What is the correct way to "gang" together and keep in sync a pair os UIScrollViews?

Question

I would like to have a pair of scrollviews - call them scrollA and scrollB - on the screen that work in parallel. When the user scroll/zooms scrollA scrollB mimics that behavior, zooming and panning identically. And vice versa.

Is this possible? Thanks in advance.

Cheers, Doug

Answer 1

Key value coding, if you don't know that then pick up Cocoa programming for Mac OS X, and learning objective-C by stephen kochan (spelling & punctuation?)

Check the docs for a way to get the position of the scroll view (like the index of a tableview) then set up KVC. Then you'll understand how the iPod keeps your scroll position after quits.

Answer 2

I think the best way would be to set the delegate of each scrollview to your controller then implement the '- (void)scrollViewDidScroll:(UIScrollView *)scrollView' in your controller, inside the method you'll want to call 'setContentOffset:animated:' on the scrollview that didn't scroll, to get the correct contentOffset you can use the UIScrollView.contentOffset property of the UIScrollView that was scrolled which will be passed to your implementation of - (void)scrollViewDidScroll:(UIScrollView *)scrollView in your controller

Answer 3

As per JoePasq's answer, I would use KVO for this and register observers for whichever key/value pairs you'd like to mimic in each UIScrollView. It would look something like this (untested):

// Do this during initialisation of scrollView2
[scrollView1 addObserver:self
              forKeyPath:@"contentOffset"
                 options:NSKeyValueObservingOptionNew
                 context:NULL];

[scrollView1 addObserver:self
              forKeyPath:@"zoomScale"
                 options:NSKeyValueObservingOptionNew
                 context:NULL];

// Implement this method on scrollView2
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {

  [self setValue:[change valueForKey:NSKeyValueChangeNewKey] forKey:keyPath];
}

Answer 4

Does this work for the flick gesture as well?

From my experiences, the contentOffset is updated after the flick gesture deceleration is done.. so even with KVO it will still be observing old contentOffsets

am i right?