Reactive Cocoa - View visibility based on scrollview content offset

Question

I'm trying to change the visibility of a view based on the current content offset y value of a scrollview using Reactive Cocoa. My thought was to create an observer on an NSNumber property that would get changed in the scrollview delegate's scrollViewDidScroll. Here's the code that I've added to viewDidLoad:

RAC(self.headerView, hidden) = [RACObserve(self, offset) subscribeNext:^(NSNumber *value) {
    return @([value integerValue] > 0);
}];

Setting a breakpoint on the return statement, I see that this is called once, but never again. What am I missing?

Answer 1

The solution you describe, implementing a method that calls -sendNext: on a subject, is so common that there's a method that does that all for you: -rac_signalForSelector:. In the case of delegates, you can call -rac_signalForSelector: on the delegate, like so:

RAC(self.headerView, hidden) = [[(id)scrollView.delegate
    rac_signalForSelector:@selector(scrollViewDidScroll:)]
    reduceEach:^(UIScrollView *scrollView) {
        return @(scrollView.contentOffset.y > 0);
    }];

One gotcha that comes up with delegates is that you must make sure you invoke any -rac_signalForSelector: calls before assigning the delegate.