View with continuous scroll; both horizontal and vertical

Question

I have been struggling with this assignment for quite some time now. What I would like to develop is a scrollview or collectionview which scrolls continuously both vertical and horizontal.

Here is an image of how I think this should look like. The transparent boxes are the views/cells which are re-loaded from the memory. As soon as a view/cell gets outside of the screen, it should be reused for upcoming new cell.. just like how a UITableViewController works.

I know that a UICollectionView can only be made to infinite scroll horizontal OR vertical, not both. However, I don't know how to do this using a UIScrollView. I tried the code attached to an answer on this question and I can get it to re-create views (e.g. % 20) but that's not really what I need.. besides, its not continuous.

I know it is possible, because the HBO Go app does this.. I want exactly the same functionality.

My Question: How can I achieve my goal? Are there any guides/tutorials that can show me how? I can't find any.

Answer 1

You can get infinite scrolling, by using the technique of re-centering the UIScrollView after you get a certain distance away from the center. First, you need to make the contentSize big enough that you can scroll a bit, so I return 4 times the number of items in my sections and 4 times the number of sections, and use the mod operator in the cellForItemAtIndexPath method to get the right index into my array. You then have to override layoutSubviews in a subclass of UICollectionView to do the re-centering (this is demonstrated in the WWDC 2011 video, "Advanced Scroll View Techniques"). Here is the controller class that has the collection view (set up in IB) as a subview:

#import "ViewController.h"
#import "MultpleLineLayout.h"
#import "DataCell.h"

@interface ViewController ()
@property (weak,nonatomic) IBOutlet UICollectionView *collectionView;
@property (strong,nonatomic) NSArray *theData;
@end

@implementation ViewController

- (void)viewDidLoad {
    self.theData = @[@[@"1",@"2",@"3",@"4",@"5"], @[@"6",@"7",@"8",@"9",@"10"],@[@"11",@"12",@"13",@"14",@"15"],@[@"16",@"17",@"18",@"19",@"20"]];
    MultpleLineLayout *layout = [[MultpleLineLayout alloc] init];
    self.collectionView.collectionViewLayout = layout;
    self.collectionView.showsHorizontalScrollIndicator = NO;
    self.collectionView.showsVerticalScrollIndicator = NO;
    layout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
    self.view.backgroundColor = [UIColor blackColor];
    [self.collectionView registerClass:[DataCell class] forCellWithReuseIdentifier:@"DataCell"];
    [self.collectionView reloadData];
}


- (NSInteger)collectionView:(UICollectionView *)view numberOfItemsInSection:(NSInteger)section {
    return 20;
}

- (NSInteger)numberOfSectionsInCollectionView: (UICollectionView *)collectionView {
    return 16;
}

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

    DataCell *cell = [collectionView  dequeueReusableCellWithReuseIdentifier:@"DataCell" forIndexPath:indexPath];
    cell.label.text = self.theData[indexPath.section %4][indexPath.row %5];
    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
   // UICollectionViewCell *item = [collectionView cellForItemAtIndexPath:indexPath];
    NSLog(@"%@",indexPath);

}

Here is the UICollectionViewFlowLayout subclass:

#define space 5
#import "MultpleLineLayout.h"

@implementation MultpleLineLayout { // a subclass of UICollectionViewFlowLayout
    NSInteger itemWidth;
    NSInteger itemHeight;
}

-(id)init {
    if (self = [super init]) {
        itemWidth = 60;
        itemHeight = 60;
    }
    return self;
}

-(CGSize)collectionViewContentSize {
    NSInteger xSize = [self.collectionView numberOfItemsInSection:0] * (itemWidth + space); // "space" is for spacing between cells.
    NSInteger ySize = [self.collectionView numberOfSections] * (itemHeight + space);
    return CGSizeMake(xSize, ySize);
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)path {
    UICollectionViewLayoutAttributes* attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:path];
    attributes.size = CGSizeMake(itemWidth,itemHeight);
    int xValue = itemWidth/2 + path.row * (itemWidth + space);
    int yValue = itemHeight + path.section * (itemHeight + space);
    attributes.center = CGPointMake(xValue, yValue);
    return attributes;
}


-(NSArray*)layoutAttributesForElementsInRect:(CGRect)rect {
    NSInteger minRow =  (rect.origin.x > 0)?  rect.origin.x/(itemWidth + space) : 0; // need to check because bounce gives negative values  for x.
    NSInteger maxRow = rect.size.width/(itemWidth + space) + minRow;
    NSMutableArray* attributes = [NSMutableArray array];
    for(NSInteger i=0 ; i < self.collectionView.numberOfSections; i++) {
        for (NSInteger j=minRow ; j < maxRow; j++) {
            NSIndexPath* indexPath = [NSIndexPath indexPathForItem:j inSection:i];
            [attributes addObject:[self layoutAttributesForItemAtIndexPath:indexPath]];
        }
    }
    return attributes;
}

And finally, here is the subclass of UICollectionView:

-(void)layoutSubviews {
    [super layoutSubviews];
    CGPoint currentOffset = self.contentOffset;
    CGFloat contentWidth = self.contentSize.width;
    CGFloat contentHeight = self.contentSize.height;
    CGFloat centerOffsetX = (contentWidth - self.bounds.size.width)/ 2.0;
    CGFloat centerOffsetY = (contentHeight - self.bounds.size.height)/ 2.0;
    CGFloat distanceFromCenterX = fabsf(currentOffset.x - centerOffsetX);
    CGFloat distanceFromCenterY = fabsf(currentOffset.y - centerOffsetY);

    if (distanceFromCenterX > contentWidth/4.0) { // this number of 4.0 is arbitrary
        self.contentOffset = CGPointMake(centerOffsetX, currentOffset.y);
    }
    if (distanceFromCenterY > contentHeight/4.0) {
        self.contentOffset = CGPointMake(currentOffset.x, centerOffsetY);
    }
}

Answer 2

@updated for swift 3 and changed how the maxRow is calculated otherwise the last column is cutoff and can cause errors

import UIKit

class NodeMap : UICollectionViewController {
    var rows = 10
    var cols = 10

    override func viewDidLoad(){
        self.collectionView!.collectionViewLayout = NodeLayout(itemWidth: 400.0, itemHeight: 300.0, space: 5.0)
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return rows
    }

    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        return cols
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        return collectionView.dequeueReusableCell(withReuseIdentifier: "node", for: indexPath)
    }
}

class NodeLayout : UICollectionViewFlowLayout {
    var itemWidth : CGFloat
    var itemHeight : CGFloat
    var space : CGFloat
    var columns: Int{
        return self.collectionView!.numberOfItems(inSection: 0)
    }
    var rows: Int{
        return self.collectionView!.numberOfSections
    }

    init(itemWidth: CGFloat, itemHeight: CGFloat, space: CGFloat) {
        self.itemWidth = itemWidth
        self.itemHeight = itemHeight
        self.space = space
        super.init()
    }

    required init(coder aDecoder: NSCoder) {
        self.itemWidth = 50
        self.itemHeight = 50
        self.space = 3
        super.init()
    }

    override var collectionViewContentSize: CGSize{
        let w : CGFloat = CGFloat(columns) * (itemWidth + space)
        let h : CGFloat = CGFloat(rows) * (itemHeight + space)
        return CGSize(width: w, height: h)
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        let x : CGFloat = CGFloat(indexPath.row) * (itemWidth + space)
        let y : CGFloat = CGFloat(indexPath.section) + CGFloat(indexPath.section) * (itemHeight + space)
        attributes.frame = CGRect(x: x, y: y, width: itemWidth, height: itemHeight)
        return attributes
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let minRow : Int = (rect.origin.x > 0) ? Int(floor(rect.origin.x/(itemWidth + space))) : 0
        let maxRow : Int = min(columns - 1, Int(ceil(rect.size.width / (itemWidth + space)) + CGFloat(minRow)))
        var attributes : Array<UICollectionViewLayoutAttributes> = [UICollectionViewLayoutAttributes]()
        for i in 0 ..< rows {
            for j in minRow ... maxRow {
                attributes.append(self.layoutAttributesForItem(at: IndexPath(item: j, section: i))!)
            }
        }
        return attributes
    }
}

Answer 3

@rdelmar's answer worked like a charm, but I needed to do it in swift. Here's the conversion :)

class NodeMap : UICollectionViewController {
    @IBOutlet var activateNodeButton : UIBarButtonItem?
    var rows = 10
    var cols = 10
    override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return rows
    }
    override func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        return cols
    }
    override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        return collectionView.dequeueReusableCellWithReuseIdentifier("node", forIndexPath: indexPath)
    }
    override func viewDidLoad() {
        self.collectionView!.collectionViewLayout = NodeLayout(itemWidth: 100.0, itemHeight: 100.0, space: 5.0)
    }
}

class NodeLayout : UICollectionViewFlowLayout {
    var itemWidth : CGFloat
    var itemHeight : CGFloat
    var space : CGFloat
    init(itemWidth: CGFloat, itemHeight: CGFloat, space: CGFloat) {
        self.itemWidth = itemWidth
        self.itemHeight = itemHeight
        self.space = space
        super.init()
    }
    required init(coder aDecoder: NSCoder) {
        self.itemWidth = 50
        self.itemHeight = 50
        self.space = 3
        super.init()
    }
    override func collectionViewContentSize() -> CGSize {
        let w : CGFloat = CGFloat(self.collectionView!.numberOfItemsInSection(0)) * (itemWidth + space)
        let h : CGFloat = CGFloat(self.collectionView!.numberOfSections()) * (itemHeight + space)
        return CGSizeMake(w, h)
    }
    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes! {
        let attributes = UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)
        let x : CGFloat = CGFloat(indexPath.row) * (itemWidth + space)
        let y : CGFloat = CGFloat(indexPath.section) + CGFloat(indexPath.section) * (itemHeight + space)
        attributes.frame = CGRectMake(x, y, itemWidth, itemHeight)
        return attributes
    }
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
        let minRow : Int = (rect.origin.x > 0) ? Int(floor(rect.origin.x/(itemWidth + space))) : 0
        let maxRow : Int = Int(floor(rect.size.width/(itemWidth + space)) + CGFloat(minRow))
        var attributes : Array<UICollectionViewLayoutAttributes> = [UICollectionViewLayoutAttributes]()
        for i in 0...self.collectionView!.numberOfSections()-1 {
            for j in minRow...maxRow {
                attributes.append(self.layoutAttributesForItemAtIndexPath(NSIndexPath(forItem: j, inSection: i)))
            }
        }
        return attributes
    }
}

Answer 4

Resetting the contentOffset probably is the best solution figured out so far.

A few steps should be taken to achieve this:

1. Pad extra items at both the left and right side of the original data set to achieve larger scrollable area; This is similar to having a large duplicated data set, but difference is the amount;
2. At start, the collection view’s contentOffset is calculated to show only the original data set (drawn in black rectangles);
3. When the user scrolls right and contentOffset hits the trigger value, we reset contentOffset to show same visual results; but actually different data; When the user scrolls left, the same logic is used.

So, the heavy lifting is in calculating how many items should be padded both on the left and right side. If you take a look at the illustration, you will find that a minimum of one extra screen of items should be padded on left and also, another extra screen on the right. The exact amount padded depends on how many items are in the original data set and how large your item size is.

I wrote a post on this solution:

http://www.awsomejiang.com/2018/03/24/Infinite-Scrolling-and-the-Tiling-Logic/