How to add auto layout on UICollectionView programmatically? - ios

Question

I create a uicollectionview programmatically, and I want to add auto layout constraint on the collection view so that I can change the frame later. But the following code is not work.
I am trying to initialize the collectionview like this:
self.rootCollectionView = [[UICollectionView alloc]initWithFrame:CGRectMake(0, 100, self.bounds.size.width, self.bounds.size.height - 100) collectionViewLayout:[[UICollectionViewFlowLayout alloc]init]];
self.rootCollectionView.dataSource = self;
self.rootCollectionView.delegate = self;
self.rootCollectionView.backgroundColor = [UIColor clearColor];
[self addSubview:self.rootCollectionView];
[self.rootCollectionView registerClass:[CASlideSwitchViewCell class] forCellWithReuseIdentifier:#"CASlideSwitchViewCell"];
self.topConstraint = 100;
float topCon = self.topConstraint;//self.topConstraint is a CGFloat property,
UICollectionView * cv = self.rootCollectionView;
cv.translatesAutoresizingMaskIntoConstraints = NO;
NSArray * constraintArray = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|-0-[cv]-0-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(cv)];
NSArray *constraintArray2 = [NSLayoutConstraint constraintsWithVisualFormat:#"V:|-topCon-[cv]-0-|" options:0 metrics:#{#"topCon":#(topCon)} views:NSDictionaryOfVariableBindings(cv)];
[self addConstraints:constraintArray];
[self addConstraints:constraintArray2];
then after I receive the data, I am trying to change topConstraint based on the data. Like this:
if (number <= 1) {
self.topScrollView.hidden = YES;
self.topConstraint = 0;
}else{
self.topScrollView.hidden = NO;
self.topConstraint = 100;
}
[self updateConstraints];
[self.rootCollectionView.collectionViewLayout invalidateLayout];
[self.rootCollectionView reloadData];
However, the top margin of the UICollectionView is always 100.
Do I miss something? Or it is not right to add auto layout like that?

Answer 1

The self.topConstraint is a float not a NSLayoutConstraint so you can not update the constraint. You can instead of using
NSArray *constraintArray2 = [NSLayoutConstraint constraintsWithVisualFormat:#"V:|-topCon-[cv]-0-|" options:0 metrics:#{#"topCon":#(topCon)} views:NSDictionaryOfVariableBindings(cv)];
You should use
NSLayoutConstraint *topLayoutContraint = [NSLayoutConstraint constraintWithItem:cv
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeTop
multiplier:1.0
constant:100];
NSArray *constraintArray2 = #[
topLayoutContraint,
[NSLayoutConstraint constraintWithItem:cv
attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeBottom
multiplier:1.0
constant:0]
];
then you can
if (number <= 1) {
self.topScrollView.hidden = YES;
topLayoutContraint.constant = 0;
} else {
self.topScrollView.hidden = NO;
topLayoutContraint.constant = 100;
}
[self setNeedsLayout];
[self.rootCollectionView reloadData];