UIScrollView behave strange - ios

Question

I set UIScrollView's subview's height to 768,and UIScrollView's contentSize is 1024*768 .But when I swipe up or swipe down,the subview(UIImageView) will scroll up or down a little.I don't know why.
here is my code
-(void)viewDidLoad
{
[super viewDidLoad];
// scrollView is an outlet of UIScrollView
[scrollView setBackgroundColor:[UIColor blackColor]];
[scrollView setCanCancelContentTouches:NO];
scrollView.indicatorStyle = UIScrollViewIndicatorStyleWhite;
scrollView.clipsToBounds = YES; // default is NO, we want to restrict drawing within our scrollview
scrollView.scrollEnabled = YES;
// pagingEnabled property default is NO, if set the scroller will stop or snap at each photo
// if you want free-flowing scroll, don't set this property.
scrollView.pagingEnabled = YES;
// load all the images from our bundle and add them to the scroll view
NSUInteger i;
imageCount=3;
for (i = 1; i <= imageCount; i++)
{
NSString *imageName = [NSString stringWithFormat:#"tu%d_s.png", i];
UIImage *image = [UIImage imageNamed:imageName];
if(!image)
{
NSLog(#"%# is nil",imageName);
}
UIImageView *TempImageView = [[UIImageView alloc] initWithImage:image];
// setup each frame to a default height and width, it will be properly placed when we call "updateScrollList"
CGRect rect = TempImageView.frame;
rect.size.height = 768;
rect.size.width = 1024;
TempImageView.frame = rect;
TempImageView.tag = i; // tag our images for later use when we place them in serial fashion
[scrollView addSubview:TempImageView];
NSLog(#"viewDidLoad TempImageView width %f,height %f,x %f,y %f",TempImageView.frame.size.width,TempImageView.frame.size.height,TempImageView.frame.origin.x,TempImageView.frame.origin.y);
}
[self layoutScrollImages]; // now place the photos in serial layout within the scrollview
}
- (void)layoutScrollImages
{
UIImageView *view = nil;
NSArray *subviews = [scrollView subviews];
// reposition all image subviews in a horizontal serial fashion
CGFloat curXLoc = 0;
for (view in subviews)
{
if ([view isKindOfClass:[UIImageView class]] && view.tag > 0)
{
CGRect frame = view.frame;
frame.origin = CGPointMake(curXLoc, 0);
view.frame = frame;
curXLoc += (1024);
}
NSLog(#"shang2 layoutScrollImages width %f,height %f,x %f,y %f",view.frame.size.width,view.frame.size.height,view.frame.origin.x,view.frame.origin.y);
}
// set the content size so it can be scrollable
[scrollView setContentSize:CGSizeMake((imageCount * 1024), 768)];
NSLog(#"shang2 content width %f,height %f,x %f,y %f",scrollView.contentSize.width,scrollView.contentSize.height,scrollView.frame.origin.x,scrollView.frame.origin.y);
}
here is Xcode log
2013-04-03 20:50:23.939 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
2013-04-03 20:50:23.952 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
2013-04-03 20:50:23.995 animation[1464:c07] viewDidLoad TempImageView width 1024.000000,height 768.000000,x 0.000000,y 0.000000
2013-04-03 20:50:23.995 animation[1464:c07] shang2 layoutScrollImages width 7.000000,height 5.000000,x 741.000000,y 1019.000000
2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 0.000000,y 0.000000
2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 1024.000000,y 0.000000
2013-04-03 20:50:23.996 animation[1464:c07] shang2 layoutScrollImages width 1024.000000,height 768.000000,x 2048.000000,y 0.000000
2013-04-03 20:50:23.996 animation[1464:c07] shang2 content width 3072.000000,height 768.000000,x 0.000000,y 0.000000

Answer 1

If your view controller containing the scrollview has either a UINavigationController or UITabBarController, the view will resize itself to fit the bounds which after the 44 pixels from the navigation controller would be 724, therefore the scrollview can scroll 44 pixels.