/*

     File: InfiniteScrollView.m

 Abstract: This view tiles UILabel instances to give the effect of infinite scrolling side to side.

  Version: 1.2

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 */

#import "InfiniteScrollView.h"

@interface InfiniteScrollView ()

@property (nonatomic, strong) NSMutableArray *visibleLabels;

@property (nonatomic, strong) UIView *labelContainerView;

@end

@implementation InfiniteScrollView

- (id)initWithCoder:(NSCoder *)aDecoder

{

    if ((self = [super initWithCoder:aDecoder]))

    {

        self.contentSize = CGSizeMake(5000, self.frame.size.height);

        _visibleLabels = [[NSMutableArray alloc] init];

        _labelContainerView = [[UIView alloc] init];

        self.labelContainerView.frame = CGRectMake(0, 0, self.contentSize.width, self.contentSize.height/2);

        [self addSubview:self.labelContainerView];

        [self.labelContainerView setUserInteractionEnabled:NO];

        // hide horizontal scroll indicator so our recentering trick is not revealed

        [self setShowsHorizontalScrollIndicator:NO];

    }

    return self;

}

#pragma mark - Layout

// recenter content periodically to achieve impression of infinite scrolling

- (void)recenterIfNecessary

{

    CGPoint currentOffset = [self contentOffset];

    CGFloat contentWidth = [self contentSize].width;

    CGFloat centerOffsetX = (contentWidth - [self bounds].size.width) / 2.0;

    CGFloat distanceFromCenter = fabs(currentOffset.x - centerOffsetX);

    if (distanceFromCenter > (contentWidth / 4.0))

    {

        self.contentOffset = CGPointMake(centerOffsetX, currentOffset.y);

        // move content by the same amount so it appears to stay still

        for (UILabel *label in self.visibleLabels) {

            CGPoint center = [self.labelContainerView convertPoint:label.center toView:self];

            center.x += (centerOffsetX - currentOffset.x);

            label.center = [self convertPoint:center toView:self.labelContainerView];

        }

    }

}

- (void)layoutSubviews

{

    [super layoutSubviews];

    [self recenterIfNecessary];

    // tile content in visible bounds

    CGRect visibleBounds = [self convertRect:[self bounds] toView:self.labelContainerView];

    CGFloat minimumVisibleX = CGRectGetMinX(visibleBounds);

    CGFloat maximumVisibleX = CGRectGetMaxX(visibleBounds);

    [self tileLabelsFromMinX:minimumVisibleX toMaxX:maximumVisibleX];

}

#pragma mark - Label Tiling

- (UILabel *)insertLabel

{

    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 500, 80)];

    [label setNumberOfLines:3];

    [label setText:@"1024 Block Street\nShaffer, CA\n95014"];

    [self.labelContainerView addSubview:label];

    return label;

}

- (CGFloat)placeNewLabelOnRight:(CGFloat)rightEdge

{

    UILabel *label = [self insertLabel];

    [self.visibleLabels addObject:label]; // add rightmost label at the end of the array

    CGRect frame = [label frame];

    frame.origin.x = rightEdge;

    frame.origin.y = [self.labelContainerView bounds].size.height - frame.size.height;

    [label setFrame:frame];

    return CGRectGetMaxX(frame);

}

- (CGFloat)placeNewLabelOnLeft:(CGFloat)leftEdge

{

    UILabel *label = [self insertLabel];

    [self.visibleLabels insertObject:label atIndex:0]; // add leftmost label at the beginning of the array

    CGRect frame = [label frame];

    frame.origin.x = leftEdge - frame.size.width;

    frame.origin.y = [self.labelContainerView bounds].size.height - frame.size.height;

    [label setFrame:frame];

    return CGRectGetMinX(frame);

}

- (void)tileLabelsFromMinX:(CGFloat)minimumVisibleX toMaxX:(CGFloat)maximumVisibleX

{

    // the upcoming tiling logic depends on there already being at least one label in the visibleLabels array, so

    // to kick off the tiling we need to make sure there's at least one label

    if ([self.visibleLabels count] == 0)

    {

        [self placeNewLabelOnRight:minimumVisibleX];

    }

    // add labels that are missing on right side

    UILabel *lastLabel = [self.visibleLabels lastObject];

    CGFloat rightEdge = CGRectGetMaxX([lastLabel frame]);

    while (rightEdge < maximumVisibleX)

    {

        rightEdge = [self placeNewLabelOnRight:rightEdge];

    }

    // add labels that are missing on left side

    UILabel *firstLabel = self.visibleLabels[0];

    CGFloat leftEdge = CGRectGetMinX([firstLabel frame]);

    while (leftEdge > minimumVisibleX)

    {

        leftEdge = [self placeNewLabelOnLeft:leftEdge];

    }

    // remove labels that have fallen off right edge

    lastLabel = [self.visibleLabels lastObject];

    while ([lastLabel frame].origin.x > maximumVisibleX)

    {

        [lastLabel removeFromSuperview];

        [self.visibleLabels removeLastObject];

        lastLabel = [self.visibleLabels lastObject];

    }

    // remove labels that have fallen off left edge

    firstLabel = self.visibleLabels[0];

    while (CGRectGetMaxX([firstLabel frame]) < minimumVisibleX)

    {

        [firstLabel removeFromSuperview];

        [self.visibleLabels removeObjectAtIndex:0];

        firstLabel = self.visibleLabels[0];

    }

}

@end