最近在做一个需求，统一管理列表页面的预加载和统计上报。简单点说，就是用户在滚动列表页时，当列表停止滚动时，请求当前可见的内容的详情接口，同时将当前可见内容的标题、id等信息上报埋点统计。

直接说结果，过程就不讲了

直接勾UIScrollView的滚动停止事件，

[UIScrollView _scrollViewDidEndDraggingForDelegateWithDeceleration:]

[UIScrollView _scrollViewDidEndDeceleratingForDelegate]

准备一个Category将这两个方法替换了，添加我们自己的逻辑就OK了

完整代码：

UIScrollView+JSBCLibrary.h

#import <UIKit/UIKit.h>

NS_ASSUME_NONNULL_BEGIN

@interface UIScrollView (JSBCLibrary)

@end

NS_ASSUME_NONNULL_END

UIScrollView+JSBCLibrary.m

#import "UIScrollView+JSBCLibrary.h"
#import <objc/runtime.h>

@implementation UIScrollView (JSBCLibrary)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        SEL originalSelector = NSSelectorFromString(@"_scrollViewDidEndDeceleratingForDelegate");
        SEL swizzledSelector = @selector(jsbc_scrollViewDidEndDeceleratingForDelegate);

        [self exchangeMethod:originalSelector swizzledSelector:swizzledSelector];
        
        SEL originalSelector1 = NSSelectorFromString(@"_scrollViewDidEndDraggingForDelegateWithDeceleration:");
        SEL swizzledSelector1 = @selector(jsbc_scrollViewDidEndDraggingForDelegateWithDeceleration:);

        [self exchangeMethod:originalSelector1 swizzledSelector:swizzledSelector1];
        
    });
    
}

+ (void)exchangeMethod:(SEL)originalSelector swizzledSelector:(SEL)swizzledSelector {
    Class class = [self class];

    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
    
    BOOL didAddMethod = class_addMethod(class,
                                        originalSelector,
                                        method_getImplementation(swizzledMethod),
                                        method_getTypeEncoding(swizzledMethod));

    if (didAddMethod) {
        class_replaceMethod(class,
                            swizzledSelector,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

- (void)jsbc_scrollViewDidEndDraggingForDelegateWithDeceleration:(BOOL)deceleration {
    [self jsbc_scrollViewDidEndDraggingForDelegateWithDeceleration:deceleration];
    if (!deceleration) {
        [self jsbcDidEndScroll];
        NSLog(@"------- 停止了手指拖动 %@",self);
    }
}

- (void)jsbc_scrollViewDidEndDeceleratingForDelegate {
    [self jsbc_scrollViewDidEndDeceleratingForDelegate];
    [self jsbcDidEndScroll];
    NSLog(@"------- 停止了惯性滚动 %@",self);
}

- (void)jsbcDidEndScroll {
    
    if ([self isKindOfClass:UITableView.class]) {
        UITableView *tableView = (UITableView *)self;
        [[tableView visibleCells] enumerateObjectsUsingBlock:^(__kindof UITableViewCell * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
            NSIndexPath *indexPath = [tableView indexPathForCell:obj];
            //通过indexPath获取当前可见区域cell坐标
            //TODO:在这里写自己的逻辑
            
         }];
    }
    else if ([self isKindOfClass:UICollectionView.class]) {
        UICollectionView *collectionView = (UICollectionView *)self;
        [[collectionView visibleCells] enumerateObjectsUsingBlock:^(__kindof UICollectionViewCell * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
            NSIndexPath *indexPath = [collectionView indexPathForCell:obj];
            //通过indexPath获取当前可见区域cell坐标
            //TODO:在这里写自己的逻辑
            
         }];
    }
}

@end

备注说明

• 这里不适合hookUIScrollView的代理方法，因为代理是一对一的，业务层会受影响
• 如果有小伙伴在Hook_scrollViewDidEndDraggingForDelegateWithDeceleration:方法后，发现运行奔溃，很有可能是因为参数修饰符出现了问题，这里不能用id而应该用BOOL
• jsbc_scrollViewDidEndDraggingForDelegateWithDeceleration:(BOOL)deceleration中的deceleration值为NO时，才是停止手指拖动