iOS 下拉刷新-上拉加载原理

前言

讲下拉刷新及上拉加载之前先给大家解释UIScrollView的几个属性

• contentSize是UIScrollView可以滚动的区域。
• contentOfinset 苹果官方文档的解释是"内容视图嵌入到封闭的滚动视图的距离，我的理解是他实际上就是scrollView的Content View相对于scrollView的外壳的边距，他其实和CSS中的pading属性有点相似。
• contentOffset是UIScrollView当前显示区域的顶点相对于frame顶点的偏移量，例如上面的例子如果拉到最下面，则contentOffset就是(0, 480)，也就是y偏移了480.

下拉刷新及上拉加载原理

• 下拉刷新实际上是监测UIScrollView的contentOffset的y值，当他往下拉动时，UIScrollView的contentOffset是一直减小的，然后把通过动画把它的contentInset值控制成一个定值，然后通过设置延时来把UIScrollView的contentInset的值恢复原点。啥话不说，上代码

//下拉刷新
  
    if (scrollView.contentOffset.y < -100) {
        
        [UIView animateWithDuration:1.0 animations:^{
            
            self.scrollView.contentInset = UIEdgeInsetsMake(100, 0, 0, 0);
            
        } completion:^(BOOL finished) {
            
            NSLog(@"发起下拉刷新");
            
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                
                [UIView animateWithDuration:1.0 animations:^{
                    
                    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);

                }];
                
            });
            
        }];
    
    }

• 上拉加载其实原理和下拉刷新基本是一样的，只不过判断的contentOffset的值不同，如果scrollView.bounds.size.height +  scrollView.contentOffset.y >scrollView.contentSize.height，说明你执行了上拉操作，然后实现起来基本就和下拉刷新是一样的。

//上拉加载
    
    if (scrollView.bounds.size.height +  scrollView.contentOffset.y >scrollView.contentSize.height) {
        
        [UIView animateWithDuration:1.0 animations:^{
            
            self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 50, 0);
            
        } completion:^(BOOL finished) {
            
            NSLog(@"发起上拉加载");
            
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                
                [UIView animateWithDuration:1.0 animations:^{
                    
                    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);
                    
                }];
            });
        }];
        
        
    }

下面奉上完整的demo代码，大家看不懂的可以私聊我，要是想深入了解的话可以去github搜索MJRefresh看完整的别人封装好的第三方

#import "ViewController.h"

@interface ViewController ()<UIScrollViewDelegate>

@property (nonatomic, strong) UIScrollView * scrollView;

@end

@implementation ViewController

- (UIScrollView *)scrollView {
    
    if (!_scrollView) {
        _scrollView = [[UIScrollView alloc] initWithFrame:self.view.bounds];
        
        //contentSize是可滑动的区域
        _scrollView.contentSize = CGSizeMake(0, 900);
        _scrollView.backgroundColor = [UIColor grayColor];
        _scrollView.delegate = self;
        
        //greenView实际上就是UIScrollView的content View
        UIView * greenView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, 900)];
        greenView.backgroundColor = [UIColor greenColor];
        [_scrollView addSubview:greenView];
        
    }
    return _scrollView;
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
    
    
    //下拉刷新-当下拉刷新时，contentOffset实际上就是greenView相对于屏幕左上角的偏移量。
    if (scrollView.contentOffset.y < -100) {
        
        [UIView animateWithDuration:1.0 animations:^{
            
            self.scrollView.contentInset = UIEdgeInsetsMake(100, 0, 0, 0);
            
        } completion:^(BOOL finished) {
            
            NSLog(@"发起下拉刷新");
            
            //设置延时时间为2秒
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                
                [UIView animateWithDuration:1.0 animations:^{
                    //恢复之前的contentInset，让greenView回到原来的地方
                    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);

                }];
                
            });
            
        }];
    
    }
    
    //上拉加载
    
    if (scrollView.bounds.size.height +  scrollView.contentOffset.y >scrollView.contentSize.height) {
        
        [UIView animateWithDuration:1.0 animations:^{
            
            self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 50, 0);
            
        } completion:^(BOOL finished) {
            
            NSLog(@"发起上拉加载");
            
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                
                [UIView animateWithDuration:1.0 animations:^{
                    
                    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);
                    
                }];
            });
        }];
        
        
    }
    
}



- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self.view addSubview:self.scrollView];
    
    
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end