Variable width & height of UICollectionViewCell us

问题:

Here is an design issue in the app that uses AutoLayout, UICollectionView and UICollectionViewCell that has automatically resizable width & height depending on AutoLayout constraints and its content (some text).

It is a UITableView list like, with each cell that has it's own width & height calculated separately for each row dependant on its content. It is more like iOS Messages build in app (or WhatsUp).

It is obvious that app should make use of func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize.

Issue is that within that method, app cannot call func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell nor dequeueReusableCellWithReuseIdentifier(identifier: String, forIndexPath indexPath: NSIndexPath!) -> AnyObject to instantiate cell, populate it with specific content and calculate its width & height. Trying to do that will result in an indefinite recursion calls or some other type of app crash (at least in iOS 8.3).

The closest way to fix this situation seems to copy definition of the cell into view hierarchy to let Auto-layout resize "cell" automatically (like cell to have the same width as parent collection view), so app can configure cell with specific content and calculate its size. This should definitely not be the only way to fix it because of duplicated resources.

All of that is connected with setting UILabel.preferredMaxLayoutWidth to some value that should be Auto-Layout controllable (not hardcoded) that could depend on screen width & height or at least setup by Auto-layout constraint definition, so app can get multiline UILabel intrinsic size calculated.

I would not like to instantiate cells from XIB file since Storyboards should be today's industry standard and I would like to have as less intervention in code.

EDIT:

The basic code that cannot be run is listed bellow. So only instantiating variable cellProbe (not used) crashes the app. Without that call, app runs smoothly.

var onceToken: dispatch_once_t = 0

class ViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {

    override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 1
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {

        dispatch_once(&onceToken) {

            let cellProbe = collectionView.dequeueReusableCellWithReuseIdentifier("first", forIndexPath: NSIndexPath(forRow: 0, inSection: 0)) as! UICollectionViewCell
        }

        return CGSize(width: 200,height: 50)
    }

    override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("first", forIndexPath: NSIndexPath(forRow: 0, inSection: 0)) as! UICollectionViewCell

        return cell
    }
}

回答1:

If you are using constraints, you don't need to set a size per cell. So you should remove your delegate method func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize

Instead you need to set a size in estimatedItemSize by setting this to a value other than CGSizeZero you are telling the layout that you don't know the exact size yet. The layout will then ask each cell for it's size and it should be calculated when it's required.

let layout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
layout.estimatedItemSize = someReasonableEstimatedSize()

回答2:

Just like when dynamically size Table View Cell height using Auto Layout you don't need call

func tableView(_ tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell

in

optional func tableView(_ tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat

the proper way is create a local static cell for height calculation like a class method of the custom cell

+ (CGFloat)cellHeightForContent:(id)content tableView:(UITableView *)tableView {
    static CustomCell *cell;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        cell = [tableView dequeueReusableCellWithIdentifier:[CustomCell cellIdentifier]];
    });

    Item *item = (Item *)content;
    configureBasicCell(cell, item);

    [cell setNeedsLayout];
    [cell layoutIfNeeded];
    CGSize size = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    return size.height + 1.0f; // Add 1.0f for the cell separator height
}

and I seriously doubt storyboard is some industry standard. xib is the best way to create custom cell like view including tableViewCell and CollectionViewCell

回答3:

To calculate the CollectionView cell's size dynamically, just set the flow layout property to any arbitrary value:

 let flowLayout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
 flowLayout.estimatedItemSize = CGSize(width: 0, height: 0)