Creating a UICollectionView programmatically

Question

I'm looking for a guide or tutorial that will show me how to set up a simple UICollectionView using only code.

I'm wading through the documentation on Apples site, and I'm using the reference manual as well.

But I would really benefit from a simple guide that can show me how to set up a UICollectionView without having to use Storyboards or XIB/NIB files - but unfortunately when I search about, all I can find is tutorials that feature the Storyboard.

Answer 1

Swift 5

XCode 11.5

import UIKit

// 1. When creating this view, instanciate this class with the param "collectionViewLayout: UICollectionViewFlowLayout".

class BespokeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {
    var cellId = "AwesomeCell"
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 2. Register a reusable cell:
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellId)
    }
    
    // 3. Return number of items:
    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 5
    }
    
    // 4. Define the reusable cell:
    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath)
        // You can cast type above as such: ...for: indexPath) as! BespokeCell
        return cell
    }
    
    // 5. Define the size of the cell. This depends on protocol 'UICollectionViewDelegateFlowLayout' to work:
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let width = view.frame.width // In this example the width is the same as the whole view.
        let height = CGFloat(200)
        return CGSize(width: width, height: height)
    }
}

To use your bespoke class, don't forget the collectionViewLayout param:

let layout = UICollectionViewFlowLayout()
let myViewController = WorkoutViewController(collectionViewLayout: layout)

If you're not using the storyboard, you need to attach this view to the window. That's a different problem. Systems up to iOS 12 will handle the window logic on the AppDelegate file. On iOS 13+, this logic is handled on the SceneDelegate file.

window?.rootViewController = someController

Good luck!

Answer 2

For whose want create a Custom Cell :

CustomCell.h

#import <UIKit/UIKit.h>

@interface HeaderCollectionViewCell : UICollectionViewCell
@property (strong,nonatomic) UIImageView *image;
@end

CustomCell.m

#import "HeaderCollectionViewCell.h"

@implementation HeaderCollectionViewCell
#define IMAGEVIEW_BORDER_LENGTH 5

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        [self setup];
    }
    return self;
}


-(void)setup{
    _image = [[UIImageView alloc] initWithFrame:(CGRectInset(self.bounds, IMAGEVIEW_BORDER_LENGTH, IMAGEVIEW_BORDER_LENGTH))];
    [self addSubview:_image];

}
@end

UIViewController.h

#import <UIKit/UIKit.h>


@interface HomeViewController : UIViewController<UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>
@property (strong,nonatomic) UICollectionView *collectionView;

@end

UIViewController.m

#import "HomeViewController.h"
#import "HomeView.h"
#import "HeaderCollectionViewCell.h"

@interface HomeViewController ()
@property (nonatomic) NSString *cellID;

@end

@implementation HomeViewController



- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = UIColor.whiteColor;

    _cellID = @"id";

    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    _collectionView = [[UICollectionView alloc] initWithFrame:self.view.frame collectionViewLayout:layout];


    [_collectionView registerClass:[HeaderCollectionViewCell class] forCellWithReuseIdentifier:_cellID];



    [_collectionView setDataSource:self];
    [_collectionView setDelegate:self];

    _collectionView.backgroundColor = UIColor.redColor;


    [self.view addSubview:_collectionView];



}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
    return 4;
}

- (__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{

    HeaderCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:_cellID forIndexPath:indexPath];

    cell.image.image = [UIImage imageNamed:@"premium-icon"];

    return cell;
}

-(UITabBarItem*) tabBarItem{
    return [[UITabBarItem alloc] initWithTitle:@"Início" image:[UIImage imageNamed:@"home-icon"] tag:0];
}


@end

Answer 3

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)
    }
}

Answer 4

colection view exam

    #import "CollectionViewController.h"
#import "BuyViewController.h"
#import "CollectionViewCell.h"

@interface CollectionViewController ()
{
    NSArray *mobiles;
    NSArray  *costumes;
    NSArray *shoes;
    NSInteger selectpath;
    NSArray *mobilerate;
    NSArray *costumerate;
    NSArray *shoerate;
}
@end

@implementation CollectionViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.title = self.receivename;
    mobiles = [[NSArray alloc]initWithObjects:@"7.jpg",@"6.jpg",@"5.jpg", nil];
    costumes = [[NSArray alloc]initWithObjects:@"shirt.jpg",@"costume2.jpg",@"costume1.jpg", nil];
    shoes = [[NSArray alloc]initWithObjects:@"shoe.jpg",@"shoe1.jpg",@"shoe2.jpg", nil];
    mobilerate = [[NSArray alloc]initWithObjects:@"10000",@"11000",@"13000",nil];
    costumerate = [[NSArray alloc]initWithObjects:@"699",@"999",@"899", nil];
    shoerate = [[NSArray alloc]initWithObjects:@"599",@"499",@"300", nil];
}
- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return 3;
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellId = @"cell";
    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellId forIndexPath:indexPath];

    UIImageView *collectionImg = (UIImageView *)[cell viewWithTag:100];

    if ([self.receivename isEqualToString:@"Mobiles"])
    {
        collectionImg.image = [UIImage imageNamed:[mobiles objectAtIndex:indexPath.row]];
    }
    else if ([self.receivename isEqualToString:@"Costumes"])
    {
        collectionImg.image = [UIImage imageNamed:[costumes objectAtIndex:indexPath.row]];
    }
    else
    {
        collectionImg.image = [UIImage imageNamed:[shoes objectAtIndex:indexPath.row]];
    }
    return cell;
}

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath

{
    selectpath = indexPath.row;
    [self performSegueWithIdentifier:@"buynow" sender:self];
}

    // In a storyboard-based application, you will often want to do a little
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"buynow"])
    {
        BuyViewController *obj = segue.destinationViewController;
        if ([self.receivename isEqualToString:@"Mobiles"])
        {
            obj.reciveimg = [mobiles objectAtIndex:selectpath];
            obj.labelrecive = [mobilerate objectAtIndex:selectpath];

        }
        else if ([self.receivename isEqualToString:@"Costumes"])
        {
            obj.reciveimg = [costumes objectAtIndex:selectpath];
            obj.labelrecive = [costumerate objectAtIndex:selectpath];
        }
        else
        {
            obj.reciveimg = [shoes objectAtIndex:selectpath];
            obj.labelrecive = [shoerate objectAtIndex:selectpath];
        }
        //     Get the new view controller using [segue destinationViewController].
        //     Pass the selected object to the new view controller.
    }
}

@end

.h file

@interface CollectionViewController :
UIViewController<UICollectionViewDelegate,UICollectionViewDataSource>
@property (strong, nonatomic) IBOutlet UICollectionView *collectionView;
@property (strong,nonatomic) NSString *receiveimg;
@property (strong,nonatomic) NSString *receivecostume;
@property (strong,nonatomic)NSString *receivename;

@end

Answer 5

swift 4 code


//
//  ViewController.swift
//  coolectionView
//




import UIKit

class ViewController: UIViewController , UICollectionViewDataSource, UICollectionViewDelegate,UICollectionViewDelegateFlowLayout{
    @IBOutlet weak var collectionView: UICollectionView!

    var items = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48"]

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return self.items.count
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
    {
        if indexPath.row % 3 != 0
        {
        return CGSize(width:collectionView.frame.width/2 - 7.5 , height: 100)
        }
        else
        {
            return CGSize(width:collectionView.frame.width - 10 , height: 100 )
        }
    }


    // make a cell for each cell index path
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        // get a reference to our storyboard cell
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CollectionViewCell1234", for: indexPath as IndexPath) as! CollectionViewCell1234

        // Use the outlet in our custom class to get a reference to the UILabel in the cell
        cell.lbl1.text = self.items[indexPath.item]
        cell.backgroundColor = UIColor.cyan // make cell more visible in our example project
        cell.layer.borderColor = UIColor.black.cgColor
        cell.layer.borderWidth = 1
        cell.layer.cornerRadius = 8

        return cell
    }
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        // handle tap events
        print("You selected cell #\(indexPath.item)!")
    }



}

Answer 6

You can handle custom cell in uicollection view see below code.

- (void)viewDidLoad
{
  UINib *nib2 = [UINib nibWithNibName:@"YourCustomCell" bundle:nil];
    [CollectionVW registerNib:nib2 forCellWithReuseIdentifier:@"YourCustomCell"];


    UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
    [flowLayout setItemSize:CGSizeMake(200, 230)];
    flowLayout.minimumInteritemSpacing = 0;
    [flowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
    [CollectionVW setCollectionViewLayout:flowLayout];

    [CollectionVW reloadData];
}

#pragma mark - COLLECTIONVIEW
#pragma mark Collection View CODE

-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return Array.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"YourCustomCell";
    YourCustomCell *cell = (YourCustomCell *)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];

    cell.MainIMG.image=[UIImage imageNamed:[Array objectAtIndex:indexPath.row]];


    return cell;
}

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
{
}

#pragma mark Collection view layout things
// Layout: Set cell size
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CGSize mElementSize;
    mElementSize=CGSizeMake(kScreenWidth/3.4, 150);
    return mElementSize;
}


- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section
{
    return 5.0;
}

// Layout: Set Edges
- (UIEdgeInsets)collectionView: (UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section
{
    if (isIphone5 || isiPhone4)
    {
        return UIEdgeInsetsMake(15,15,5,15);  // top, left, bottom, right
    }
    else if (isIphone6)
    {
        return UIEdgeInsetsMake(15,15,5,15);  // top, left, bottom, right
    }
    else if (isIphone6P)
    {
        return UIEdgeInsetsMake(15,15,5,15);  // top, left, bottom, right
    }

    return UIEdgeInsetsMake(15,15,5,15);  // top, left, bottom, right
}

Answer 7

Swift 3

class TwoViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout, UICollectionViewDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        let flowLayout = UICollectionViewFlowLayout()

        let 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
    {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "collectionCell", for: indexPath as IndexPath)

        cell.backgroundColor = UIColor.green
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
    {
        return CGSize(width: 50, height: 50)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets
    {
        return UIEdgeInsets(top: 5, left: 5, bottom: 5, right: 5)
    }

}

Answer 8

1. Building off @Warewolf's answer, the next step is to create your own custom cell.

   Go to File -> New -> File -> User Interface -> Empty -> Call this nib "customNib".

2. In your customNib drag a UICollectionView Cell in. Give it reuse cell identifier @"Cell".

3. File -> New -> File -> Cocoa Touch Class -> Class named "CustomCollectionViewCell" subclass if UICollectionViewCell.

4. Go back to the custom nib, click cell and make this custom class "CustomCollectionViewCell".

5. Go to your viewDidLoad viewcontroller and instead of

   [_collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cellIdentifier"];

   have

   UINib *nib = [UINib nibWithNibName:@"customNib" bundle:nil]; [_collectionView registerNib:nib forCellWithReuseIdentifier:@"Cell"];

6. Also, change (to your new cell identifier)

   UICollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];

Answer 9

    #pragma mark -
    #pragma mark - UICollectionView Datasource and Delegates

    -(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
    {
        return 1;
    }

    -(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
    {
        return Arr_AllCulturalButtler.count;
    }

    -(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
    {
        static NSString *coll=@"FromCulturalbutlerCollectionViewCell";
        FromCulturalbutlerCollectionViewCell *cell=[collectionView dequeueReusableCellWithReuseIdentifier:coll forIndexPath:indexPath];
        cell.lbl_categoryname.text=[[Arr_AllCulturalButtler objectAtIndex:indexPath.row] Category_name];
        cell.lbl_date.text=[[Arr_AllCulturalButtler objectAtIndex:indexPath.row] event_Start_date];
        cell.lbl_location.text=[[Arr_AllCulturalButtler objectAtIndex:indexPath.row] Location_name];
        [cell.Img_Event setImageWithURL:[APPDELEGATE getURLForMediumSizeImage:[(EventObj *)[Arr_AllCulturalButtler objectAtIndex:indexPath.row] Event_image_name]] placeholderImage:nil usingActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
        cell.button_Bookmark.selected=[[Arr_AllCulturalButtler objectAtIndex:indexPath.row] Event_is_bookmarked];
        [cell.button_Bookmark addTarget:self action:@selector(btn_bookmarkClicked:) forControlEvents:UIControlEventTouchUpInside];
        cell.button_Bookmark.tag=indexPath.row;


        return cell;
    }
    - (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath
    {

        [self performSegueWithIdentifier:SEGUE_CULTURALBUTLER_KULTURELLIS_DETAIL sender:self];
    }

// stroy board navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([segue.identifier isEqualToString:@"Overview_Register"])
    {
        WDRegisterViewController *obj=(WDRegisterViewController *)[segue destinationViewController];
        obj.str_Title=@"Edit Profile";
        obj.isRegister=NO;
    }
}

            [self performSegueWithIdentifier:@"Overview_Measure" sender:nil];



    UIStoryboard *sb = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    WDPeekViewController *Peek = (WDPeekViewController *)[sb instantiateViewControllerWithIdentifier:@"WDPeekViewController"];
 [self.navigationController pushViewController:tabBarController animated:YES];

Answer 10

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
}

Answer 11

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---

Answer 12

Apple Docs:

- (id)initWithFrame:(CGRect)frame 
      collectionViewLayout:(UICollectionViewLayout *)layoutParameters

Use this method when initializing a collection view object programmatically. If you specify nil for the layout parameter, you must assign a layout object to the collectionViewLayout property before displaying the collection view onscreen. If you do not, the collection view will be unable to present any items onscreen.

This method is the designated initializer.

This method is used to initialize the UICollectionView. here you provide frame and a UICollectionViewLayout object.

UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc]init];

At the end, add UICollectionView as a subview to your view.

Now collection view is added pro grammatically. You can go on learning.
Happy learning!! Hope it helps you.