Creating a UICollectionView programmatically

Header file:--

@interface ViewController : UIViewController<UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>{    UICollectionView *_collectionView;}

Implementation File:--

- (void)viewDidLoad{     [super viewDidLoad];     self.view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];     UICollectionViewFlowLayout *layout=[[UICollectionViewFlowLayout alloc] init];    _collectionView=[[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:layout];    [_collectionView setDataSource:self];    [_collectionView setDelegate:self];    [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];    [_collectionView setBackgroundColor:[UIColor redColor]];    [self.view addSubview:_collectionView];    // Do any additional setup after loading the view, typically from a nib.}- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{    return 15;}// The cell that is returned must be retrieved from a call to -dequeueReusableCellWithReuseIdentifier:forIndexPath:- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{    UICollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"cellIdentifier" forIndexPath:indexPath];    cell.backgroundColor=[UIColor greenColor];    return cell;}- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{    return CGSizeMake(50, 50);}

Output---

For swift4 user:--

class TwoViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout, UICollectionViewDelegate {    override func viewDidLoad() {        super.viewDidLoad()        self.collectionView = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "collectionCell")        collectionView.delegate = self        collectionView.dataSource = self        collectionView.backgroundColor = UIColor.cyan        self.view.addSubview(collectionView)    }    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {        return 20    }    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {       var cell = collectionView.dequeueReusableCell(withReuseIdentifier: "collectionCell", for: indexPath as IndexPath)       cell.backgroundColor = UIColor.green       return cell    }    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {        return CGSize(width: 50, height: 50)    }    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets {        return UIEdgeInsets(top: 5, left: 5, bottom: 5, right: 5)    }}

For Swift 2.0

Instead of implementing the methods that are required to draw the CollectionViewCells:

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize    {        return CGSizeMake(50, 50);    }    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets    {        return UIEdgeInsetsMake(5, 5, 5, 5); //top,left,bottom,right    }

Use UICollectionViewFlowLayout

func createCollectionView() {    let flowLayout = UICollectionViewFlowLayout()    // Now setup the flowLayout required for drawing the cells    let space = 5.0 as CGFloat    // Set view cell size    flowLayout.itemSize = CGSizeMake(50, 50)    // Set left and right margins    flowLayout.minimumInteritemSpacing = space    // Set top and bottom margins    flowLayout.minimumLineSpacing = space    // Finally create the CollectionView    let collectionView = UICollectionView(frame: CGRectMake(10, 10, 300, 400), collectionViewLayout: flowLayout)    // Then setup delegates, background color etc.    collectionView?.dataSource = self    collectionView?.delegate = self    collectionView?.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: "cellID")    collectionView?.backgroundColor = UIColor.whiteColor()    self.view.addSubview(collectionView!)}

Then implement the UICollectionViewDataSource methods as required:

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {        return 20;    }func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {    var cell:UICollectionViewCell=collectionView.dequeueReusableCellWithReuseIdentifier("collectionCell", forIndexPath: indexPath) as UICollectionViewCell;    cell.backgroundColor = UIColor.greenColor();    return cell;}func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {    // #warning Incomplete implementation, return the number of sections    return 1}