How to set cell spacing and UICollectionView - UICollectionViewFlowLayout size ratio?

Question

Add these 2 lines

layout.minimumInteritemSpacing = 0
layout.minimumLineSpacing = 0

So you have:

   // Do any additional setup after loading the view, typically from a nib.
        let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
        layout.sectionInset = UIEdgeInsets(top: 20, left: 0, bottom: 10, right: 0)
        layout.itemSize = CGSize(width: screenWidth/3, height: screenWidth/3)
        layout.minimumInteritemSpacing = 0
        layout.minimumLineSpacing = 0
        collectionView!.collectionViewLayout = layout

That will remove all the spaces and give you a grid layout:

If you want the first column to have a width equal to the screen width then add the following function:

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    if indexPath.row == 0
    {
        return CGSize(width: screenWidth, height: screenWidth/3)
    }
    return CGSize(width: screenWidth/3, height: screenWidth/3);

}

Grid layout will now look like (I've also added a blue background to first cell):

For Swift 3 and XCode 8, this worked. Follow below steps to achieve this:-

{
    let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
    let width = UIScreen.main.bounds.width
    layout.sectionInset = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
    layout.itemSize = CGSize(width: width / 2, height: width / 2)
    layout.minimumInteritemSpacing = 0
    layout.minimumLineSpacing = 0
    collectionView!.collectionViewLayout = layout
}

Place this code into viewDidLoad() function.

In Certain situations, Setting the UICollectionViewFlowLayout in viewDidLoador ViewWillAppear may not effect on the collectionView.

Setting the UICollectionViewFlowLayout in viewDidAppear may cause see the changes of the cells sizes in runtime.

Another Solution, in Swift 3 :

extension YourViewController : UICollectionViewDelegateFlowLayout{

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 20, left: 0, bottom: 10, right: 0)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let collectionViewWidth = collectionView.bounds.width
        return CGSize(width: collectionViewWidth/3, height: collectionViewWidth/3)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return 0
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return 20
    }
}

If you are looking for Swift 3, Follow the steps to achieve this:

func viewDidLoad() {

    //Define Layout here
    let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()

    //Get device width
    let width = UIScreen.main.bounds.width

    //set section inset as per your requirement.
    layout.sectionInset = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)

    //set cell item size here 
    layout.itemSize = CGSize(width: width / 2, height: width / 2)

    //set Minimum spacing between 2 items
    layout.minimumInteritemSpacing = 0

    //set minimum vertical line spacing here between two lines in collectionview
    layout.minimumLineSpacing = 0

    //apply defined layout to collectionview
    collectionView!.collectionViewLayout = layout

}

This is verified on Xcode 8.0 with Swift 3.