在方向更改时更新UICollectionView补充视图的约束

Question

I'm using a header (supplementary view) in my UICollectionView . On the header I have label which has a certain distance from the left side. I'm doing my calculation and setting the constant of a constraint. Everything does work as expected.

If the orientation is now changed, the labels have the old position. Now I need to update the constraints for all my headers. Calling invalidateLayout doesn't update the constraints. How can I manually trigger the recalculation?

Edit:

This is how my layoutSubviews does look like, where the recalculation takes place:

public override void LayoutSubviews ()
{
    this.width = this.collectionViewSize.Width;
    this.itemWidth  = (nfloat)Math.Round(this.width / numberOfItemsInRow);

    leftSpacing.Constant =  this.itemWidth * this.referencePoint;
    if (leftSpacing.Constant == 0)
        leftSpacing.Constant = SectionHeader.MIN_SPACING;

    base.LayoutSubviews ();
}

As you can see this is not Objective-C but you should be able to see what I'm doing. I take the width of the collection view (set on instantiation of the supplementary view) and then I calculate the width of cell which corresponds nearly to the real size of the cells. With the referencePoint I determine the position. Here you can see the setup of my constraints:

public SectionHeader (CGRect frame) : base (frame)
{
    this.width = frame.Size.Width;
    this.itemWidth  = (nfloat)Math.Round(width / numberOfItemsInRow);

    label = new UILabel (){
        BackgroundColor = UIColor.White,
        TextAlignment = UITextAlignment.Left
    };

    label.TranslatesAutoresizingMaskIntoConstraints = false;
    AddSubview (label);

    NSMutableDictionary viewsDictionary = new NSMutableDictionary();
    viewsDictionary["label"] = label;

    this.AddConstraints(NSLayoutConstraint.FromVisualFormat("V:|[label]|",(NSLayoutFormatOptions)0,null,viewsDictionary));

    leftSpacing = NSLayoutConstraint.Create(label, NSLayoutAttribute.Left, NSLayoutRelation.Equal, this, NSLayoutAttribute.Left, 1, SectionHeader.MIN_SPACING);
    leftSpacing.Priority = 250;
    this.AddConstraint(leftSpacing);
    this.AddConstraint(NSLayoutConstraint.Create(label, NSLayoutAttribute.Right, NSLayoutRelation.Equal, this, NSLayoutAttribute.Right, 1, 0));
}

In this code the calculation for the distance from the left is nearly the same, except that frame has the height of the set headerReferenceSize . The width should be OK for the initialisation. I'm using the full height for the label, I set up the leading space and I pin it to the right.

Answer 1

You need to call layoutIfNeeded on the main view in the view controller so that the constraints are updated. Something like this should work:

- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
    // Update the constraints here for the new orientation - toOrientation
    // ...

    [UIView animateWithDuration:duration animations: ^{
        [self.view layoutIfNeeded];
    }];
}

This will animate the layout changes on orientation change.

You could add a breakpoint after the layoutIfNeeded call, to see that the frames are what you would expect.

If your constraints are not in the view controller, but in a UIView subclass, you can do the following:

- (void)awakeFromNib {
    [super awakeFromNib];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateIntrinsicConstraints) name:UIDeviceOrientationDidChangeNotification object:nil];
}

- (void)updateIntrinsicConstraints {
    // Update your constraints here
    [self.constraint setConstant:newValue];

    // You might need to call [self layoutIfNeeded] here, if you don't call it in another place
}
- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

Let me know if this worked for you!

---

Update

You should not change the frame of the SectionHeader manually. You should do this by implementing the protocol method:

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section;

You could call layoutIfNeeded in the init method of the SectionHeader after you add the constraints to the label, add a breakpoint after to see if the frame is correct.

Let me know if it works out for you!