WrappedCollectionViewCell.swift

/**
 Generic CollectionViewCell, which can be specialized to wrap a UIView.
 
 To use this, you can simply define a typealias like so:
 
 ```
 class FooView : UIVIew { /* ... */ }
 typealias FooCell = WrappedCollectionViewCell<FooView>
 ```

 The resulting `UICollectionViewCell` will hug the wrapped view
 with layout constraints, so that the cell and the wrapped view can influence
 each other's size via the normal Auto Layout mechanism. This allows either for
 the layout object to specify the cell's `itemSize`, or for the cell to declare
 its own size as a self-sizing cell, when the layout uses `estimatedItemSize`.

 Known-good: Swift 3
 */
class WrappedCollectionViewCell<T:UIView> :  UICollectionViewCell
{
  private func setup()
  {
    let wrappedView = T.init(frame: self.contentView.bounds)
    self.contentView.addSubview(wrappedView)
    wrappedView.translatesAutoresizingMaskIntoConstraints = false
    
    for vfl in ["V:|[v]|","H:|[v]|"]
    {
      let cs = NSLayoutConstraint.constraints(withVisualFormat: vfl, options: [], metrics: nil, views: ["v":wrappedView])
      self.contentView.addConstraints(cs)
    }
  }

  var wrappedView:T {
    return self.contentView.subviews.first! as! T
  }
  
  class var classReuseIdentifier:String {
    return T.description() + "CellIdentifier"
  }
  
  override class var requiresConstraintBasedLayout:Bool { return true }
  override init(frame:CGRect)
  {
    super.init(frame:frame)
    setup()
  }
  
  required init?(coder aDecoder: NSCoder) {
    super.init(coder:aDecoder)
    setup()
  }
}