说说写这篇博客的背景吧,前两天去面试,问到一个问题说的是:比如我们首页,是有3个fragment构成的,并且要是实现作用可以滑,那么这个最好的选择就是ViewPager+fragment了,但是我们知道ViewPager+fragment是做预加载的,然而这种方法有一个坏处,当前页面和预加载页面都有大量的网络请求,可能就会比较慢,这样就会造成不好打体验。其实我们忽略的一个问题,Android的fragment里面已经帮我们提供了一个方法setUserVisibleHint。setUserVisibleHint()来显示与隐藏Fragment的。
我们看一下系统的代码:
public void setUserVisibleHint(boolean isVisibleToUser) {
if (!mUserVisibleHint && isVisibleToUser && mState < STARTED) {
mFragmentManager.performPendingDeferredStart(this);
}
mUserVisibleHint = isVisibleToUser;
mDeferStart = !isVisibleToUser;
}
但是setUserVisibleHint优于onCreate调用,所以当onCreate调用完毕setUserVisibleHint就不会触发,这时需要在首个显示的fragment调用setUserVisibleHint方法。
所以我们在第一个Fragment创建成功之后,需要设置下下一个页面是否展示
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub
setUserVisibleHint(true);
super.onActivityCreated(savedInstanceState);
}
为了方便我们使用我们可以继承fragment重写一下setUserVisibleHint方法。
public abstract class BasePageFragment extends Fragment {
protected boolean isViewInitiated;
protected boolean isVisibleToUser;
protected boolean isDataInitiated;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
isViewInitiated = true;
prepareFetchData();
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
this.isVisibleToUser = isVisibleToUser;
prepareFetchData();
}
public abstract void fetchData();
public boolean prepareFetchData() {
return prepareFetchData(false);
}
public boolean prepareFetchData(boolean forceUpdate) {
if (isVisibleToUser && isViewInitiated && (!isDataInitiated || forceUpdate)) {
fetchData();
isDataInitiated = true;
return true;
}
return false;
}
}
说下这里关于prepareFetchData的判断逻辑:当前UI可见,并且fragment已经初始化完毕,如果网络数据未加载,那么请求数据,或者需要强制刷新页面,那么再去请求页面数据,这样就达到了本文开始需要的要求。
用的话也听方便的:
public class PageFragment extends BasePageFragment {
public static PageFragment newInstance(String title){
PageFragment fragment = new PageFragment();
Bundle args = new Bundle();
args.putString("key_fragment_title", title);
fragment.setArguments(args);
return fragment;
}
private String title;
private TextView tv;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
title = getArguments().getString("key_fragment_title");
Trace.d(title + ":onCreate");
}
@Override
public void onResume() {
super.onResume();
Trace.d(title + ":onResume");
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
Trace.d(title + ":onCreateView");
tv = new TextView(getActivity());
return tv;
}
@Override
public void fetchData() {<pre code_snippet_id="1723966" snippet_file_name="blog_20160621_4_911600" name="code" class="html"> //这里是网络异步请求返回
tv.setText(title); }}
}
通过setOffscreenPageLimit进行Fragment缓存
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();
}
}
好了,最近面试面到的比较坑的问题还蛮多(比如Intent底层实现,Looper底层算法,Ams等),借此机会好好看下源码,欢迎留言。