快速，以编程方式更改UICollectionViewCell和UILabel（在单元格内）的宽度

Question

I've set the width of a cell(UICollectionViewCell) to be equal to the width of the UICollectionView and I'm trying to do exactly the same thing with the UILabel that is included inside that cell. I think the code below explains exactly what I'm trying to achieve. So i've read some question here in SO and also a couple of tutorials but I'm still not sure how I can achieve this.

In a couple of questions it was saying about using collectionViewLayout but I'm really struggling on how to use it within my code. Any ideas? Thank you!

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    var cell = collectionView.dequeueReusableCellWithReuseIdentifier("myCell", forIndexPath: indexPath) as LocationViewCell
    cell.locationLabel.text = "Hello there!"

    // Set cell & label width to 100%
    let collectionViewWidth = self.collectionView.bounds.size.width
    cell.frame.size.width = collectionViewWidth // Works
    cell.locationLabel.frame.size.width = collectionViewWidth // Does NOT 

Update 1

So I added the following:

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    // Set cell width to 100%
    let collectionViewWidth = self.collectionView.bounds.size.width
    return CGSize(width: collectionViewWidth, height: 35)
}

What happens is that when the view is loaded the UILabel's width is still small. If I go to another view and then return back then it's 100%. So I have to do something in the viewDidLoad() right? I'm already using self.collectionView.reloadData() but I guess that's only for data.

Update 2

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    var cell = collectionView.dequeueReusableCellWithReuseIdentifier("locationCell", forIndexPath: indexPath) as LocationViewCell
    cell.locationLabel.text = "Hello UILabel"

    // Set cell width to 100%
    let collectionViewWidth = self.collectionView.bounds.size.width
    cell.frame.size.width = collectionViewWidth
    cell.locationLabel.frame.size.width = collectionViewWidth

    return cell
}

Answer 1

It doesn't work because by the time this method is called, the collection view already knows how big the cell should be because it has got it from the flow delegate method:

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

This is where you should be setting the size of your cells.