tableviewcell里面imageview在下载图片之前显示loading，下载后显示指定图片

在tableView:cellForRowAtIndexPath委托里面定义一个 UIActivityIndicatorView 添加到cell.imageview中并启动。

UIActivityIndicatorView *activeIndicatorView=[[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];

[cell.imageView addSubview:activeIndicatorView];//[UIImage imageNamed:@"process.png"];

[activeIndicatorView release];

[[cell.imageView.subviews lastObject]  startAnimating];

[[cell.imageView.subviews lastObject] setFrame:CGRectMake(50, 75,50,50)];

cell.imageView.image = [UIImage imageNamed:@"bg.jpg"];//设置一个default图片，要不然activityindicator不会显示

在下载完成图片的事件中移除subview，指定下载后的图片显示

移除添加uiactiveityindicatorview

[[cell.imageView.subviews lastObject] removeFromSuperview];

或者移除所有subview

[[cell.imageView subviews] makeObjectsPerformSelector:@selector(removeFromSuperview)];