where to create autolayout constraints for subclas

Question

I am trying to use autolayout for a uitableviewcell subclass I am creating and I would appreciate some advice on what method to put the layout constraint creation code. I have been searching around and all the information I find talks about adding constraints after adding subviews in the viewDidLoad method. As far as I know viewDidLoad isn't an option for a uitableviewcell subclass.

I am using interface builder to create a custom cell and dynamically allocating it in my code. Nothing special there... I subclass the uitableviewcell so I can add a custom uiview to the cell. Again, nothing particularly Earth shattering... My difficulty comes when I try to position my custom uiview with respect to a label I added to the cell in interface builder.

This is the code to create the custom uiview and add it to the cell's content view:

- (id)initWithCoder:(NSCoder *)decoder
{
    if ((self = [super initWithCoder:decoder]))
    {
        [self initSelf];
    }
    return self;
}

- (id)initWithStyle:(UITableViewCellStyle)style reuseIdentifier:(NSString *)reuseIdentifier
{
    if ((self = [super initWithStyle:style reuseIdentifier:reuseIdentifier]))
    {
        [self initSelf];
    }
    return self;
}

- (void) initSelf
{
    // Initialization code
    _badgeLabel = @"";

    if (!_customBadge)
    {
        _customBadge = [CustomBadge customBadgeWithString:self.badgeLabel];
    }

    // hide the badge until needed
    self.customBadge.hidden = YES;

    // add badge to the cell view hierarchy
    [self.customBadge setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.customBadge setContentHuggingPriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisHorizontal];
    [self.customBadge setContentHuggingPriority:UILayoutPriorityDefaultHigh forAxis:UILayoutConstraintAxisVertical];

    [self.contentView addSubview:self.customBadge];
}

If I put the constraint code at the end of initSelf nothing happens. The position of my _customBadge stays at its' default. When I put the constraint code in layoutSubviews the positioning is applied; but I am pretty sure it is the wrong place. Here's the code:

- (void) layoutSubviews
{
    [self.contentView addConstraint:[NSLayoutConstraint
                                     constraintWithItem:self.customBadge
                                     attribute:NSLayoutAttributeLeft
                                     relatedBy:NSLayoutRelationEqual
                                     toItem:self.competence
                                     attribute:NSLayoutAttributeRight
                                     multiplier:1.0
                                     constant:-14.0]];

    [self.contentView addConstraint:[NSLayoutConstraint
                                     constraintWithItem:self.customBadge
                                     attribute:NSLayoutAttributeTop
                                     relatedBy:NSLayoutRelationEqual
                                     toItem:self.competence
                                     attribute:NSLayoutAttributeTop
                                     multiplier:1.0
                                     constant:0.0]];

    [super layoutSubviews];
}

Can anyone tell me where this code should go? Surely I am creating duplicate constraints every time a layout happens.

Thanks

Answer 1

You would need to update constraints like:

-(void)updateConstraints{
 // add your constraints if not already there
 [super updateConstraints];
}

After adding your views to superview you need to call [self setNeedsUpdateConstraints] to start using them. By doing so the rendering runtime will call updateConstraints at the right time.