UIScrollview calling superviews layoutSubviews when scrolling?

Question

I added a UITableView as a subview to a custom UIView class I'm working on. However I noticed that whenever I scroll the table it calls my classes layoutSubviews. I'm pretty sure its the UIScrollview that the table is inheriting from which is actually doing this but wanted to know if there is a way to disable this functionality and if not why is it happening? I don't understand why when you scroll a scrollview it needs its superview to layout its subviews.

Code:

@implementation CustomView
- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        self.clipsToBounds = YES;

        UITableView *tableView = [[UITableView alloc] initWithFrame:CGRectMake(0.0, 15.0, 436.0, 132.0) style:UITableViewStylePlain];
        tableView.dataSource = self;
        tableView.delegate = self;
        tableView.separatorStyle = UITableViewCellSeparatorStyleNone;
        tableView.backgroundColor = [UIColor clearColor];
        tableView.showsVerticalScrollIndicator = NO;
        tableView.contentInset = UIEdgeInsetsMake(kRowHeight, 0.0, kRowHeight, 0.0);
        tableView.tag = componentIndex;

        [self addSubview:tableView];
        [tableView release];
    }
    return self;
}
- (void)layoutSubviews {
    // This is called everytime I scroll the tableview
}

@end

Answer 1

UITableView at least does appear to layout its superview. This behavior can be problematic when you have a layoutSubviews method that might be expensive (e.g. if you call some JavaScript).

The quick fix is add an intermediary subview that prevents the scroll view from laying out your superview. Instead, it will layout the intermediate subview.

This could be somewhat imperfect but it should work for most cases:

Assume UIView * intermediateView is defined as an instance variable:

-(id) initWithFrame:(CGRect)frame
{
    self = [super initWithFrame: frame];
    if (self)
    {
        UIScrollView * theScrollView; // = your scroll view or table view
        intermediateView = [[[UIView alloc] initWithFrame:CGRectZero] autorelease];
        // Ensures your intermediate view will resize its subviews.
        intermediateView.autoresizesSubviews = YES;

        // Ensure when the intermediate view is resized that the scroll view
        // is given identical height and width.
        theScrollView.autoresizingMask = UIViewAutoresizingFlexibleWidth |
                                         UIViewAutoresizingFlexibleHeight;
        [intermediateView addSubview: theScrollView];

        // Ensure the frame of the scroll view is exactly the bounds of your
        // intermediate view.
        theScrollView.frame = bottomContainerView.bounds;
        [self addSubview: intermediateView];
    }
    return self;
}

-(void) layoutSubviews
{
    intermediateView.frame = CGRectMake(0, 50, 42, 42); // replace with your logic
}

Answer 2

Yes, a UIScrollView does call layoutsubviews whenever it scrolls. I could've sworn this was stated in the documentation somewhere, but I guess not.

Anyways, the prevailing idea for this is that a UIScrollView should layout its stuff so that views that currently can't be seen shouldn't be laid out. As users scroll in the scroll view, it should add and remove subviews as necessary. I'm guessing this is what TableViews use to enqueue table cells that get hidden.

Is there any reason why you would care if layoutsubviews gets called or not?