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

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>

AFragment、 BFragment、 CFragment 对应的布局文件就不赘述了，可以随意自定义。

源码下载：

https://github.com/cnwutianhao/ViewPager

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

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