How to make UIScrollView respect the layout of the containing UIView? - iphone

Question

I'm using a UIView to control the layout of my view (along with a view controller). I want UIScrollView to only use half of the vertical screen. That works fine if I use the upper half of the screen, but not the bottom half.
Here's the relevant code from the UIViewController:
- (void)loadView {
CGRect fullFrame = [[UIScreen mainScreen] applicationFrame];
//trying to put the scroll view on the bottom half of the screen, but does not work.
CGRect halfFrame = CGRectMake(0, fullFrame.size.height / 2 ,
fullFrame.size.width, fullFrame.size.height / 2);
//use this instead for the scroll view to go to the top half of the screen (and work properly)
//CGRect halfFrame = CGRectMake(0, 0 , fullFrame.size.width, fullFrame.size.height / 2);
UIScrollView* sv = [[UIScrollView alloc] initWithFrame:halfFrame];
[sv setContentSize:CGSizeMake(3 * halfFrame.size.width, halfFrame.size.height)];
CGRect stencilFrame = halfFrame;
UIView *leftView = [[UIView alloc] initWithFrame:stencilFrame];
stencilFrame.origin.x += stencilFrame.size.width;
UIView *centerView = [[UIView alloc] initWithFrame:stencilFrame];
stencilFrame.origin.x += stencilFrame.size.width;
UIView *rightView = [[UIView alloc] initWithFrame:stencilFrame];
//mix up the colors
[leftView setBackgroundColor:[UIColor redColor]];
[centerView setBackgroundColor:[UIColor greenColor]];
[rightView setBackgroundColor:[UIColor blueColor]];
//add them to the scroll view
[sv addSubview:leftView];
[sv addSubview:centerView];
[sv addSubview:rightView];
//turn on paging
[sv setPagingEnabled:YES];
UIView *containerView = [[UIView alloc]initWithFrame:fullFrame];
[containerView addSubview:sv];
[self setView:containerView];
}
Thank you in advance for any advice or help.

Answer 1

I figured it out. The crux of the problem is that views within the scroll view are initialized with the same frame as the scroll view itself. When the scrollView is initialized with halfFrame, the origin is (0, half the full screen size), which is ok since that is relative to the application window itself. However, the views that are put inside the scrollView (like leftView) are initialized to halfFrame, but in this case the origin is relative to the scrollView, effectively placing them off the screen. Setting the origin to (0,0) fixes this:
CGRect stencilFrame = CGRectMake(0, 0, fullFrame.size.width , fullFrame.size.height / 2);

Answer 2

contentSize must contain the rectangle of the view inside the scroll view. That is, the total size of all scrollable controls within. The frame of the UIScrollView decides how much scrolling is needed to let the user browse everything.

Answer 3

You don't have the "full frame" available if you have a nav bar or a tab bar. In general, code that uses [UIScreen mainScreen] for layout information is probably wrong.
Additionally, the status bar can change size if (for example) a call is in progress or tethering is enabled.
Instead, use any sane value for full frame and enable autoresizing:
CGRect fullFrame = {{0,0}, {320,480}};
...
sv.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleTopMargin;
EDIT: You also probably need to subclass UIScrollView and implement -setFrame: so that it also sets the content size and -layoutSubviews to do the correct layout.