Cannot add constraints with anchors even after adding subview

Question

I'm new to iOS. Here is my code causing the error.

FSImageView *imageView = [_imageViews objectAtIndex:(NSUInteger) page];
if ((NSNull *) imageView == [NSNull null]) {
        imageView = [self dequeueImageView];
        if (imageView != nil) {
            [_imageViews exchangeObjectAtIndex:(NSUInteger) imageView.tag withObjectAtIndex:(NSUInteger) page];
            imageView = [_imageViews objectAtIndex:(NSUInteger) page];
        }
    }

    if (imageView == nil || (NSNull *) imageView == [NSNull null]) {
        imageView = [[FSImageView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, _scrollView.bounds.size.width, _scrollView.bounds.size.height)];
        UIColor *backgroundColor = barsHidden ? _backgroundColorHidden : _backgroundColorVisible;
        [imageView changeBackgroundColor:backgroundColor];
        [_imageViews replaceObjectAtIndex:(NSUInteger) page withObject:imageView];
    }

    imageView.useScaleFactor = self.useScaleFactor;
    imageView.image = _imageSource[page];
if (imageView.superview == nil) {
        [_scrollView addSubview:imageView];

        if (@available(iOS 11.0, *)) {
            [imageView.topAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.topAnchor
                                                constant:0.0].active = YES;
            [imageView.bottomAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.bottomAnchor
                                                   constant:0.0].active = YES;
        } else {
            // Fallback on earlier versions
        }
    }

The exception occurs at Anchor stuffs. It says..

Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors and because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.'

Bhavin Kansagara Bhavin Kansagara · Accepted Answer · 2018-08-16T11:09:12

Looking at your code at this point

[imageView.topAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.topAnchor
                                            constant:0.0].active = YES;
[imageView.bottomAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.bottomAnchor
                                               constant:0.0].active = YES;

You are trying to activate the constraint with respect to the superView(i.e your self.view) of superView(i.e your scrollView) That's illegal

Your structure is like UIView -> ScrollView -> imageView. and your are setting anchor from your UIView, you should try setting it relative to ScrollView.

EDIT

Your should try this:

[imageView.topAnchor constraintEqualToAnchor:_scrollView.topAnchor
                                                constant:0.0].active = YES;
[imageView.bottomAnchor constraintEqualToAnchor:_scrollView.bottomAnchor
                                                   constant:0.0].active = YES;

also, set for .leftAnchor and .rightAnchor.

EDIT

[_scrolLView.topAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.topAnchor
                                                constant:0.0].active = YES;
[_scrolLView.bottomAnchor constraintEqualToAnchor:self.view.safeAreaLayoutGuide.bottomAnchor
                                                   constant:0.0].active = YES;

also, set for .leftAnchor and .rightAnchor.

Try and share the results.

Hope it helps.

Cannot add constraints with anchors even after adding subview

1 Answers