通过底部导航栏更改片段时恢复片段状态

Question

单击导航栏中的项目时，我有底部导航栏，我正在替换片段。我有 3 个片段 A、B、C，因此单击 b 项 B 片段已加载，并且在 B 中我正在调用 3-4 个 API。所以现在如果我去 C 然后再次来到 B 一个新的 B 片段实例被创建并且再次调用这些 API 我如何保存片段实例状态而不是在更改片段时再次调用 API。这是我的代码。

mBottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            int id = item.getItemId();
            Fragment currentLoaded = fgMan.findFragmentById(R.id.container_body);
            switch (id) {
                case R.id.nearby_fragment:
                    if (!(currentLoaded instanceof SpotFeedMapFragment)) {
                        removeScroll();
                        mNearByFragment = fgMan.findFragmentByTag(NEARBY_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(NEARBY_FRAGMENT_TAG) : mNearByFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mNearByFragment, NEARBY_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.nearby_fragment));
                    }
                    break;
                case R.id.route_fragment:
                    if (!(currentLoaded instanceof BusLocationsFragment)) {
                        if (!inParent) {
                            mRl.removeView(fixLayout);
                            p.addRule(RelativeLayout.BELOW, toolbar.getId());
                            scrollView.setLayoutParams(p);
                            scrollView.addView(fixLayout);
                            mRl.addView(scrollView);
                            inParent = true;
                        }
                        //mFragment = new BusLocationsFragment();
                        mBusLocFragment = fgMan.findFragmentByTag(BUS_LOC_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(BUS_LOC_FRAGMENT_TAG) : mBusLocFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mBusLocFragment, BUS_LOC_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.app_name));
                    }
                    break;
                case R.id.newsfeed_activity:
                    if (!(currentLoaded instanceof NewsFeedActivity)) {
                        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP_MR1) {
                            removeScroll();
                        }
                        mNewsFeedFragment = fgMan.findFragmentByTag(NEWSFEED_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(NEWSFEED_FRAGMENT_TAG) : mNewsFeedFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mNewsFeedFragment, NEWSFEED_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.news));
                    }
                    break;
            }
            return true;
        }
    });

我已经在onCreateofMainActivity上面初始化了fragments成员变量

Answer 1

您应该使用 FragmentPagerAdapter 来启动 Fragment，这样当您想在它们之间切换时，会保存 Fragment 的状态。

CutomViewPager viewPager = (CustomViewPager) findViewById(R.id.viewpager1);
ViewPagerAdapter adapter = new ViewPagerAdapter (MainActivity.this.getSupportFragmentManager());
adapter.addFragment(new SpotFeedMapFragment(), "title");
adapter.addFragment(new BusLocationsFragment(), "title");
adapter.addFragment(new NewsFeedActivity(), "title");
viewPager.setAdapter(adapter);

然后在选择的底部导航中，您可以通过简单的命令设置片段

viewPager.setCurrentItem(n);

我的viewpager类如下：

public class CustomViewPager extends ViewPager {

private boolean isPagingEnabled;

public CustomViewPager(Context context) {
    super(context);
    this.isPagingEnabled = true;
}

public CustomViewPager(Context context, AttributeSet attrs) {
    super(context, attrs);
    this.isPagingEnabled = true;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    return this.isPagingEnabled && super.onTouchEvent(event);
}

//for samsung phones to prevent tab switching keys to show on keyboard
@Override
public boolean executeKeyEvent(KeyEvent event) {
    return isPagingEnabled && super.executeKeyEvent(event);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    return this.isPagingEnabled && super.onInterceptTouchEvent(event);
}

public void setPagingEnabled(boolean enabled) {
    this.isPagingEnabled = enabled;
}
}

在 xml 中，而不是你需要的片段的空布局：

<com.package.util.CustomViewPager
    android:id="@+id/viewpager1"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

自定义 FragmentPagerAdapter 的代码：

private class ViewPagerAdapter extends FragmentPagerAdapter {
    private final SparseArray<WeakReference<Fragment>> instantiatedFragments = new SparseArray<>();
    private final List<Fragment> mFragmentList = new ArrayList<>();
    private final List<String> mFragmentTitleList = new ArrayList<>();

    ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        return mFragmentList.get(position);
    }

    @Override
    public int getCount() {
        return mFragmentList.size();
    }

    void addFragment(Fragment fragment, String title) {
        mFragmentList.add(fragment);
        mFragmentTitleList.add(title);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        final Fragment fragment = (Fragment) super.instantiateItem(container, position);
        instantiatedFragments.put(position, new WeakReference<>(fragment));
        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        instantiatedFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    @Nullable
    Fragment getFragment(final int position) {
        final WeakReference<Fragment> wr = instantiatedFragments.get(position);
        if (wr != null) {
            return wr.get();
        } else {
            return null;
        }
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return mFragmentTitleList.get(position);
    }
}

Answer 2

要恢复/保留片段的状态，您应该使用 ViewPager2，因为它是 ViewPager 的更新版本。

您将获得my GitHub repository 上的代码，其中三个菜单项在底部导航栏中具有更多功能。我也在此处提供了一个简单的描述，其中 两个菜单项位于 底部导航栏。

分步指南（以恢复/保留 EditText 的状态为例）：

第 1 步：

在您的 build.gradle（应用模块）文件中添加 依赖项：

dependencies {

    def nav_version = "2.3.0"
    implementation "androidx.navigation:navigation-fragment:$nav_version"
    implementation "androidx.navigation:navigation-ui:$nav_version"

    implementation 'androidx.viewpager2:viewpager2:1.0.0'

}

第 2 步：

将 menu_bottom_navigation.xml 添加到 res/menu：（您也可以将 icons 添加到 菜单项）

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_first"
        android:checked="true"
        android:title="First"
        app:showAsAction="always" />
    <item
        android:id="@+id/menu_second"
        android:checked="false"
        android:title="Second"
        app:showAsAction="always" />

</menu>

第 3 步：

将 activity_main.xml 添加到 res/layout：（将菜单添加到 BottomNavigationView 并放置 ViewPager2）

<?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:id="@+id/activityRoot"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="bottom"
    android:orientation="vertical"
    android:animateLayoutChanges="true"
    tools:context=".MainActivity">

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/viewpager2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/bottom_navigation"
        android:layout_alignParentTop="true"
        android:layout_weight="1"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_navigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_gravity="bottom"
        android:fitsSystemWindows="true"
        app:itemIconSize="20dp"
        android:background="#A8DD44"
        app:menu="@menu/menu_bottom_navigation" />

</LinearLayout>

第 4 步：

将 fragment_first.xml 添加到 res/layout：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:layout_margin="20dp"
    tools:context="com.example.rough.Fragment.FirstFragment">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="First Fragment"
        android:layout_centerInParent="true"
        android:textSize="30sp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Write something &amp; it will stay"
        android:ems="13"/>

</LinearLayout>

第 5 步：

将 fragment_second.xml 添加到 res/layout：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="20dp"
    android:orientation="vertical"
    tools:context="com.example.rough.Fragment.SecondFragment">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Second Fragment"
        android:layout_centerInParent="true"
        android:textSize="30sp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Write something &amp; it will stay"
        android:ems="13"/>

</LinearLayout>

第 6 步：

ViewPagerAdapter.java：

public class ViewPagerAdapter extends FragmentStateAdapter {
    private final List<Fragment> mFragmentList = new ArrayList<>();

    public ViewPagerAdapter(@NonNull FragmentManager fragmentManager, Lifecycle b ) {
        super(fragmentManager,b);
    }

    public void addFragment(Fragment fragment) {
        mFragmentList.add(fragment);
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        return mFragmentList.get(position);
    }

    @Override
    public int getItemCount() {
        return mFragmentList.size();
    }

}

第 7 步：

FirstFragment.java：

public class FirstFragment extends Fragment {


    public FirstFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_first, container, false);
    }

}

第 8 步：

SecondFragment.java：

public class SecondFragment extends Fragment {


    public SecondFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_second, container, false);
    }

}

第 9 步：

MainActivity.java：

public class MainActivity extends AppCompatActivity {

    BottomNavigationView bottomNavigationView;

    private ViewPager2 viewPager2;

    FirstFragment firstFragment;
    SecondFragment secondFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        viewPager2 = findViewById(R.id.viewpager2);
        bottomNavigationView = findViewById(R.id.bottom_navigation);

        bottomNavigationView.setOnNavigationItemSelectedListener(
                new BottomNavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                        switch (item.getItemId()) {
                            case R.id.menu_first:
                                viewPager2.setCurrentItem(0,false);
                                break;
                            case R.id.menu_second:
                                viewPager2.setCurrentItem(1,false);
                                break;
                        }
                        return false;
                    }
                });

        viewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                super.onPageScrolled(position, positionOffset, positionOffsetPixels);

                switch (position) {
                    case 0:
                        bottomNavigationView.getMenu().findItem(R.id.menu_first).setChecked(true);
                        break;
                    case 1:
                        bottomNavigationView.getMenu().findItem(R.id.menu_second).setChecked(true);
                        break;
                }
            }
        });

        setupViewPager(viewPager2);

    }

    private void setupViewPager(ViewPager2 viewPager) {

        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager(), getLifecycle());

        firstFragment =new FirstFragment();
        secondFragment =new SecondFragment();

        adapter.addFragment(firstFragment);
        adapter.addFragment(secondFragment);

        viewPager.setAdapter(adapter);
    }

}

Answer 3

我使用了bottom navigation bar，并通过自定义 viewpager 来做到这一点，并禁用了滑动导航。每次用户单击底部项目时，在 viewpager 中设置相关片段。 Fragment的Viewpager控制状态，所以不需要控制状态。

自定义 ViewPager

public class BottomNavigationViewPager extends ViewPager {

    private boolean enabled;

    public BottomNavigationViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.enabled = false;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (this.enabled) {
            return super.onTouchEvent(event);
        }

        return false;
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        if (this.enabled) {
            return super.onInterceptTouchEvent(event);
        }

        return false;
    }

    /**
     * Enable or disable the swipe navigation
     * @param enabled
     */
    public void setPagingEnabled(boolean enabled) {
        this.enabled = enabled;
    }
}

如果你仍然想控制片段的状态，你可以在这个链接中看到我的回答 How to save fragment state in android?

Answer 4

使用导航组件的解决方案

使用这些版本，多栈支持仅在这些版本中可用

versions.fragment = "1.4.0-alpha01"
versions.navigation = "2.4.0-alpha01"

首先，您需要为底部导航中的每个片段创建导航图

文件名：first.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation 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:id="@+id/first"
    app:startDestination="@id/homeFragment"
    tools:ignore="UnusedNavigation">

    <fragment
        android:id="@+id/homeFragment"
        android:label="@string/fragment_A_title"
        android:name="com.app.company.HomeFragment"
    >
    </fragment>
</navigation>

为所有标签创建导航图并使用这些 ID 设置底部导航菜单

<item android:title="@string/title_one"
    android:id="@+id/first"
    android:icon="@drawable/ic_icon_24"/>

<item android:title="@string/title_two"
    android:id="@+id/second"
    android:icon="@drawable/ic_icon_24"/>

.
.

.

将这些导航图包含在主导航图中并在 FragmentContainerView 中使用此主图

<include app:graph="@navigation/first"/>
<include app:graph="@navigation/second"/>
<include app:graph="@navigation/third"/>

设置底部导航

  val navHostFragment = supportFragmentManager.findFragmentById(R.id.fragment_host) as NavHostFragment
        navController = navHostFragment.navController


        val controller = binding.bottonNavigation.setupWithNavController(navController)


        appBarConfig = AppBarConfiguration(
            setOf(
                R.id.homeFragment,
                R.id.secondFragment,
                R.id.thirdFragment
            )
        )
        setupActionBarWithNavController(navController, appBarConfig)

更多信息请参考以下链接

Navigation multiple back stacks

Answer 5

最简单的解决方案是在 Fragment B 中覆盖“OnCreate()”方法，并在“OnCreate()”方法而不是“OnCreateView()”中调用 API。

希望它对你有用！