常见的一些UIScrollViewDelegate代理回调可以参考https://www.cnblogs.com/cchHers/p/12423838.html，下面列出的主要是一些特殊情况。
1. scrollViewWillBeginDecelerating和scrollViewDidEndDecelerating并不是必须的
你可以尝试使用后附的代码在模拟器或手机上进行测试：当你手指接触屏幕并拖动一段距离，稍作停顿再松手，你会发现只执行到scrollViewWillEndDragging和scrollViewDidEndDragging就停止了。
因此，除非你确定你的滑动列表一定会有减速动画（比如，启用了Paging且保证用户不会精准停留至无需结束弹性动画的位置），你可以尝试以scrollViewWillBeginDecelerating或scrollViewDidEndDecelerating作为滑动结束的标志，否则请不要将这两个回调作为唯一结束标志。
2. 区别手指滑动和命令滑动
如果你通过命令控制UIScrollView的contentOffset，比如
[self.collectionView setContentOffset:CGPointZero animated:YES];你需要注意，通过setContentOffset方式进行滚动时会有下面几种情况
| Animated: YES | Animated: NO | |
| ContentOffset与上次相同 | 无 | 无 |
| ContentOffset与上次不同 | [scrollViewDidScroll] 多次 [scrollViewDidEndScrollingAnimation] 一次 | [scrollViewDidScroll] 一次 |
你应该注意到涉及到手指滑动的一些回调一律不会被调用，只会涉及到scrollViewDidScroll和scrollViewDidEndScrollingAnimation这两种。如果你的App中存在通过命令滚动的情况则需要特别关心。
3. 轻点Status Bar至顶
点击Status Bar是一种比较tricky的操作，用户可以通过这种方式快速滚动至页面的顶端，它只会调用scrollViewDidScroll和scrollViewDidScrollToTop这两种，并且scrollViewDidScrollToTop的触发时机是在滚动结束后，类似于scrollViewDidEndDecelerating。在一些需要对用户滑动监控的场景下（比如回到顶部时刷新信息流）同样需要进行特别关注，因为在这种场景下监听scrollViewDidEndDecelerating是无效的。
附录
可以使用下面的代码自行测试
#import "UIKit/UIkit.h"
#import "ViewController.h"
@interface ViewController () <UICollectionViewDelegate, UICollectionViewDataSource, UIScrollViewDelegate>
@property (nonatomic, strong) UITextView *logTextView;
@property (nonatomic, strong) UICollectionView *collectionView;
@property (nonatomic, strong) NSMutableArray *logMessages;
@property (nonatomic, strong) UIButton *backToTopAnimatedButton;
@property (nonatomic, strong) UIButton *backToTopNoAnimationButton;
@property (nonatomic, strong) UIButton *scrollToRandomOffsetAnimatedButton;
@property (nonatomic, strong) UIButton *scrollToRandomOffsetNoAnimationButton;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Setup logTextView
self.logTextView = [[UITextView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, 300)];
self.logTextView.editable = NO;
[self.view addSubview:self.logTextView];
// Initialize logMessages array
self.logMessages = [NSMutableArray array];
// Setup UICollectionView
UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
layout.itemSize = CGSizeMake(100, 100); // Set the size of each cell
self.collectionView = [[UICollectionView alloc] initWithFrame:CGRectMake(0, 300, self.view.frame.size.width, self.view.frame.size.height - 300) collectionViewLayout:layout];
self.collectionView.delegate = self;
self.collectionView.dataSource = self;
[self.collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"cell"];
// [self.collectionView setPagingEnabled:YES];
[self.view addSubview:self.collectionView];
// Setup buttons
[self setupButtons];
}
// UICollectionView DataSource
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
return 100;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];
UIImageView *imageView = [[UIImageView alloc] initWithFrame:cell.contentView.bounds];
imageView.image = [UIImage imageNamed:@"example_image"]; // Use your image name here
[cell.contentView addSubview:imageView];
return cell;
}
// UIScrollViewDelegate Methods
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewDidScroll]"];
}
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewWillBeginDragging]"];
}
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset {
[self logMessage:@"[scrollViewWillEndDragging]"];
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
[self logMessage:@"[scrollViewDidEndDragging]"];
}
- (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewShouldScrollToTop]"];
return YES;
}
- (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewDidScrollToTop]"];
}
- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewWillBeginDecelerating]"];
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewDidEndDecelerating]"];
}
- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView {
[self logMessage:@"[scrollViewDidEndScrollingAnimation]"];
}
// Helper method to log messages
- (void)logMessage:(NSString *)message {
[self.logMessages addObject:message];
if (self.logMessages.count > 20) {
[self.logMessages removeObjectAtIndex:0];
}
NSString *logText = [self.logMessages componentsJoinedByString:@"\n"];
self.logTextView.text = logText;
}
// Setup buttons
- (void)setupButtons {
CGFloat buttonWidth = self.view.frame.size.width / 2;
CGFloat buttonHeight = 50;
self.backToTopAnimatedButton = [self createButtonWithTitle:@"Back to Top (Animated)" action:@selector(backToTopAnimated)];
self.backToTopAnimatedButton.frame = CGRectMake(0, self.view.frame.size.height - 100, buttonWidth, buttonHeight);
[self.view addSubview:self.backToTopAnimatedButton];
self.backToTopNoAnimationButton = [self createButtonWithTitle:@"Back to Top (No Animation)" action:@selector(backToTopNoAnimation)];
self.backToTopNoAnimationButton.frame = CGRectMake(buttonWidth, self.view.frame.size.height - 100, buttonWidth, buttonHeight);
[self.view addSubview:self.backToTopNoAnimationButton];
self.scrollToRandomOffsetAnimatedButton = [self createButtonWithTitle:@"Random Offset (Animated)" action:@selector(scrollToRandomOffsetAnimated)];
self.scrollToRandomOffsetAnimatedButton.frame = CGRectMake(0, self.view.frame.size.height - 50, buttonWidth, buttonHeight);
[self.view addSubview:self.scrollToRandomOffsetAnimatedButton];
self.scrollToRandomOffsetNoAnimationButton = [self createButtonWithTitle:@"Random Offset (No Animation)" action:@selector(scrollToRandomOffsetNoAnimation)];
self.scrollToRandomOffsetNoAnimationButton.frame = CGRectMake(buttonWidth, self.view.frame.size.height - 50, buttonWidth, buttonHeight);
[self.view addSubview:self.scrollToRandomOffsetNoAnimationButton];
}
// Create a button with title and action
- (UIButton *)createButtonWithTitle:(NSString *)title action:(SEL)action {
UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
[button setTitle:title forState:UIControlStateNormal];
[button addTarget:self action:action forControlEvents:UIControlEventTouchUpInside];
button.backgroundColor = [UIColor lightGrayColor];
[button setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
return button;
}
// Button Actions
- (void)backToTopAnimated {
[self.collectionView setContentOffset:CGPointZero animated:YES];
[self logMessage:@"Back to top with animation"];
}
- (void)backToTopNoAnimation {
[self.collectionView setContentOffset:CGPointZero animated:NO];
[self logMessage:@"Back to top without animation"];
}
- (void)scrollToRandomOffsetAnimated {
CGFloat maxOffsetY = self.collectionView.contentSize.height - self.collectionView.frame.size.height;
CGFloat randomOffsetY = arc4random_uniform((unsigned int)maxOffsetY);
[self.collectionView setContentOffset:CGPointMake(0, randomOffsetY) animated:YES];
[self logMessage:[NSString stringWithFormat:@"Scrolled to random offset (%.2f) with animation", randomOffsetY]];
}
- (void)scrollToRandomOffsetNoAnimation {
CGFloat maxOffsetY = self.collectionView.contentSize.height - self.collectionView.frame.size.height;
CGFloat randomOffsetY = arc4random_uniform((unsigned int)maxOffsetY);
[self.collectionView setContentOffset:CGPointMake(0, randomOffsetY) animated:NO];
[self logMessage:[NSString stringWithFormat:@"Scrolled to random offset (%.2f) without animation", randomOffsetY]];
}
@end