关于fragment懒加载文章，网上有很多，大家可根据自己项目的情况来选择，在此总结一下我在项目中用到的方法。
为什么使用fragment懒加载？
ViewPager默认加载当前页面，而且预加载当前页面的前后两个页面数据（即同时会加载3个页面的数据），如果3个页面都有网络请求，就会影响性能，影响用户体验，由此，懒加载就尤为重要了，可以在滑到哪个fragment的时候，该fragment再进行网络请求。
直接上代码：
第1步：创建BaseFragment
public abstract class BaseFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(setContentView(), container, false);
findViewById(rootView);
return rootView;
}
/*
* 初始化布局：
* findViewById() 或者 ButterKnife.bind(this, view);
* @param rootView 布局UI
*/
protected abstract void findViewById(View rootView);
/*
* 记载布局文件：
* R.layout.xxx
*/
protected abstract int setContentView();
}
第2步：创建LazyBaseFragment 继承BaseFragment
public abstract class LazyBaseFragment extends BaseFragment {
/**
* 是否初始化布局
*/
protected boolean isViewInitiated;
/**
* 当前界面是否可见
*/
protected boolean isVisibleToUser;
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
isViewInitiated = true;
isCanLoadData();
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
// 是否对用户可见
this.isVisibleToUser = isVisibleToUser;
if (isVisibleToUser) {
isCanLoadData();
}
}
/*
* 执行数据加载： 条件是view初始化完成并且对用户可见
*/
private void isCanLoadData() {
if (isViewInitiated && isVisibleToUser) {
lazyLoad();
// 加载过数据后，将isViewInitiated和isVisibleToUser设置成false，防止重复加载数据
isViewInitiated = false;
isVisibleToUser = false;
}
}
protected abstract void lazyLoad();
}
第3步：创建FragmentOne、FragmentTwo，FragmentThree三个类继承LazyBaseFragment，下面以FragmentOne为例；
public class FragmentOne extends LazyBaseFragment {
private static final String TAG = "lazy";
private Context context;
***此处省略其他方法***
@Override
protected void findViewById(View rootView) {
TextView tv = (TextView) rootView.findViewById(R.id.id_fragment_content);
tv.setText("fragment 1");
}
@Override
protected int setContentView() {
return R.layout.fragment_item;
}
@Override
protected void lazyLoad() {
//此处模拟网络请求过程
Log.e(TAG, "fragment 1 开始加载数据。。。");
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
Log.e(TAG, "fragment 1 数据加载完成!");
}
}, 3000);
}
}
第4步：创建fragment_item.xml，就一个TextView。
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/id_fragment_content"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</android.support.constraint.ConstraintLayout>
第5步：接下来就是创建主页面了，先创建activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v4.view.ViewPager
android:id="@+id/id_viewpager"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
<android.support.design.widget.BottomNavigationView
android:id="@+id/id_bottom_menu"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:background="#fff"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:menu="@menu/navigation" />
</android.support.constraint.ConstraintLayout>
navigation.xml
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:id="@+id/navigation_f1"
android:title="F1" />
<item
android:id="@+id/navigation_f2"
android:title="F2" />
<item
android:id="@+id/navigation_f3"
android:title="F3" />
</menu>
然后是MainActivity.java
public class MainActivity extends AppCompatActivity {
ViewPager idViewPager;
BottomNavigationView idBottomMenu;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
idBottomMenu = (BottomNavigationView) findViewById(R.id.id_bottom_menu);
idBottomMenu.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.navigation_f1:
idViewPager.setCurrentItem(0);
return true;
case R.id.navigation_f2:
idViewPager.setCurrentItem(1);
return true;
case R.id.navigation_f3:
idViewPager.setCurrentItem(2);
return true;
}
return false;
}
});
idViewPager = (ViewPager) findViewById(R.id.id_viewpager);
idViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
idBottomMenu.getMenu().getItem(position).setChecked(true);
}
@Override
public void onPageScrollStateChanged(int state) {
}
});
MyAdapter adapter = new MyAdapter(getSupportFragmentManager());
adapter.addFragment(new FragmentOne());
adapter.addFragment(new FragmentTwo());
adapter.addFragment(new FragmentThree());
idViewPager.setAdapter(adapter);
//要设置viewpager的limit为2，因为滑动到fragment3的时候，fragment3的左侧有2个页面了
idViewPager.setOffscreenPageLimit(2);
}
//这里自定义的adapter要继承FragmentPagerAdapter
class MyAdapter extends FragmentPagerAdapter {
private List<Fragment> fragments = new ArrayList<>();
public MyAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
return fragments.get(position);
}
@Override
public int getCount() {
return fragments.size();
}
public void addFragment(Fragment fragment) {
fragments.add(fragment);
}
}
}
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