Objective-C setContentOffset方法

Question

I am trying to convert code written in objective-c into Swift language. In following lines:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

if (scrollView.contentOffset.y < self.mapView.frame.size.height*-1 ) {
    [scrollView setContentOffset:CGPointMake(scrollView.contentOffset.x,   self.mapView.frame.size.height*-1)];

}
}

I encountered this parameter which should be, due to documentation, evaluated to boolean type

self.mapView.frame.size.height*-1

Can anybody tell me what's going on here and how this expression is evaluated to boolean? the pointer is the thing that unables me to convert it into Swift, could you help how to do so?

Thanks in advance

Answer 1

The reason for this error is because of no space between operator and object for an expression.

On compilation time compiler breaks this lines self.mapView.frame.size.height* -1 . And what it gets self.mapView.frame.size.height* and -1 . So compiler mis understand as two separate parameters here.

A space between self.mapView.frame.size.height and * required to tell the compiler that this is an expression to multiply self.mapView.frame.size.height with -1 value.

So your relevant Swift code will be as below:

func scrollViewDidScroll(scrollView: UIScrollView) {
    if (scrollView.contentOffset.y < self.mapView.frame.size.height * -1 ) {
        scrollView .setContentOffset(CGPointMake(scrollView.contentOffset.x, self.mapView.frame.size.height * -1), animated: true)
    }
}

Answer 2

self.mapView.frame.size.height * -1

这是一个乘法。