前言

講下拉刷新及上拉加載之前先給大家解釋UIScrollView的幾個屬性

• contentSize是UIScrollView可以滾動的區域。
• contentOfinset 蘋果官方文檔的解釋是"內容視圖嵌入到封閉的滾動視圖的距離，我的理解是他實際上就是scrollView的Content View相對於scrollView的外殼的邊距，他其實和CSS中的pading屬性有點相似。
• contentOffset是UIScrollView當前顯示區域的頂點相對於frame頂點的偏移量，例如上面的例子如果拉到最下面，則contentOffset就是(0, 480)，也就是y偏移了480.

下拉刷新及上拉加載原理

• 下拉刷新實際上是監測UIScrollView的contentOffset的y值，當他往下拉動時，UIScrollView的contentOffset是一直減小的，然後把通過動畫把它的contentInset值控制成一個定值，然後通過設置延時來把UIScrollView的contentInset的值恢復原點。啥話不說，上代碼

//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(@"發起下拉刷新");
            
            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);

                }];
                
            });
            
        }];
    
    }

• 上拉加載其實原理和下拉刷新基本是一樣的，只不過判斷的contentOffset的值不同，如果scrollView.bounds.size.height +  scrollView.contentOffset.y >scrollView.contentSize.height，說明你執行了上拉操作，然後實現起來基本就和下拉刷新是一樣的。

//上拉加載
    
    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(@"發起上拉加載");
            
            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代碼，大家看不懂的可以私聊我，要是想深入了解的話可以去github搜索MJRefresh看完整的別人封裝好的第三方

#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是可滑動的區域
        _scrollView.contentSize = CGSizeMake(0, 900);
        _scrollView.backgroundColor = [UIColor grayColor];
        _scrollView.delegate = self;
        
        //greenView實際上就是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-當下拉刷新時，contentOffset實際上就是greenView相對於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(@"發起下拉刷新");
            
            //設置延時時間為2秒
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                
                [UIView animateWithDuration:1.0 animations:^{
                    //恢復之前的contentInset，讓greenView回到原來的地方
                    self.scrollView.contentInset = UIEdgeInsetsMake(0, 0, 0, 0);

                }];
                
            });
            
        }];
    
    }
    
    //上拉加載
    
    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(@"發起上拉加載");
            
            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