预期效果
如图1所示:
UIScrollView进行嵌套组合，最外层是一个UIScrollView，为了说明方便，称为父UIScrollView
里面嵌套着头部区、悬停区和一个UIScrollView，称为子UIScrollView
要求头部区展示在屏幕时，滚动内容为父UIScrollView，一旦头部区域消失在屏幕中时，这个时候滚动的内容为子UIScrollView，悬停区悬停在屏幕顶部。如图2 所示
解决方案
方案1
通过监听父UIScrollView的contentOffset，如果偏移量小于于头部区的高度，则设置父UIScrollView的scrollEnabled为YES，设置子UIScrollView的scrollEnable为NO。如果偏移量大于头部区的高度，则设置父UIScrollView的scrollEnabled为NO，设置子UIScrollView的scrollEnable为YES.
这个方案存在最大的问题是一旦快速滑动屏幕，到临界值那边会卡住，需要重新施加一次滑动手势，才能继续滚动。
方案2
让一次手势同时拥有多个响应者，即一次滑动手势无论子UIScrollView或者父UIScrollView都能作为响应者进行滚动，然后我们根据自己的需要进行重置不需要的滚动效果。
要达到手势能够被多个UIView进行响应，需要在父UIScrollView实现 UIGestureRecognizerDelegate 代理方法
//支持手势同步到其他接收者
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
return YES;
}
在他们所在的父View或ViewController中实现 UIScrollViewDelegate 的
-(void)scrollViewDidScroll:(UIScrollView *)scrollView
默认只能滚动父UIScrollView，子UIScrollView在未被允许滚动的情况，偏移量会被重置为0。 比对父UIScrollView的偏移量和头部区的高度，如果偏移量大于等于头部区的高度，则开始重置父头部区的高度的偏移量为头部区高度，子UIScrollView不再重置偏移量为0.
具体代码实现
定义一个父UIScrollView为 FatherScrollView
// FatherScrollView.h
#import <UIKit/UIKit.h>
@interface FatherScrollView : UIScrollView<UIGestureRecognizerDelegate>
@end
//FatherScrollView.m
#import "FatherScrollView.h"
@implementation FatherScrollView
//支持手势同步到其他接收者
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
return YES;
}
@end
定义一个ViewController进行内容展示
#import "ViewController.h"
#import "FatherScrollView.h"
@interface ViewController ()<UIScrollViewDelegate>
//父UIScrollView
@property (nonatomic, strong) FatherScrollView *fatherscrollView;
//头部区
@property (nonatomic, strong) UIView *headerView;
//子UIScrollView
@property (nonatomic, strong) UIScrollView *childScrollView;
//父UIScrollView是否能滚动
@property (nonatomic, assign) BOOL enableFatherViewScroll;
//子UIScrollView是否能滚动
@property (nonatomic, assign) BOOL enableChildViewScroll;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = UIColor.whiteColor;
self.enableChildViewScroll = NO;
self.enableFatherViewScroll = YES;
self.automaticallyAdjustsScrollViewInsets = NO;
self.fatherscrollView = [[FatherScrollView alloc] initWithFrame:self.view.bounds];
self.fatherscrollView.delegate = self;
self.fatherscrollView.bounces = YES;
self.fatherscrollView.backgroundColor = UIColor.grayColor;
self.fatherscrollView.contentSize = CGSizeMake(0, self.view.bounds.size.height+200);
[self.view addSubview:self.fatherscrollView];
self.headerView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, 200)];
self.headerView.backgroundColor = UIColor.greenColor;
[self.fatherscrollView addSubview:self.headerView];
UILabel *label = [[UILabel alloc] init];
label.text = @"悬浮标题";
label.textAlignment = NSTextAlignmentCenter;
label.frame = CGRectMake(0, 160, self.view.bounds.size.width, 40);
label.backgroundColor = UIColor.whiteColor;
[self.headerView addSubview:label];
self.childScrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 200, self.view.bounds.size.width, self.view.bounds.size.height)];
self.childScrollView.delegate = self;
self.childScrollView.contentSize = CGSizeMake(0, self.view.bounds.size.height * 1.5);
self.childScrollView.backgroundColor = UIColor.redColor;
[self.fatherscrollView addSubview:self.childScrollView];
UILabel *contentLabel = [[UILabel alloc] init];
contentLabel.font = [UIFont systemFontOfSize:50];
contentLabel.text = @"子滚动内容";
contentLabel.textAlignment = NSTextAlignmentCenter;
contentLabel.frame = CGRectMake(0, 0, self.view.bounds.size.width, self.view.bounds.size.height);
[self.childScrollView addSubview:contentLabel];
}
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
CGFloat contentOffset = 160 - [UIApplication sharedApplication].statusBarFrame.size.height;
if (scrollView == self.fatherscrollView) {
if (!self.enableFatherViewScroll) {
scrollView.contentOffset = CGPointMake(0, contentOffset);
self.enableChildViewScroll = YES;
} else {
if (scrollView.contentOffset.y >= contentOffset) {
scrollView.contentOffset = CGPointMake(0, contentOffset);
if (self.enableFatherViewScroll) {
self.enableFatherViewScroll = NO;
self.enableChildViewScroll = YES;
}
}
}
} else {
if (!self.enableChildViewScroll) {
scrollView.contentOffset = CGPointMake(0, 0);
} else {
if (scrollView.contentOffset.y <= 0) {
self.enableChildViewScroll = NO;
self.enableFatherViewScroll = YES;
}
}
}
}
@end
开发中的问题
1、这种嵌套的设计，往往会包含有多个子UIScrollView并且可以进行左右滑动切换，子UIScrollView区域往往会当独封闭，如果都在UIViewController这边进行处理的话，会有大量的耦合代码出现
2、不得不为父UIScrollView创建一个新的UIScrollView在子类，实现 (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer
针对上面问题，我这边对UIScrollView嵌套方案进行了简要的封装，方便开发中直接使用
Demo地址：CCNestScrollView
