberOfPreloadedPages / 2) * numberOfElementsPerLine;
int end = thresholdEnd > numberOfElements ? numberOfElements : thresholdEnd;
for (int i = start; i < end; i++) {
[self createElementAtIndex:i];
// [self performSelectorInBackground:#selector(createElementAtIndex:) withObject:[NSNumber numberWithInt:i]];
for (int i = TAG_CONFLICT; i < start + TAG_CONFLICT; i++) {
[self removeElementAtIndex:i];
}
for (int i = end + TAG_CONFLICT; i < numberOfElements + TAG_CONFLICT; i++) {
[self removeElementAtIndex:i];
}
}
self.isLoaded = YES;
NSLog(#"start: %d, end: %d", start, end);
}
- (void)removeElementAtIndex:(int)index {
UIView *view = [base viewWithTag:index];
if (view != nil && ![view isKindOfClass:[UIImageView class]]) {
[view removeFromSuperview];
}
}
- (void)createElementAtIndex:(int)index {
if (![base viewWithTag:index+TAG_CONFLICT]) {
int x = leftRightPadding + index % numberOfElementsPerLine * (sizeElement.width + flexibleSpace);
int y = topBottomPadding + index / numberOfElementsPerLine * (sizeElement.height + flexibleSpace);
UIView *element = [[UIView alloc] initWithFrame:CGRectMake(x, y, sizeElement.width, sizeElement.height)];
[self.dataSource gallery:self view:element atIndex:index];
if (border) {
element.layer.borderWidth = 1.0;
element.layer.borderColor = [borderColor CGColor];
}
element.layer.masksToBounds = YES;
UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:#selector(elementTapped:)];
tap.numberOfTapsRequired = 1;
[element addGestureRecognizer:tap];
[element setTag:index+TAG_CONFLICT];
[element setAlpha:0];
[base addSubview:element];
[UIView animateWithDuration:0.3 animations:^{
element.alpha = 1;
}];
}
}
#pragma mark - ScrollView delegates
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
initialOffset = scrollView.contentOffset.y;
currentPage = (initialOffset - topBottomPadding) / (sizeElement.height + topBottomPadding);
direction = initialOffset > base.contentOffset.y ? BOTTOM : TOP;
[self loadDataInScrollView]; // Here i've tried different ways to send in background
}

Answer 1

The error message tells you what the problem is and what to do. You cannot mutate a mutable array while you are enumerating that array (i.e. in the middle of a for-loop). You need to walk through the array and make a separate nonmutable list (array) of the things you want to change. After the loop is completely over, do another loop where you use that list to make the changes.
Also, you must never touch the interface in a background thread. You are calling loadDataInScrollView in the background. Thus loadDataInScrollView calls removeElementAtIndex in the background. Thus removeElementAtIndex calls removeFromSuperview in the background - that is totally illegal.
So you're going to have to completely rearchitect your approach here on those two grounds.