自动布局约束：无法同时满足约束

Question

I have an issue with adding a NSLayoutConstraint. I'd like to update the height of an UICollectionView so that all cells fit in the UICollectionView without scrolling. This is because I have put the UICollectionView in a UIScrollView, together with other UI Elements.

I have set the constraints in the interface builder, and I resize the UICollectionView on viewDidLoad, when I know how many items should be displayed. I do this with

[self allBooksCollectionViewSetConstraints];

I have set

[allBooksCollectionView setTranslatesAutoresizingMaskIntoConstraints:NO];

This is my code

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation) fromInterfaceOrientation {
    [self allBooksCollectionViewConstraints];
}

-(NSInteger)allBooksCollectionViewHeight
{
    float booksPerRow;
    if (UIDeviceOrientationIsPortrait([UIDevice currentDevice].orientation))
    {
        booksPerRow = 6.0;
    }
    if (UIDeviceOrientationIsLandscape([UIDevice currentDevice].orientation))
    {
        booksPerRow = 8.0;
    }
    //calculate right height do display all cells
    NSInteger cvHeight = (ceil((float)[allBooksCollectionView numberOfItemsInSection:0]/booksPerRow)*200.0)+49.0;
    return cvHeight;
}

-(void)allBooksCollectionViewSetConstraints
{
    NSInteger cvHeight = [self allBooksCollectionViewHeight];

    [scrollView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:[NSString stringWithFormat:@"V:[view(%d)]", cvHeight] options:0 metrics:nil views:@{@"view":allBooksCollectionView}]];
}

I have tried removing the UICollectionView constraints from the UIScrollview, but it doesn't change a thing.

[scrollView removeConstraints:allBooksCollectionView.constraints];

On orientation change I get the following error:

Unable to simultaniously satisfy constraints ... (
"<NSLayoutConstraint:0x9aa49f0 V:[UICollectionView:0xc0b6000(849)]>",
"<NSLayoutConstraint:0xb2b15d0 V:[UICollectionView:0xc0b6000(1049)]>"
)

Will attempt to recover by breaking constraint <NSLayoutConstraint:0xb2b15d0 V:[UICollectionView:0xc0b6000(1049)]>

But the other constraint needs to be broken! Not this one, because 1049 is cvHeight.

How can I fix this?

Answer 1

I have tried removing the UICollectionView constraints from the UIScrollview, but it doesn't change a thing.

[scrollView removeConstraints:allBooksCollectionView.constraints];

This line of code is wrong. None of the constraints returned from collectionView.constraints will be on the scrollview, so this call will do nothing. You should store the constraints you care about in a property or instance variable:

if (collectionViewHeightConstraints)
{
    [scrollView removeConstraints:collectionViewHeightConstraints];
}
collectionViewHeightConstraints = [NSLayoutConstraint constraintsWithVisualFormat:[NSString stringWithFormat:@"V:[view(%d)]", cvHeight] options:0 metrics:nil views:@{@"view":allBooksCollectionView}];
[scrollView addConstraints:collectionViewHeightConstraints];