Dynamic UICollectionView header size based on UILa

问题:

I've read a bunch of posts on adding header to UICollectionView. In an iOS 7+ app in Swift, I'm trying to add a header with a UILabel in it whose height should adjust based on the height of UILabel. The UILabel has lines = 0.

I've set up the header in IB with AutoLayout

The ViewController implements UICollectionViewDelegate, UICollectionViewDataSource. I didn't set up a custom class for the header but am using these two functions:

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
      //description is a String variable defined in the class
    let size:CGSize = (description as NSString).boundingRectWithSize(CGSizeMake(CGRectGetWidth(collectionView.bounds) - 20.0, 180.0), options: NSStringDrawingOptions.UsesLineFragmentOrigin, attributes: [NSFontAttributeName: UIFont(name: "Helvetica Neue", size: 16.0)], context: nil).size
    return CGSizeMake(CGRectGetWidth(collectionView.bounds), ceil(size.height))
}

func collectionView(collectionView: UICollectionView!, viewForSupplementaryElementOfKind kind: String!, atIndexPath indexPath: NSIndexPath!) -> UICollectionReusableView! {
    var reusableview:UICollectionReusableView = UICollectionReusableView()
    if (kind == UICollectionElementKindSectionHeader) {
                    //listCollectionView is an @IBOutlet UICollectionView defined at class level, using collectionView crashes
            reusableview = listCollectionView.dequeueReusableSupplementaryViewOfKind(UICollectionElementKindSectionHeader, withReuseIdentifier: "ListHeader", forIndexPath: indexPath) as UICollectionReusableView
            let label = reusableview.viewWithTag(200) as UILabel  //the UILabel within the header is tagged with 200
            label.text = description   //description is a String variable defined in the class
        }
    }
    return reusableview
}

The displaying of the text seems to be working but the height calculation doesn't seem to be working (see screenshot below). Also, I don't think I can access the UILabel via the collectionView...referenceSizeForHeaderInSection function either. Any suggestions on how to calculate CGSize correctly?

回答1:

This is how I did it:

let labels = [
"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc ac lorem enim. Curabitur rhoncus efficitur quam, et pretium ipsum. Nam eu magna at velit sollicitudin fringilla nec nec nisi. Quisque nec enim et ipsum feugiat pretium. Vestibulum hendrerit arcu ut ipsum gravida, ut tincidunt justo pellentesque. Etiam lacus ligula, aliquet at lorem vel, ullamcorper commodo turpis. Nullam commodo sollicitudin mauris eu faucibus.",
"Lorem ipsum dolor",
"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc ac lorem enim. Curabitur rhoncus efficitur quam, et pretium ipsum. Nam eu magna at velit sollicitudin fringilla nec nec nisi. Quisque nec enim et ipsum feugiat pretium."]

The basic idea is to create an identical UILabel to the one that will be shown in the section header. That label will be used to set the desired size for the header in the referenceSizeForHeaderInSection method.

I have a label outlet called label in my UICollectionReusableView subclass (MyHeaderCollectionReusableView), which I use for my section header view by assigning it in the storyboard (setting "MyHeader" as Reuse Identifier for the section view). That mentioned label has the horizontal and vertical space constraints to the section header borders in order to autolayout correctly.

override func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return 3
    }

override func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {

        let headerView =
        collectionView.dequeueReusableSupplementaryViewOfKind(kind,
            withReuseIdentifier: "MyHeader",
            forIndexPath: indexPath)
            as MyHeaderCollectionReusableView

        headerView.label.text = labels[indexPath.section]

        return headerView

    }

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        // that -16 is because I have 8px for left and right spacing constraints for the label.
        let label:UILabel = UILabel(frame: CGRectMake(0, 0, collectionView.frame.width - 16, CGFloat.max))
        label.numberOfLines = 0
        label.lineBreakMode = NSLineBreakMode.ByWordWrapping
       //here, be sure you set the font type and size that matches the one set in the storyboard label
        label.font = UIFont(name: "Helvetica", size: 17.0)
        label.text = labels[section]
        label.sizeToFit()

// Set some extra pixels for height due to the margins of the header section.  
//This value should be the sum of the vertical spacing you set in the autolayout constraints for the label. + 16 worked for me as I have 8px for top and bottom constraints.
        return CGSize(width: collectionView.frame.width, height: label.frame.height + 16)
    }

回答2:

Like the questioner, I had a UICollectionView that contained a header with a single label, whose height I wanted to vary. I created an extension to UILabel to measure the height of a multiline label with a known width:

public extension UILabel {
    public class func size(withText text: String, forWidth width: CGFloat) -> CGSize {
        let measurementLabel = UILabel()
        measurementLabel.text = text
        measurementLabel.numberOfLines = 0
        measurementLabel.lineBreakMode = .byWordWrapping
        measurementLabel.translatesAutoresizingMaskIntoConstraints = false
        measurementLabel.widthAnchor.constraint(equalToConstant: width).isActive = true
        let size = measurementLabel.systemLayoutSizeFitting(UILayoutFittingCompressedSize)
        return size
    }
}

Note: the above is in Swift 3 syntax.

Then I implement the header size method of UICollectionViewDelegateFlowLayout as:

extension MyCollectionViewController : UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        let text = textForHeader(inSection: section)
        var size =  UILabel.size(withAttributedText: text, forWidth: collectionView.frame.size.width)
        size.height = size.height + 16
        return size
    }
}

The work of calculating the header size is delegated to the above UILabel extension. The +16 is a experimentally derived fixed offset (8 + 8) that is based on margins and could be obtained programmatically.

All that's needed in the header callback is just to set the text:

override func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
    if kind == UICollectionElementKindSectionHeader, let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: headerIdentifier, for: indexPath) as?  MyCollectionHeader {
        let text = textForHeader(inSection: section)
        headerView.label.text = text
        return headerView
    }
    return UICollectionReusableView()
}

回答3:

The idea is to have a template header instance in memory to calculate the desired height before creating the result header view. You should move your section header view to a separate .nib file, setup all autolayout constraints and instantiate the template in your viewDidLoad method like this:

class MyViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

  @IBOutlet var collectionView : UICollectionView?
  private var _templateHeader : MyHeaderView

  override func viewDidLoad() {
    super.viewDidLoad()

    let nib = UINib(nibName: "HeaderView", bundle:nil)
    self.collectionView?.registerNib(nib, forCellWithReuseIdentifier: "header_view_id")

    _templateHeader = nib.instantiateWithOwner(nil, options:nil)[0] as! MyHeaderView
  }

}

Then you will be able to calculate the header size (height in my example) in your flow layout delegate method:

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {

    _templateHeader.lblTitle.text = "some title here"
    _templateHeader.lblDescription.text = "some long description"

    _templateHeader.setNeedsUpdateConstraints();
    _templateHeader.updateConstraintsIfNeeded()

    _templateHeader.setNeedsLayout();
    _templateHeader.layoutIfNeeded();

    let computedSize = _templateHeader.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize)

    return CGSizeMake(collectionView.bounds.size.width, computedSize.height);
}

And then create and return your regular header view as always, since you have already calculated its size in flow layout delegate method:

func collectionView(collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, atIndexPath indexPath: NSIndexPath) -> UICollectionReusableView {

    switch kind {

    case UICollectionElementKindSectionHeader:

        let headerView = collectionView.dequeueReusableSupplementaryViewOfKind(kind, withReuseIdentifier: "header_view_id", forIndexPath: indexPath) as! MyHeaderView
        headerView.lblTitle.text = "some title here"
        headerView.lblDescription.text = "some long description"

        headerView.setNeedsUpdateConstraints()
        headerView.updateConstraintsIfNeeded()

        headerView.setNeedsLayout()
        headerView.layoutIfNeeded()

        return headerView
    default:
        assert(false, "Unexpected kind")
    }

}

Forgot to say about one important moment - your header view should have an autolayout constraint on its contentView to fit the collection view width (plus or minus the desired margins).

回答4:

I had luck using Vladimir's method, but I had to set the frame of the template view to have equal width to my collection view.

    templateHeader.bounds = CGRectMake(templateHeader.bounds.minX, templateHeader.bounds.minY, self.collectionView.bounds.width, templateHeader.bounds.height)

Additionally, my view has several resizable components, and having a template view seems robust enough to deal with any changes. Still feels like there should be an easier way.

回答5:

You have to implement the UICollectionViewDelegate method referenceSizeForHeaderInSection.

There you have to calculate the height without using the label by calling boundingRectWithSize:options:context: on the string with the appropriate attributes.