UIScrollView Infinite Scrolling

Question

I\'m attempting to setup a scrollview with infinite (horizontal) scrolling.

Scrolling forward is easy - I have implemented scrollViewDidScroll, and when the contentO

Answer 1

So one issue is that setting the contentOffset while in the scrollViewDidScroll: delegate method will cause the delegate method to fire again while you are inside it. So what I do is remove the delegate > setContentOffset > set the delegate again.

Here's my code:

#import "ViewController.h"

@interface ViewController () 
@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (strong, nonatomic) NSMutableArray *labels;
@property (weak, nonatomic) UILabel *originLabel;

- (void)addScrollViewLabels;
- (void)adjustOrigins:(float)deltaX;

@end


@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    // just some starting size
    CGSize size = self.scrollView.frame.size;
    CGSize contentSize = CGSizeMake(size.width * 4, size.height);
    [self.scrollView setContentSize:contentSize];

    // just some starting offset
    CGPoint contentOffset = CGPointMake((contentSize.width / 2), 0);
    [self.scrollView setContentOffset:contentOffset];

    [self addScrollViewLabels];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGSize contentSize = scrollView.contentSize;
    CGSize size = scrollView.frame.size;
    CGPoint contentOffset = scrollView.contentOffset;

    const float kContentOffsetBuffer = 100;
    const float kContentSizeGrowth = (4 * size.width);

    BOOL shouldGrowContentSize = (contentOffset.x > (contentSize.width - size.width - kContentOffsetBuffer))
                                || (contentOffset.x < (kContentOffsetBuffer));
    if (shouldGrowContentSize) {
        // the contentOffset has reached a point where we need to grow the contentSize
        CGSize adjustedContentSize = CGSizeMake(contentSize.width + kContentSizeGrowth, contentSize.height);
        [self.scrollView setContentSize:adjustedContentSize];

        if(contentOffset.x < (kContentOffsetBuffer)) {
            // the growth needs to happen on the left side which means that we need to adjust the contentOffset to compensate for the growth.
            // this is not necessary when growth happens to the right since the contentOffset is the same.
            CGPoint adjustedContentOffset = CGPointMake(contentOffset.x + kContentSizeGrowth, contentOffset.y);
            [self.scrollView setDelegate:nil];
            [self.scrollView setContentOffset:adjustedContentOffset];
            [self.scrollView setDelegate:self];
            [self adjustOrigins:kContentSizeGrowth];
        }

        [self addScrollViewLabels];
    }
}


- (void)addScrollViewLabels {
    const float kOriginY = 100;

    if (!self.labels) {
        self.labels = [NSMutableArray array];
        float originX = [self.scrollView contentOffset].x;
        UILabel *label0 = [[UILabel alloc] initWithFrame:CGRectMake(originX, kOriginY, 100, 30)];
        label0.text = @"0";
        [self.scrollView addSubview:label0];
        [self.labels addObject:label0];
        self.originLabel = label0;
    }

    CGSize contentSize = [self.scrollView contentSize];
    const float kIncrementAmount = 75;
    NSInteger indexOfOriginLabel = [self.labels indexOfObject:self.originLabel];

    // add labels to the right
    UILabel *lastLabel = [self.labels lastObject];
    float lastOriginX = lastLabel.frame.origin.x;
    for (float x = (lastOriginX + kIncrementAmount); (x < (contentSize.width)) ; x += kIncrementAmount) {
        UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(x, kOriginY, 100, 30)];
        NSInteger indexFromOrigin = ([self.labels count] - indexOfOriginLabel);
        label.text = [@(indexFromOrigin) description];
        [self.scrollView addSubview:label];
        [self.labels addObject:label];
        [label setNeedsDisplay];
    }

    // add labels to the left
    UILabel *firstLabel = [self.labels firstObject];
    float firstOriginX = firstLabel.frame.origin.x;
    for (float x = (firstOriginX - kIncrementAmount); (x >= 0) ; x -= kIncrementAmount) {
        UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(x, kOriginY, 100, 30)];
        NSInteger indexFromOrigin = -(indexOfOriginLabel + 1);
        label.text = [@(indexFromOrigin) description];
        [self.scrollView addSubview:label];
        [self.labels insertObject:label atIndex:0];
        indexOfOriginLabel++;
        [label setNeedsDisplay];
    }
}

- (void)adjustOrigins:(float)deltaX {
    for (UILabel *label in self.labels) {
        CGRect frame = label.frame;
        frame.origin.x += deltaX;
        [label setFrame:frame];
        [label setNeedsDisplay];
    }
}


@end