Question

I want to make two tables on single screen, so that if Table A is scrolled down render Table B simultaneously scrolling up. Can someone code it or provide me any easy way to do that.

Answer 1

As Wubao mentioned, you have to use the UIScrollViewDelegate. But you have to check, which scrollView is dragging / active at the moment. Because else you will have the problem that you will get delegate callbacks from both scrollviews and they will give their change to each other at the same time resulting in an endless loop / in endless scrolling.

In detail, you have to check: - (void)scrollViewDidScroll:(UIScrollView *)scrollView

But you have to remember the previous offset, so you do know the change of the value.
(Or your views have the same height. Then you could just use contentSize.height-offset as the offset for the other view.

I’ll try to write it down a little bit (untested):

@interface ViewController () <UITableViewDelegate,UIScrollViewDelegate>

// instances of your tableviews
@property (nonatomic, strong) UITableView *tableLeft;
@property (nonatomic, strong) UITableView *tableRight;

// track active table
@property (nonatomic, strong) UIScrollView* activeScrollView;

// helpers for contentoffset tracking
@property (nonatomic, assign) CGFloat lastOffsetLeft;
@property (nonatomic, assign) CGFloat lastOffsetRight;

@end


@implementation ViewController

- (void) viewDidLoad
{
  [super viewDidLoad];
  self.tableLeft.delegate  = self;
  self.tableRight.delegate = self;
}

– (void) scrollViewWillBeginDragging: (UIScrollView*) scrollView
{
   self.activeScrollView = scrollView;
   self.tableViewRight.userInterActionEnabled = (self.tableViewRight == scrollView);
   self.tableViewLeft.userInterActionEnabled  = (self.tableViewLeft  == scrollView);
}

- (void) scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
   if(!decelerate) {
     self.activeScrollView = nil;
     self.tableViewRight.userInterActionEnabled = YES;
     self.tableViewLeft.userInterActionEnabled  = YES;
   }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
     self.activeScrollView = nil;
     self.tableViewRight.userInterActionEnabled = YES;
     self.tableViewLeft.userInterActionEnabled  = YES;
}

- (void) scrollViewDidScroll:(UIScrollView *)scrollView
{
  if(self.activeScrollView == self.tableViewLeft)
  {
    CGFloat changeLeft  = self.tableViewLeft.contentOffset.y  - self.lastOffsetLeft;
    self.tableViewRight.contentOffset.y += changeLeft;
  }
  else if (self.activeScrollView == self.tableViewRight)
  {
    CGFloat changeRight = self.tableViewRight.contentOffset.y - self.lastOffsetRight;
    self.tableViewLeft.contentOffset.y += changeRight;
  }

  self.lastOffsetLeft  = self.tableViewLeft.contentOffset.y;
  self.lastOffsetRight = self.tableViewRight.contentOffset.y;
}

@end

Thats it basically. It also locks the not active scrollview. Because scrolling both would result in ugly behaviour. Also contentOffset.y += changeLeft; wont work probably. You have to create a new CGPoint/CGSize.