BottomNavigationView后退按钮行为应该像Youtube App一样工作但崩溃

Question

重现步骤:

1. 启动一个新的Android项目,选择" BottomNavigationView":

2. 用以下方法替换MainActivity:

   class MainActivity : AppCompatActivity() {
   
   private var fragmentIds = ArrayList<Int>()
   
   val fragmentA: FragmentA = FragmentA()
   private val fragmentB = FragmentB()
   private val fragmentC = FragmentC()
   
   private fun getFragment(fragmentId: Int): Fragment {
       when (fragmentId) {
           R.id.navigation_home -> {
               return fragmentA
           }
           R.id.navigation_dashboard -> {
               return fragmentB
           }
           R.id.navigation_notifications -> {
               return fragmentC
           }
       }
       return fragmentA
   }
   
   private fun updateView(fragmentId: Int) {
       var exists = false
       fragmentIds
               .filter { it == fragmentId }
               .forEach { exists = true }
   
       if (exists) {
           fragmentIds.remove(fragmentId)
           showTabWithoutAddingToBackStack(getFragment(fragmentId))
       } else {
           fragmentIds.add(fragmentId)
           showTab(getFragment(fragmentId))
       }
   }
   
   private val onNavigationItemClicked = BottomNavigationView.OnNavigationItemSelectedListener { item ->
       when (item.itemId) {
           R.id.navigation_home -> {
               updateView(R.id.navigation_home)
               return@OnNavigationItemSelectedListener true
           }
           R.id.navigation_dashboard -> {
               updateView(R.id.navigation_dashboard)
               return@OnNavigationItemSelectedListener true
           }
           R.id.navigation_notifications -> {
               updateView(R.id.navigation_notifications)
               return@OnNavigationItemSelectedListener true
           }
       }
       false
   }
   
   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_main)
       showTabWithoutAddingToBackStack(fragmentA)
   
       navigation.setOnNavigationItemSelectedListener(onNavigationItemClicked)
   
   }
   
   private fun showTab(fragment: Fragment) {
       supportFragmentManager
               .beginTransaction()
               .replace(R.id.main_container, fragment, fragment::class.java.simpleName)
               .addToBackStack(fragment::class.java.simpleName)
               .commit()
   }
   
   fun showTabWithoutAddingToBackStack(fragment: Fragment) {
       supportFragmentManager
               .beginTransaction()
               .replace(R.id.main_container, fragment, fragment::class.java.simpleName)
               .commit()
   }
   
   fun setBottomTab(id: Int) {
       navigation.setOnNavigationItemSelectedListener(null)
       navigation.selectedItemId = id
       // currentTab = id
       navigation.setOnNavigationItemSelectedListener(onNavigationItemClicked)
   }
   }
   

3. 创建3个新类FragmentA,FragmentB和FragmentC:

   class FragmentA : Fragment() {
   
   override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
       setHasOptionsMenu(true)
       return inflater.inflate(R.layout.fragment_a, container, false)
   }
   
   override fun onResume() {
       super.onResume()
       val act = activity as MainActivity
       act.setBottomTab(R.id.navigation_home)
   }
   }
   

使用这个xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment A" />
</LinearLayout>

1. 启动应用程序
2. 按"仪表板" - 显示片段B.
3. 按"通知" - 显示片段C.
4. 按"仪表板" - 显示片段B.
5. 按"主页" - 显示片段A.
6. 按返回按钮 - 显示片段B.
7. 按返回按钮 - 应显示片段C - 应用程序崩溃
8. 按返回按钮 - 应显示片段A - 应用程序崩溃
9. 按返回按钮 - 应用程序关闭. - 应用程序崩溃了

这是一段演示上述步骤的视频

堆栈跟踪:

12-06 12:58:35.899 25903-25903/com.example.jimclermonts.bottomnavigationview E/InputEventSender: Exception dispatching finished signal.
12-06 12:58:35.900 25903-25903/com.example.jimclermonts.bottomnavigationview E/MessageQueue-JNI: Exception in MessageQueue callback: handleReceiveCallback
12-06 12:58:35.912 25903-25903/com.example.jimclermonts.bottomnavigationview E/MessageQueue-JNI: java.lang.**IllegalStateException: Fragment already added**: FragmentB{3aac1d9 #1 id=0x7f080059 FragmentB}
                                                                                                     at android.support.v4.app.FragmentManagerImpl.addFragment(FragmentManager.java:1882)
                                                                                                     at android.support.v4.app.BackStackRecord.executePopOps(BackStackRecord.java:825)
                                                                                                     at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2577)
                                                                                                     at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
                                                                                                     at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
                                                                                                     at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:851)
                                                                                                     at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:794)
                                                                                                     at android.support.v4.app.FragmentActivity.onBackPressed(FragmentActivity.java:174)

Answer 1

这使您可以使用自定义后堆栈（双端队列）实现“不要在后堆栈上重复（但重新排序）我的片段”行为：

class MainActivity extends AppCompatActivity {
    private BottomNavigationView navigation;
    // initialize with number of different fragments
    private Deque<Integer> fragmentIds = new ArrayDeque<>(3);

    private FragmentA fragmentA = new FragmentA();
    private FragmentB fragmentB = new FragmentB();
    private FragmentC fragmentC = new FragmentC();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        fragmentIds.push(R.id.navigation_home);
        showTabWithoutAddingToBackStack(fragmentA);
        navigation = findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(onNavigationItemClicked);
    }

    private BottomNavigationView.OnNavigationItemSelectedListener onNavigationItemClicked = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            item.setChecked(true);
            int itemId = item.getItemId();
            if (fragmentIds.contains(itemId)) {
                fragmentIds.remove(itemId);
            }
            fragmentIds.push(itemId);
            showTabWithoutAddingToBackStack(getFragment(item.getItemId()));
            return true;
        }
    };

    private Fragment getFragment(int fragmentId) {
        switch (fragmentId) {
            case R.id.navigation_home:
                return fragmentA;
            case R.id.navigation_dashboard:
                return fragmentB;
            case R.id.navigation_notifications:
                return fragmentC;
        }
        return fragmentA;
    }

    void showTabWithoutAddingToBackStack(Fragment fragment) {
        getSupportFragmentManager().beginTransaction().replace(R.id.container, fragment, fragment.getClass().getSimpleName()).commit();
    }

    void setBottomTab(int id) {
        int itemIndex;
        switch (id) {
            case R.id.navigation_dashboard:
                itemIndex = 1;
                break;
            case R.id.navigation_notifications:
                itemIndex = 2;
                break;
            default:
            case R.id.navigation_home:
                itemIndex = 0;
        }
        navigation.getMenu().getItem(itemIndex).setChecked(true);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
            onBackPressed();
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }

    @Override
    public void onBackPressed() {
        fragmentIds.pop();
        if (!fragmentIds.isEmpty()) {
            showTabWithoutAddingToBackStack(getFragment(fragmentIds.peek()));
        } else {
            finish();
        }
    }
}