重网上下载图片是很慢的,为了不影响体验,选择延时加载图片是很好的办法。
一个tableView 列表,左边暂时没有图
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath
{
static NSString *CellIdentifier = @"myCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle
reuseIdentifier:CellIdentifier] autorelease];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
// 设置cell一些数据
AppRecord *appRecord = [self.entries objectAtIndex:indexPath.row];
cell.textLabel.text = appRecord.appName;
cell.detailTextLabel.text = appRecord.artist;
// 如果不存在图片
if (!appRecord.appIcon)
{
if (self.tableView.dragging == NO && self.tableView.decelerating == NO)//不在拖动中和减速时,开始下载图片
{
[self startIconDownload:appRecord forIndexPath:indexPath];
}
//设置图片为空白图片(等待下载)
cell.imageView.image = [UIImage imageNamed:@"Placeholder.png"];
}
//如果有图片
else
{
cell.imageView.image = appRecord.appIcon;
}
return cell;
}
关键就是[self startIconDownload:appRecord forIndexPath:indexPath];
- (void)startIconDownload:(AppRecord *)appRecord forIndexPath:(NSIndexPath *)indexPath
{
IconDownloader *iconDownloader = [imageDownloadsInProgress objectForKey:indexPath];
if (iconDownloader == nil) //已经在下载中的不用重复下载了,没有在下载中就往下走
{
iconDownloader = [[IconDownloader alloc] init];
iconDownloader.appRecord = appRecord;
iconDownloader.indexPathInTableView = indexPath;
iconDownloader.delegate = self;
[imageDownloadsInProgress setObject:iconDownloader forKey:indexPath];
[iconDownloader startDownload];
[iconDownloader release];
}
}
IconDownloader 是一个下载图片封装类
关键方法:iconDownloader.delegate = self;
[iconDownloader startDownload];
一个是委托,将来告诉self下载完成更新图片
一个是自己方法开始联网下载图片
委托调用方法,重设图片
- (void)appImageDidLoad:(NSIndexPath *)indexPath
{
IconDownloader *iconDownloader = [imageDownloadsInProgress objectForKey:indexPath];
if (iconDownloader != nil)
{
UITableViewCell *cell = [self.tableViewcellForRowAtIndexPath:iconDownloader.indexPathInTableView];
cell.imageView.image = iconDownloader.appRecord.appIcon;
}
}
类IconDownloader 中的方法
- (void)startDownload
{
self.activeDownload = [NSMutableData data];
NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:
[NSURLRequest requestWithURL:
[NSURL URLWithString:appRecord.imageURLString]] delegate:self];
self.imageConnection = conn;
[conn release];
}
最后 NSURLConnection的委托需要自己实现了。