The crash is telling you pretty explicitly what is going wrong:

UICollectionView must be initialized with a non-nil layout parameter.

If you check the documentation for UICollectionView, you'll find that the only initializer is initWithFrame:collectionViewLayout:. Further, in the parameters for that initializer, you'll see:

frame

The frame rectangle for the collection view, measured in points. The origin of the frame is relative to the superview in which you plan to add it. This frame is passed to the superclass during initialization.

layout

The layout object to use for organizing items. The collection view stores a strong reference to the specified object. Must not be nil.

I've bolded the important part. You must use initWithFrame:collectionViewLayout: to initialize your UICollectionView, and you must pass it a non-nil UICollectionViewLayout object.

One way to fix this, then, would be to simply change the order of initialization you do:

UICollectionViewFlowLayout* flowLayout = [[UICollectionViewFlowLayout alloc] init];
flowLayout.itemSize = CGSizeMake(100, 100);
[flowLayout setScrollDirection:UICollectionViewScrollDirectionHorizontal];
self.collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:flowLayout];
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];

Note that in the above example, I've assumed you want to use self.view.frame as the frame of self.collectionView. If that's not the case, insert whatever frame you want instead.

Just a swift version of Riley Avron's answer

    let layout = UICollectionViewFlowLayout()
layout.itemSize = CGSizeMake(100, 100)
let collectionView = UICollectionView(frame: CGRectZero, collectionViewLayout: layout)
self.view.addSubview(collectionView)
collectionView.delegate   = self
collectionView.dataSource = self
collectionView.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: "cellIdentifier")

Swift 3.0 and Swift 4.0

UICollectionView

The app crashed as soon as it launched when UICollectionView is initialised

initialize like below (above viewDidLoad)

let alarmCollectionView:UICollectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: UICollectionViewFlowLayout.init())
let layout:UICollectionViewFlowLayout = UICollectionViewFlowLayout.init()

In Method (inside viewDidLoad)

layout.scrollDirection = UICollectionViewScrollDirection.vertical
alarmCollectionView.setCollectionViewLayout(layout, animated: true)
alarmCollectionView.delegate = self
alarmCollectionView.dataSource = self
alarmCollectionView.backgroundColor = UIColor.clear
self.addSubview(alarmCollectionView)

Worked fine after initialising like this I am using Autolayout so using CGRect.zero or use your own CGRect

UICollectionViewController

In case if you are using UICollectionViewController instead of UICollectionView you need to add layout when initialization

 let  testNavController:UINavigationController = UINavigationController.init()
let layout:UICollectionViewFlowLayout = UICollectionViewFlowLayout.init()
let collectionView:UICollectionViewController = UICollectionViewController.init(collectionViewLayout:layout )
testNavController.pushViewController(collectionView, animated: true)

don't setCollectionViewLayout liked this:

[self.collectionView setCollectionViewLayout:flowLayout];

try to set when init:

[[UICollectionView alloc] initWithFrame:frame collectionViewLayout:defaultLayout];

In Swift

var collectionView: UICollectionView!
let layout = UICollectionViewFlowLayout()
collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
layout.scrollDirection = .vertical
view.addSubview(collectionView)

Swift 3.0, Swift 4.0 and Swift 5.0

It's work for me.

let collectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: UICollectionViewFlowLayout.init())

In Method (inside viewDidLoad)

if let flowLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {
flowLayout.scrollDirection = .horizontal
}

Swift 5 general solution

Can use UICollectionViewDelegateFlowLayout and don't forget to set your cell size

class YourViewController : UICollectionViewController, UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize.init(width: view.frame.width, height: 250)
}
override func viewDidLoad() {
super.viewDidLoad()
self.collectionView!.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
}
// initialized with a non-nil layout parameter
init() {
super.init(collectionViewLayout: UICollectionViewFlowLayout())
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}