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