Redraw / setNeedsLayout of UIScrollView subview does not work

Question

I have create a simple, custom UIView subclass, LineView, which overrides drawRect: to draw a line from the top right corner to the bottom left corner.

This LineView is placed as subview within a UIScrollView using fixed constraints for its height and width and top + leading constraints for its position.

When the UIScrollView is scrolled the width constraints is updated to to change the size of the line view:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    ...
    self.lineViewWidthConstraint.constant = someFactorRelatedToTheCurrentScrollPosition;
    [self.lineView setNeedsLayout];
}

Problem: Changing the width does NOT call the drawRect: method of the LineView. Thus the line is not redrawn but simply squeezed or stretched. Even using setNeedsLayout manually does not trigger the re-draw.

Any idea why the draw method is not called and how to fix this?

Thanks @iDhaval but how could I perform drawing in layoutSubviews? — Andrei Herford, Mar 12 '19 at 11:41
Ultimately, drawing is done on layer. So you can draw your line using CAShapeLayer and UIBezierPath. Follow this : https://stackoverflow.com/questions/21311880/drawing-uibezierpath-on-code-generated-uiview — iDhaval, Mar 12 '19 at 11:50
Thanks, adding a CALayer and updating it manually works fine and solves the problem. — Andrei Herford, Mar 12 '19 at 12:41

score 0 · Answer 1 · answered Mar 12 '19 at 11:30

0

Try with layoutIfNeeded instead of setNeedsLayout. As former is synchronous(forced) call and latter is async call.

answered Mar 12 '19 at 11:30

Krishna Kumar

1,652
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17

Thanks, but the result is still the same. – Andrei Herford Mar 12 '19 at 11:37
@AndreiHerford Try using layoutIfNeeded on scrollview instance or your superview instance. – Krishna Kumar Mar 12 '19 at 11:47

Redraw / setNeedsLayout of UIScrollView subview does not work

1 Answers1