Fragment禁止预加载
项目中经常会用到ViewPager+Fragment组合,然而,有一个很让人头疼的问题就是,去加载数据的时候由于ViewPager的内部机制所限制,所以它会默认至少预加载一个。
1、既然说是ViewPager的内部机制,那么我们可不可以设置ViewPager的预加载为0?如下:
vp.setOffscreenPageLimit(0);
No,结果发现根本行不通。。。查看源码:
private static final int DEFAULT_OFFSCREEN_PAGES = 1; /** * Set the number of pages that should be retained to either side of the * current page in the view hierarchy in an idle state. Pages beyond this * limit will be recreated from the adapter when needed. * * <p>This is offered as an optimization. If you know in advance the number * of pages you will need to support or have lazy-loading mechanisms in place * on your pages, tweaking this setting can have benefits in perceived smoothness * of paging animations and interaction. If you have a small number of pages (3-4) * that you can keep active all at once, less time will be spent in layout for * newly created view subtrees as the user pages back and forth.</p> * * <p>You should keep this limit low, especially if your pages have complex layouts. * This setting defaults to 1.</p> * * @param limit How many pages will be kept offscreen in an idle state. */ public void setOffscreenPageLimit(int limit) { if (limit < DEFAULT_OFFSCREEN_PAGES) { Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " + DEFAULT_OFFSCREEN_PAGES); limit = DEFAULT_OFFSCREEN_PAGES; } if (limit != mOffscreenPageLimit) { mOffscreenPageLimit = limit; populate(); } }
所以即使你设置为0,还是会设置默认值1,所以这个方法是行不通的。
2、Fragment里面有个setUserVisibleHint(boolean isVisibleToUser)方法,所以考虑在此去处理数据。
public class LazyFragment extends Fragment { protected PullToRefreshListView mPullToRefreshListView; protected EmptyLayout mEmptyLayout; protected ListView mListView; protected int maxId = 0; //标志位,标志已经初始化完成 protected boolean isInit = false; //是否已被加载过一次,第二次不再去请求数据 protected boolean mHasLoadedOnce; private View view; @Nullable @Override public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { view = inflater.inflate(R.layout.activity_common_list, null); initView(view); return view; } public void initView(View view) { mPullToRefreshListView = (PullToRefreshListView) view.findViewById(R.id.pull_refresh_list); mEmptyLayout = (EmptyLayout) view.findViewById(R.id.error_layout); isInit = true; isCanLoadData(); } @Override public void setUserVisibleHint(boolean isVisibleToUser) { super.setUserVisibleHint(isVisibleToUser); isCanLoadData(); } /** * 禁止预加载 */ private void isCanLoadData() { if (!isInit) { return; } if (getUserVisibleHint() && !mHasLoadedOnce) { loadData(); } } protected void loadData() { //数据加载成功后
mHasLoadedOnce = true;
}
}
在loadData()中加载数据,在数据加载成功之后将mHasLoadedOnce置为true即可。以上完美解决因为Fragment的懒加载导致的,比如:预先请求服务器接口,造成服务器额外压力等问题~
By LiYing