viewWillAppear for subviews

I have UIScrollView with multiple UIVIew subviews. I would like to update the data that is displayed by each UIView when they appear in the visible portion of the UIScrollView.

What is the 开发者_运维问答callback that gets triggered? I tried viewWillAppear, but it does not seem to get called.

Thanks. :)

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You have to do the calculation yourself. Implement scrollViewDidScroll: in your scroll view delegate and calculate manually which views are visible (e.g. by checking if CGRectIntersectsRect(scrollView.bounds, subview.frame) returns true.

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Swift 3 solution

func scrollViewDidScroll(_ scrollView: UIScrollView) {
    let viewFrame = greenView.frame

    let container = CGRect(x: scrollView.contentOffset.x, y: scrollView.contentOffset.y, width: scrollView.frame.size.width, height: scrollView.frame.size.height)


    // We may have received messages while this tableview is offscreen
    if (viewFrame.intersects(container)) {
        // Do work here
        print("view is visible")
    }
    else{
        print("nope view is not on the screen")
    }
}

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Above answers are correct if your scrollview is not in the zoomed in state. In case if your scrollview can zoom above calculation won't work as you need to consider zoom too

here is the code

CGRect visibleRect;
               visibleRect.origin = self.mapScrollView.contentOffset;
                visibleRect.size = self.mapScrollView.bounds.size;

                float theScale = 1.0 / self.mapScrollView.zoomScale;
                visibleRect.origin.x *= theScale;
                visibleRect.origin.y *= theScale;
                visibleRect.size.width *= theScale;
                visibleRect.size.height *= theScale;
                if(CGRectIntersectsRect(visibleRect, btnPin.frame)){
                      ...
                }

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A slight refinement. I wanted to know the amount of the view that was displayed in the scrollview:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView;
{
    // Figure out how much of the self.userFeedbackView is displayed.
    CGRect frame = CGRectIntersection(self.scrollView.bounds, self.userFeedbackView.frame);
    CGFloat proportion = (frame.size.height*frame.size.width)/(self.userFeedbackView.frameWidth*self.userFeedbackView.frameHeight);
    NSLog(@"%f; %@", proportion, NSStringFromCGRect(frame));
}

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Ole Begemann's answer in swift 5,

func scrollViewDidScroll(_ scrollView: UIScrollView) {
        if scrollView.bounds.intersects(subViewFrame){
            // do something
        }
}

Note: Pls make sure that the subViewFrame is calculated with respect to the scrollView's frame.