设计用于下载和显示图像的异步过程

Question

I have a JSON feed which contains URLs for images. I am using NSURLConnection to download the JSON feed extract the URLs. I want to download all the images asynchronously. I subclassed UIImage and sent that class a URL which it downloads, one image at at a time, in an asynchronous manner.

First, is that a good way to do it? Second, I'd like to show four images at a time. Shouldn't I download every set of four together instead of downloading one by one?

My second concern is that I have two NSURLConnection s. That's probably bad. Should I use the very same NSURLConnection to download the JSON feed and at the same time get the image?

I am trying to display four images at a time, with a next button that displays the next four on the next line.

Answer 1

I am not sure UIImage is expected to be subclassed. If you need good design practice, you should have a look at TopPaid sample code from Apple, as it is showing how to properly download a feed and then asynchronously download images. Take a close look at the IconDownloader class from this project, which is a class handling image downloading, and notifying its delegate when it's finished.

As far as I know there is no problem with having multiple NSURLConnections at a time. You might run into trouble if the number of connections becomes very large, because this could saturate the number of open file descriptors allowed on iPhone, or more likely create a memory warning. In your case if you only have 2 connections, you don't have any problem.