UIScrollView实现切换图片，图片间有间距

效果图：

one.gif

默认情况下通过UIScrollView实现图片轮播。图片间是无缝连接的。

two.gif

要想实现iOS相册中的间距效果，就需要用到
CGRectInset(CGRect rect, CGFloat dx, CGFloat dy)
这个功能了

subView.frame = CGRectInset(frame, dx, dy);

这里的意思是设置subView的frame为父视图的frame，但是宽缩小dx，高缩小dy

先上代码

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 100, 375, 400)];
    _scrollView.backgroundColor = [UIColor lightGrayColor];
    [self.view addSubview:_scrollView];
    
    [_scrollView setContentSize:CGSizeMake(375 * 9, 0)];
    _scrollView.pagingEnabled = YES;
    
    CGFloat dx = 20.0f; //间距，这里指一边，实际效果是两页间的间距加起来，即dx*2
    CGFloat itemWidth = _scrollView.bounds.size.width + dx * 2.0; //每个元素所占宽
    
    for (int i = 0; i < 9; i++) {
        UIView *subView = [[UIView alloc] initWithFrame:CGRectMake(375 * i, 0, 375, 400)];
        subView.backgroundColor = [self arcColor];
        [_scrollView addSubview:subView];
        
        CGRect frame = _scrollView.bounds;
        frame.origin.x = itemWidth * i;
        frame.origin.y = 0;
        frame.size.width = itemWidth;
        
        subView.frame = CGRectInset(frame, dx, 0);  //缩小至适应屏幕
    }
    
    _scrollView.frame = CGRectMake(-dx, 100, itemWidth, 400);   
    CGSize pageViewSize = _scrollView.bounds.size;
    [_scrollView setContentSize:CGSizeMake(itemWidth * 9, pageViewSize.height)];    //将间距计算进size中
}

- (UIColor *)arcColor
{
    return [UIColor colorWithRed:arc4random() % 256 / 255.0 green:arc4random() % 256 / 255.0 blue:arc4random() % 256 / 255.0 alpha:1];
}

原理：设置scrollView的contentSize.width为默认情况下的width加上增加的间距值

itemWidth是指scrollView上每页的宽度
最后还有一个很重要的功能，
subView.frame = CGRectInset(frame, dx, 0); //缩小至适应屏幕

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