举例讲解iOS中延迟加载和上拉刷新/下拉加载的实现

@model类  #import <Foundation/Foundation.h>    @interface NewsItem : NSObject    @property (nonatomic,copy) NSString * newsTitle;  @property (nonatomic,copy) NSString * newsPicUrl;  @property (nonatomic,retain) UIImage * newsPic; //  存储每个新闻自己的image对象    - (id)initWithDictionary:(NSDictionary *)dic;    //  处理解析  + (NSMutableArray *)handleData:(NSData *)data;  @end      #import "NewsItem.h"  #import "ImageDownloader.h"    @implementation NewsItem    - (void)dealloc  {      self.newsTitle = nil;      self.newsPicUrl = nil;      self.newsPic = nil;      [super dealloc];  }    - (id)initWithDictionary:(NSDictionary *)dic  {      self = [super init];      if (self) {              self.newsTitle = [dic objectForKey:@"title"];          self.newsPicUrl = [dic objectForKey:@"picUrl"];                    //从本地沙盒加载图像          ImageDownloader * downloader = [[[ImageDownloader alloc] init] autorelease];          self.newsPic = [downloader loadLocalImage:_newsPicUrl];        }        return self;  }    + (NSMutableArray *)handleData:(NSData *)data;  {            //解析数据          NSError * error = nil;          NSDictionary * dic = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:&error];          NSMutableArray * originalArray = [dic objectForKey:@"news"];            //封装数据对象          NSMutableArray * resultArray = [NSMutableArray array];                for (int i=0 ;i<[originalArray count]; i++) {              NSDictionary * newsDic = [originalArray objectAtIndex:i];              NewsItem * item = [[NewsItem alloc] initWithDictionary:newsDic];              [resultArray addObject:item];              [item release];          }            return resultArray;    }    @end 

@图片下载类  #import <Foundation/Foundation.h>      @class NewsItem;      @interface ImageDownloader : NSObject      @property (nonatomic,copy) NSString * imageUrl;  @property (nonatomic,retain) NewsItem * newsItem; //下载图像所属的新闻      //图像下载完成后，使用block实现回调  @property (nonatomic,copy) void (^completionHandler)(void);      //开始下载图像  - (void)startDownloadImage:(NSString *)imageUrl;      //从本地加载图像  - (UIImage *)loadLocalImage:(NSString *)imageUrl;      @end          #import "ImageDownloader.h"  #import "NewsItem.h"      @implementation ImageDownloader      - (void)dealloc  {      self.imageUrl = nil;      Block_release(_completionHandler);      [super dealloc];  }          #pragma mark - 异步加载  - (void)startDownloadImage:(NSString *)imageUrl  {          self.imageUrl = imageUrl;          // 先判断本地沙盒是否已经存在图像，存在直接获取，不存在再下载，下载后保存      // 存在沙盒的Caches的子文件夹DownloadImages中      UIImage * image = [self loadLocalImage:imageUrl];          if (image == nil) {              // 沙盒中没有，下载          // 异步下载,分配在程序进程缺省产生的并发队列          dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{                  // 多线程中下载图像              NSData * imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageUrl]];                  // 缓存图片              [imageData writeToFile:[self imageFilePath:imageUrl] atomically:YES];                  // 回到主线程完成UI设置              dispatch_async(dispatch_get_main_queue(), ^{                      //将下载的图像，存入newsItem对象中                  UIImage * image = [UIImage imageWithData:imageData];                  self.newsItem.newsPic = image;                      //使用block实现回调，通知图像下载完成                  if (_completionHandler) {                      _completionHandler();                  }                                });                        });      }        }    #pragma mark - 加载本地图像  - (UIImage *)loadLocalImage:(NSString *)imageUrl  {        self.imageUrl = imageUrl;          // 获取图像路径      NSString * filePath = [self imageFilePath:self.imageUrl];          UIImage * image = [UIImage imageWithContentsOfFile:filePath];          if (image != nil) {          return image;      }        return nil;  }    #pragma mark - 获取图像路径  - (NSString *)imageFilePath:(NSString *)imageUrl  {      // 获取caches文件夹路径      NSString * cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];          // 创建DownloadImages文件夹      NSString * downloadImagesPath = [cachesPath stringByAppendingPathComponent:@"DownloadImages"];      NSFileManager * fileManager = [NSFileManager defaultManager];      if (![fileManager fileExistsAtPath:downloadImagesPath]) {              [fileManager createDirectoryAtPath:downloadImagesPath withIntermediateDirectories:YES attributes:nil error:nil];      }      #pragma mark 拼接图像文件在沙盒中的路径,因为图像URL有"/",要在存入前替换掉,随意用"_"代替      NSString * imageName = [imageUrl stringByReplacingOccurrencesOfString:@"/" withString:@"_"];      NSString * imageFilePath = [downloadImagesPath stringByAppendingPathComponent:imageName];          return imageFilePath;  }    @end 

@这里只给出关键代码,网络请求,数据处理,自定义cell自行解决    #pragma mark - Table view data source    - (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView  {      // Return the number of sections.      return 1;  }    - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section  {      // Return the number of rows in the section.      if (_dataArray.count == 0) {          return 10;      }      return [_dataArray count];  }    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath  {      static NSString *cellIdentifier = @"Cell";      NewsListCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier ];      if (!cell) {          cell = [[[NewsListCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];      }        NewsItem * item = [_dataArray objectAtIndex:indexPath.row];        cell.titleLabel.text = item.newsTitle;        //判断将要展示的新闻有无图像        if (item.newsPic == nil) {          //没有图像下载          cell.picImageView.image = nil;                    NSLog(@"dragging = %d,decelerating = %d",self.tableView.dragging,self.tableView.decelerating);          // ??执行的时机与次数问题          if (self.tableView.dragging == NO && self.tableView.decelerating == NO) {              [self startPicDownload:item forIndexPath:indexPath];          }        }else{          //有图像直接展示          NSLog(@"1111");          cell.picImageView.image = item.newsPic;        }            cell.titleLabel.text = [NSString stringWithFormat:@"indexPath.row = %ld",indexPath.row];        return cell;  }    - (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath  {      return [NewsListCell cellHeight];  }    //开始下载图像  - (void)startPicDownload:(NewsItem *)item forIndexPath:(NSIndexPath *)indexPath  {      //创建图像下载器      ImageDownloader * downloader = [[ImageDownloader alloc] init];        //下载器要下载哪个新闻的图像，下载完成后，新闻保存图像      downloader.newsItem = item;        //传入下载完成后的回调函数      [downloader setCompletionHandler:^{            //下载完成后要执行的回调部分，block的实现          //根据indexPath获取cell对象，并加载图像  #pragma mark cellForRowAtIndexPath-->没看到过          NewsListCell * cell = (NewsListCell *)[self.tableView cellForRowAtIndexPath:indexPath];          cell.picImageView.image = downloader.newsItem.newsPic;        }];        //开始下载      [downloader startDownloadImage:item.newsPicUrl];        [downloader release];  }      - (void)loadImagesForOnscreenRows  {  #pragma mark indexPathsForVisibleRows-->没看到过      //获取tableview正在window上显示的cell，加载这些cell上图像。通过indexPath可以获取该行上需要展示的cell对象      NSArray * visibleCells = [self.tableView indexPathsForVisibleRows];      for (NSIndexPath * indexPath in visibleCells) {          NewsItem * item = [_dataArray objectAtIndex:indexPath.row];          if (item.newsPic == nil) {              //如果新闻还没有下载图像，开始下载              [self startPicDownload:item forIndexPath:indexPath];          }      }  }    #pragma mark - 延迟加载关键  //tableView停止拖拽，停止滚动  - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate  {      //如果tableview停止滚动，开始加载图像      if (!decelerate) {            [self loadImagesForOnscreenRows];      }       NSLog(@"%s__%d__|%d",__FUNCTION__,__LINE__,decelerate);  }    - (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView  {      //如果tableview停止滚动，开始加载图像      [self loadImagesForOnscreenRows];    }

// 下拉刷新的原理  - (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView  {      if (scrollView.contentOffset.y < - 100) {                    [UIView animateWithDuration:1.0 animations:^{                            //  frame发生偏移,距离顶部150的距离(可自行设定)              self.tableView.contentInset = UIEdgeInsetsMake(150.0f, 0.0f, 0.0f, 0.0f);          } completion:^(BOOL finished) {                                        }];      }  }    // 上拉加载的原理  - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate  {            NSLog(@"%f",scrollView.contentOffset.y);      NSLog(@"%f",scrollView.frame.size.height);      NSLog(@"%f",scrollView.contentSize.height);            if (scrollView.contentOffset.y + scrollView.frame.size.height >= scrollView.contentSize.height) {                    NSLog(@"%d %s",__LINE__,__FUNCTION__);          [UIView commitAnimations];                    [UIView animateWithDuration:1.0 animations:^{              //  frame发生的偏移量,距离底部往上提高60(可自行设定)              self.tableView.contentInset = UIEdgeInsetsMake(0, 0, 60, 0);          } completion:^(BOOL finished) {                                      }];        }  }