问题背景：

需要在UITableView中的每一行下载图片，之前使用placeholder，下载好后存在cache中。

解决方案：

方案一：

使用SDWebImage：https://github.com/rs/SDWebImage

如何安装及使用在git页面有详细解释，具体使用的代码：

#import <SDWebImage/UIImageView+WebCache.h>

...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *MyIdentifier = @"MyIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

    if (cell == nil)
    {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
                                       reuseIdentifier:MyIdentifier] autorelease];
    }

    // Here we use the new provided setImageWithURL: method to load the web image
    [cell.imageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
                   placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

    cell.textLabel.text = @"My Text";
    return cell;
}

方案二：

使用iOS自有的异步多线程，下载好图片后存入cache，再使用delegate函数更新tableview：

- (void)reloadRowsAtIndexPaths:(NSArray *)indexPaths withRowAnimation:(UITableViewRowAnimation)animation

@interface ViewController ()

@property (strong, nonatomic)  NSMutableArray *imageArray;
@property (strong, nonatomic) NSCache *imageCache;
@property (weak, nonatomic) IBOutlet UITableView *myCustomTableView;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.imageCache = [[NSCache alloc] init];
}

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    // Number of rows is the number of time zones in the region for the specified section.
    return 32;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSLog(@"cellForRowAtIndexPath %d", indexPath.row);
    
    static NSString *MyIdentifier = @"customIdentifier";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
    if (cell == nil) {
        cell = [[CustomTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault  reuseIdentifier:MyIdentifier];
    }
    
    if ([cell isKindOfClass:[CustomTableViewCell class]])
    {
        CustomTableViewCell *customCell = (CustomTableViewCell *)cell;
        NSData *imageData = [self.imageCache objectForKey:[NSString stringWithFormat:@"%d", indexPath.row]];
        
        if(imageData != nil){
            customCell.customImageView.image = [UIImage imageWithData:imageData];
        }
        else{
            customCell.customImageView.image = [UIImage imageNamed:@"test1"];
            
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long) NULL), ^(void){
                UIImage *image = [self getPictureFromServer];
                
                if (image != nil) {
                    NSData *tmpImageData = UIImagePNGRepresentation(image);
                    [self.imageCache setObject:tmpImageData forKey:[NSString stringWithFormat:@"%d", indexPath.row]];
                    NSLog(@"download image for %d", indexPath.row);
                    
                    [self performSelectorOnMainThread:@selector(reloadTableViewDataAtIndexPath:) withObject:indexPath waitUntilDone:NO];
                }
                else{
                    // do nothing ..
                }
            });
        }
    }
    else{
        // do nothing ..
    }

    return cell;
}

- (void) reloadTableViewDataAtIndexPath: (NSIndexPath *)indexPath{
    NSLog(@"MyWineViewController: in reload collection view data for index: %d", indexPath.row);

    NSArray* indexArray = [NSArray arrayWithObjects:indexPath, nil];
    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationFade];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationAutomatic];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationBottom];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationLeft];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationMiddle];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationNone];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationRight];
//    [self.myCustomTableView reloadRowsAtIndexPaths:indexArray withRowAnimation:UITableViewRowAnimationTop];
}

下载图片

- (UIImage *) getPictureFromServer{
    UIImage *image = nil;
    NSString *urlString = [NSString stringWithFormat:@"http://yourPicture<span style="font-family: Arial, Helvetica, sans-serif;">.</span><span style="font-family: Arial, Helvetica, sans-serif;">jpg"];</span>
    
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
    [request setURL:[NSURL URLWithString:urlString]];
    [request setHTTPMethod:@"GET"];
    
    NSURLResponse *response;
    NSError *connectionError;
    NSData *returnData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&connectionError];

    if (connectionError == NULL) {
        NSInteger statusCode = [(NSHTTPURLResponse *)response statusCode];
        NSLog(@"getPictureFromServer - Response Status code: %ld", (long)statusCode);
        
        if (statusCode == 200) {
            image = [UIImage imageWithData:returnData];
        }
        else{
            // do nothing..
        }
    }
    else{
        // do nothing..
    }
    
    return image;
}

在方案二的基础上，变动在于下载好图片后load整个tableview，缺点时会引起滑动tableview时阻塞。

- (void) reloadTableViewData{
    NSLog(@"MyWineViewController: in reload collection view data");
    [self.myCustomTableView reloadData];
}

http://stackoverflow.com/questions/15290974/uitableview-on-screen-image-download-lazy-loading

http://stackoverflow.com/questions/7040740/reload-spefic-uitableview-cell-ios

iOS UITableView中异步加载图片 － 解决方案