Question

我是一名尝试制作android应用程序的初学者android开发人员。我的应用程序有一个包含4个片段的viewpager。现在，每当我从主要活动（viewpager活动）移动到另一个活动并返回时，主要活动就会再次开始，并且viewpager显示第一个片段，而不是用户先前显示的片段。我在下面提供了我的代码。请提供帮助，以便我可以解决此Viewpager不必要地回滚到第一个片段的问题。

public class MainActivity extends AppCompatActivity {

    private static final int RC_SIGN_IN = 123;
    private FirebaseAuth mFirebaseAuth;
    private FirebaseAuth.AuthStateListener mAuthStateListener;
    public String ADMIN_CHANNEL_ID = "ADMIN";


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
        final PbFragmentPagerAdapter adapter = new PbFragmentPagerAdapter(getSupportFragmentManager());
        final TabLayout tabLayout = (TabLayout) findViewById(R.id.sliding_tabs);
        mFirebaseAuth = FirebaseAuth.getInstance();
        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if(user!=null){
                    viewPager.setAdapter(adapter);
                    tabLayout.setupWithViewPager(viewPager);
                    viewPager.setOffscreenPageLimit(4);
                    adapter.fetchFragmentByPosition(viewPager, viewPager.getCurrentItem());

                }
                else{startActivityForResult(
                        AuthUI.getInstance()
                                .createSignInIntentBuilder()
                                .setAvailableProviders(Arrays.asList(
                                        new AuthUI.IdpConfig.EmailBuilder().build(),
                                        new AuthUI.IdpConfig.GoogleBuilder().build()
                                        ))
                                   .setLogo(R.drawable.ic_fui_icon_round)
                                .build(),
                        RC_SIGN_IN);
                }
            }
        };

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            // Create the NotificationChannel
            CharSequence name = getString(R.string.notifications_admin_channel_name);
            String description = getString(R.string.notifications_admin_channel_description);
            int importance = NotificationManager.IMPORTANCE_HIGH;
            NotificationChannel mChannel = new NotificationChannel(ADMIN_CHANNEL_ID, name, importance);
            mChannel.setDescription(description);
            mChannel.enableLights(true);
            mChannel.setLightColor(R.color.colorPrimaryDark);
            mChannel.enableVibration(true);
            NotificationManager notificationManager = (NotificationManager) getSystemService(
                    NOTIFICATION_SERVICE);
            notificationManager.createNotificationChannel(mChannel);
        }

    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode== RC_SIGN_IN){
            if(resultCode==RESULT_OK){
                Toast.makeText(MainActivity.this, "Signed In Successfully!",Toast.LENGTH_SHORT).show();
            }
                else if (resultCode==RESULT_CANCELED){
                    finish();
                }
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.main_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.sign_out_menu:
                AuthUI.getInstance().signOut(this);
                return true;
            case R.id.share_menu:
                shareApp();
                return true;
            case R.id.terms_menu:
                openTermsActivity();
                return true;
            case R.id.privacy_menu:
                openPrivacyActivity();
                return true;
            case R.id.about_menu:
                openAboutActivity();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }

    public void openAboutActivity() {
        Intent intent = new Intent(this, AboutActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
        startActivity(intent);
    }
    public void openTermsActivity() {
        Intent intent = new Intent(this, TermsActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
        startActivity(intent);
    }
    public void openPrivacyActivity() {
        Intent intent = new Intent(this, PrivacyActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
        startActivity(intent);
    }
    public void shareApp() {

        Intent shareIntent = new Intent();
        shareIntent.setAction(Intent.ACTION_SEND);
        shareIntent.putExtra(Intent.EXTRA_TEXT,
                "Hi, I would like to recommend the xyz app to you..);
        shareIntent.setType("text/plain");
        startActivity(shareIntent);
    }

    @Override
    protected void onPause() {
        super.onPause();
        mFirebaseAuth.removeAuthStateListener(mAuthStateListener);
    }

    @Override
    protected void onResume() {
        super.onResume();
        mFirebaseAuth.addAuthStateListener(mAuthStateListener);
    }

    public class PbFragmentPagerAdapter extends SmartFragmentStatePagerAdapter {
        final int PAGE_COUNT = 4;
        private String tabTitles[] = new String[] { "Open Trades", "Past Trades", "Books", "Broker" };


        public PbFragmentPagerAdapter(FragmentManager fm) {
            super(fm);
        }


        @Override
        public Fragment getItem(int position) {
            switch (position) {
                case 0:
                    return new OpenTradesFragment();
                case 1:
                    return new PastTradesFragment();
                case 2:
                    return new BooksFragment();
                case 3:
                    return new BrokersFragment();
                default:
                    return null;
            }
        }

        @Override
        public int getCount() {
            return PAGE_COUNT;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return tabTitles[position];
        }

    }

}

//下面的代码用于寻呼机适配器

public abstract class SmartFragmentStatePagerAdapter extends FragmentStatePagerAdapter {
    // Sparse array to keep track of registered fragments in memory
    private SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

    public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    // Register the fragment when the item is instantiated
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        registeredFragments.put(position, fragment);
        return fragment;
    }

    // Unregister when the item is inactive
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        registeredFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    // Returns the fragment for the position (if instantiated)
    public Fragment getRegisteredFragment(int position) {
        return registeredFragments.get(position);
    }

    public Fragment fetchFragmentByPosition(ViewGroup pagerInstance, int position) {
        Fragment existingInstance = registeredFragments.get(position);
        if (existingInstance != null) {
            return existingInstance;
        } else {
            return (Fragment)instantiateItem(pagerInstance, position);
        }
    }
}

共享下面的XML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:layout_centerVertical="true"
    android:orientation="vertical"
    android:id="@+id/main_activity_layout"
    tools:context="in.permabull.permabull.MainActivity">

    <android.support.design.widget.TabLayout
        android:id="@+id/sliding_tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabSelectedTextColor="@color/colorPrimaryDark"
        app:tabTextColor="@color/colorGrey"
        app:tabPadding="8dp"
        app:tabMode="scrollable">

    </android.support.design.widget.TabLayout>

        <TextView
        android:layout_width="match_parent"
        android:layout_height="1dp"
        android:background="@color/colorLightGrey"/>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="0px"
        android:layout_weight="1">

    </android.support.v4.view.ViewPager>



</LinearLayout>

Answer 1

如预期的那样，这是一个简单的问题。.（使用onPageSelected）解决了以下问题：

DELETE FROM TABLE A WHERE A.KEY IN (SELECT KEY FROM TABLE B WHERE STATUS = ‘DELETED’)

其中tabPosition是一个全局int变量，当按下后退/主页按钮时，该变量有助于存储viewpager所在的fragment（tab）编号。

我的应用中的Viewpager活动重新启动

1 个答案: