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在scrollViewDidScroll代理方法里计算子控件距离中心点的差值，再用transform放大该子控件，越接近中心点放大比例越高

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    if (scrollView == _myScrollView) {
        int centerX = scrollView.contentOffset.x + scrollView.frame.size.width / 2;
                
        for (UIImageView *imageV in _imageViews) {
            CGFloat distanceX = fabs(imageV.center.x - centerX); // 子控件距中心点差值
            if (distanceX < imageV.frame.size.width / 2) {
                // 0-1，差值越小（越靠近中心点），distanceScale越大
                CGFloat distanceScale = 1 - (distanceX / (imageV.frame.size.width / 2));
                // 设置中心子控件缩放比例，这里设置最多放大1.2倍，所以用1 + 0.2 * 差值比例
                CGFloat currentScale = 1 + distanceScale * 0.2;
                
                imageV.transform = CGAffineTransformMakeScale(currentScale, currentScale);
                [_myScrollView bringSubviewToFront:imageV];
            }
        }
    }
}

完整代码：

#import "ViewController.h"

@interface ViewController () <UIScrollViewDelegate>

@property (nonatomic, strong) UIScrollView *myScrollView;

@property (nonatomic, strong) NSMutableArray *imageViews;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self initSubviews];
}

- (void)initSubviews {
    _myScrollView = [[UIScrollView alloc]initWithFrame:CGRectMake(0, 200, [UIScreen mainScreen].bounds.size.width, 220)];
    _myScrollView.backgroundColor = UIColor.lightGrayColor;
    _myScrollView.delegate = self;
    _myScrollView.contentSize = CGSizeMake(20 + 180 * 10, 0);
    _myScrollView.clipsToBounds = NO;
    
    _imageViews = [NSMutableArray array];
    for (int i = 0; i < 10; i++) {
        UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(20 + i * 180, 20, 160, 180)];
        imageView.backgroundColor = [self randomColor];
        imageView.userInteractionEnabled = YES;
        [_myScrollView addSubview:imageView];
        
        [_imageViews addObject:imageView];
    }
    
    [self.view addSubview:_myScrollView];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    if (scrollView == _myScrollView) {
        int centerX = scrollView.contentOffset.x + scrollView.frame.size.width / 2;
                
        for (UIImageView *imageV in _imageViews) {
            CGFloat distanceX = fabs(imageV.center.x - centerX); // 子控件距中心点差值
            if (distanceX < imageV.frame.size.width / 2) {
                // 0-1，差值越小（越靠近中心点），distanceScale越大
                CGFloat distanceScale = 1 - (distanceX / (imageV.frame.size.width / 2));
                // 设置中心子控件缩放比例，这里设置最多放大1.2倍，所以用1 + 0.2 * 差值比例
                CGFloat currentScale = 1 + distanceScale * 0.2;
                
                imageV.transform = CGAffineTransformMakeScale(currentScale, currentScale);
                [_myScrollView bringSubviewToFront:imageV];
            }
        }
    }
}

- (UIColor *)randomColor {
    NSInteger aRedValue = arc4random() % 255;
    NSInteger aGreenValue = arc4random() % 255;
    NSInteger aBlueValue = arc4random() % 255;
    UIColor *randColor = [UIColor colorWithRed:aRedValue / 255.0f green:aGreenValue / 255.0f blue:aBlueValue / 255.0f alpha:1.0f];
    return randColor;
}

@end