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

I'm trying to add UICollectionView to ViewController, and I need to have 3 cells 'per row' without blank space between cells (it should look like a grid). Cell width should be one third of screen size, so I thought that the layout.item width should be the same. But then I get this:

If I reduce that size (by 7 or 8 pixels e.g.), it's better, but the third cell in row is not completely visible, and I still have that blank space (top & bottom, and left & right) .

class ViewController: UIViewController, UICollectionViewDelegateFlowLayout, UICollectionViewDataSource {
    var collectionView: UICollectionView?
    var screenSize: CGRect!
    var screenWidth: CGFloat!
    var screenHeight: CGFloat!

    override func viewDidLoad() {
        super.viewDidLoad()

        screenSize = UIScreen.mainScreen().bounds
        screenWidth = screenSize.width
        screenHeight = screenSize.height

        // 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)
        collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
        collectionView!.dataSource = self
        collectionView!.delegate = self
        collectionView!.registerClass(CollectionViewCell.self, forCellWithReuseIdentifier: "CollectionViewCell")
        collectionView!.backgroundColor = UIColor.greenColor()
        self.view.addSubview(collectionView!)
    }

    func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return 1
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 20
    }

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("CollectionViewCell", forIndexPath: indexPath) as CollectionViewCell
        cell.backgroundColor = UIColor.whiteColor()
        cell.layer.borderColor = UIColor.blackColor().CGColor
        cell.layer.borderWidth = 0.5
        cell.frame.size.width = screenWidth / 3
        cell.frame.size.height = screenWidth / 3

        cell.textLabel?.text = "\(indexPath.section):\(indexPath.row)"
        return cell
    }
}

Solution 1:

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):

Solution 2:

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.