iOS UITableView中异步加载图片 － 解决方案

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

初始化：

@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];}

TableView delegate函数：

- (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;}

reloadTableViewDataAtIndexPath：

- (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];}

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;}

方案三：（不推荐）

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];}

参考链接：

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

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