Loading and removing ViewPager pages_html/css_WEB-ITnose

Today I used ViewPager when making carousel image ads, and explored the loading and removal of pages when it slides.

There are destroyItem method and instantiateItem method in PagerAdapter, the former is used to remove the page, and the latter is used to load the page.

[Note] If the current page is page 0, the program will not call the instantiateItem method; similarly, if When the current page is views.size() (the number of views loaded in ViewPager) - 1, the instantiateItem method will not be called.

You can write a simple ViewPager demo (such as a navigation page) for observation and use Logcat to print information.

(If you don’t know how to write a navigation page yet, please read ViewPager to implement a navigation page)

Information output in the destroyItem method:

Log.d("MainActivity", "Remove page" position);

Information output in the instantiateItem method:

Log.d( "MainActivity", "Loading page" position);

Information output in onPageSelected:

Log.d("MainActivity", "Current selected page" arg0) ;

Then I slide the navigation page from left to right, and from right to left (note: there are 5 pages in total):

Information printing:

03-22 15:22:45.713: D/MainActivity(4835): 加载页面 003-22 15:22:45.713: D/MainActivity(4835): 加载页面 103-22 15:22:48.768: D/MainActivity(4835): 当前选择页面 103-22 15:22:49.408: D/MainActivity(4835): 加载页面 203-22 15:22:51.558: D/MainActivity(4835): 当前选择页面 203-22 15:22:52.198: D/MainActivity(4835): 移除页面 003-22 15:22:52.203: D/MainActivity(4835): 加载页面 303-22 15:22:53.988: D/MainActivity(4835): 当前选择页面 303-22 15:22:54.508: D/MainActivity(4835): 移除页面 103-22 15:22:54.508: D/MainActivity(4835): 加载页面 403-22 15:22:55.578: D/MainActivity(4835): 当前选择页面 403-22 15:22:55.928: D/MainActivity(4835): 移除页面 203-22 15:22:58.958: D/MainActivity(4835): 当前选择页面 303-22 15:22:59.503: D/MainActivity(4835): 加载页面 203-22 15:23:01.288: D/MainActivity(4835): 当前选择页面 203-22 15:23:01.928: D/MainActivity(4835): 加载页面 103-22 15:23:01.928: D/MainActivity(4835): 移除页面 403-22 15:23:02.613: D/MainActivity(4835): 当前选择页面 103-22 15:23:03.243: D/MainActivity(4835): 加载页面 003-22 15:23:03.248: D/MainActivity(4835): 移除页面 303-22 15:23:05.138: D/MainActivity(4835): 当前选择页面 003-22 15:23:05.773: D/MainActivity(4835): 移除页面 2

When entering the program, page 0 to be displayed is loaded, then page 1 is preloaded, and the instantiateItem method is called twice.

Naturally, you have to call it twice when you first enter. First, you have to load your own picture, and then you have to load the next picture.

At this time, of course, page No. 2 is preloaded.

Remove page 0 and load page 3

Remove page 1 and load page 4

Remove page 2 without preloading Because it's the end.

Forward dialing completed.

At this point, we can also see the general rules.

Every time you slide, you will retain the two pages adjacent to the current page (preload the page that has not been loaded, and save the page that has not been loaded). loaded page), remove the page that is not adjacent but has been loaded, that is to say, only three pages exist at a time.

The same principle applies to reverse toggle. There is no need to draw the schematic diagram of reverse toggle. Everyone should understand it here.

After knowing this sliding feature, we can use ViewPager more flexibly.

Okay, let’s talk about this for today. If there is anything wrong, please criticize and correct me!