Why bother lazy loading and purging pages, you ask? Well, in this example, it won’t matter too much if you load all the pages at the start, since there are only five and they won’t be large enough to eat up too much memory. But imagine you had 100 pages and each image was 5MB in size. That would take up 500MB of memory if you loaded all the pages at once! Your app would quickly exceed the amount of memory available and be killed by the operating system. Lazy loading means that you’ll only have a certain number of pages in memory at any given time.
原文：http://blog.csdn.net/leochang130731/article/details/46415329