How to properly observe the contentOffset property of my scrollView subclass?

Question

In my iOS application, I observe changes in the contentOffset property of my scrollView subclass. My observer handler looks like this:

- (void)observeContentOffsetHandler:(id)aContentOffset { NSLog(@"%@", aContentOffset); } 

I chose the parameter for the method arbitrarily as id for simplicity.

My NSLog'ging is as follows:

 -[MyScrollView observeContentOffsetHandler:] [Line 111] NSPoint: {296, 375} -[MyScrollView observeContentOffsetHandler:] [Line 111] NSPoint: {296, 389} -[MyScrollView observeContentOffsetHandler:] [Line 111] NSPoint: {295, 401} -[MyScrollView observeContentOffsetHandler:] [Line 111] NSPoint: {291, 415} 

I need to use the x and y values, but I have no idea how to get to them. I tried pouring the identifier in CGPoint, no. I tried changing the parameter to CGPoint, nope.

UPDATE

Deeper and deeper. @mgold no joy. This is how I set up the observation:

 self.contentOffsetObserver = [[[Observer alloc] initWithTarget:self action:@selector(observeContentOffsetHandler:)] autorelease]; [self.myScrollViewSubclass addObserver:self.contentOffsetObserver forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:NULL]; 

The observer is a convenient class that I use to simplify observation. Note the observer callback observContentOffsetHandler:. When I change the signature of this method to the current one:

 - (void)observeContentOffsetHandler:(id)aContentOffset 

in the @mgold CGPoint suggestion:

 - (void)observeContentOffsetHandler:(CGPoint)aContentOffset 

This is not true, as NSLog shows with all zeros for aContentOffset:

 -[MyScrollController observeContentOffsetHandler:] [Line 74] aContentOffset 0 0 -[MyScrollController observeContentOffsetHandler:] [Line 74] aContentOffset 0 0 -[MyScrollController observeContentOffsetHandler:] [Line 74] aContentOffset 0 0 -[MyScrollController observeContentOffsetHandler:] [Line 74] aContentOffset 0 0 

Not sure if my move is here.

Answer 1

Got it. Correct method signature:

 - (void)observeContentOffsetHandler:(NSValue *)aContentOffset 

CGPoint extraction is then trivial:

 CGPoint pt = [aContentOffset CGPointValue]; 

Cheers
Doug

Answer 2

Since you have a subclass of UIScrollView, you have access to layoutSubViews
It is called every time the value of contentOffset changes.

This is the “right way” to get changes as they occur. Do not use KVO. Yes contentOffset is CGPoint .... if you haven’t talked about NSScrollView ..... but even then the basic idea remains the same.

Override layoutSubviews .... remember super call

OR

register your ViewController as a scrollView delegate and implement scrollView: didScroll

Answer 3

contentoffset really CGPoint , which is a C structure with CGFloat x and y . So simple

 aContentOffset.x aContentOffset.y 

Since you are a subclass of UIScrollView , you also have a contentoffset property, simply saying.