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>
答案 0 :(得分：0)
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。