【 Android 】ViewPager + TabLayout + Fragment 数据初始化问题

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在 ViewPager 和 Fragment 配合使用的时候，ViewPager 会使用预加载机制，使得我们在没有切换到到对应页面时，就已经加载好了，这是个非常不好的用户体验。

所以本示例项目就诞生了。

关键字：setUserVisibleHint

实例：

示例结构：
使用Activity嵌套Fragment

MainActivity

public class MainActivity extends AppCompatActivity { private ArrayList<Fragment> mFragments; @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); initFragment(); initView();
} private void initFragment() {
mFragments = new ArrayList<>(); AFragment aFragment = AFragment.newInstance();
BFragment bFragment = BFragment.newInstance();
CFragment cFragment = CFragment.newInstance(); mFragments.add(aFragment);
mFragments.add(bFragment);
mFragments.add(cFragment);
} private void initView() {
final TabLayout tabLayout = findViewById(R.id.tab_layout);
final ViewPager viewPager = findViewById(R.id.viewpager); viewPager.setAdapter(new FragmentStatePagerAdapter(getSupportFragmentManager()) {
@Override
public Fragment getItem(int position) {
return mFragments.get(position);
} @Override
public int getCount() {
return mFragments.size();
}
}); viewPager.setOffscreenPageLimit(mFragments.size()); viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout)); tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
@Override
public void onTabSelected(TabLayout.Tab tab) {
Log.d(TAG, "onTabSelected: "); viewPager.setCurrentItem(tabLayout.getSelectedTabPosition(), true);
} @Override
public void onTabUnselected(TabLayout.Tab tab) {
Log.d(TAG, "onTabUnselected: ");
} @Override
public void onTabReselected(TabLayout.Tab tab) {
Log.d(TAG, "onTabReselected: ");
}
});
}
}

AFragment

public class AFragment extends Fragment { public AFragment() {
// Requires empty public constructor
} public static AFragment newInstance() {
return new AFragment();
} @Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_a, container, false); initView(view); return view;
} private void initView(View view) { }
}

BFragment

public class BFragment extends Fragment { private boolean isFirst = true; public BFragment() {
// Requires empty public constructor
} public static BFragment newInstance() {
return new BFragment();
} @Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
} @Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater,
@Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_b, container, false); initView(view); return view;
} @Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser); if (isVisibleToUser && isFirst) {
Log.d(TAG, "BFragment 首次加载"); isFirst = false;
}
} private void initView(View view) { }
}

CFragment和BFragment相似，就不赘述了。

MainActivity对应的布局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".ui.activity.MainActivity"> <android.support.design.widget.TabLayout
android:id="@+id/tab_layout"
android:layout_width="match_parent"
android:layout_height="50dp"
app:tabMode="scrollable"
app:tabSelectedTextColor="@color/colorAccent"> <android.support.design.widget.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="A" /> <android.support.design.widget.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="B" /> <android.support.design.widget.TabItem
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="C" /> </android.support.design.widget.TabLayout> <android.support.v4.view.ViewPager
android:id="@+id/viewpager"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:overScrollMode="never"
android:scrollbars="none" /></LinearLayout>