关于 android 导航架构组件的问题

Question

我有一个问题。我正在使用 android 导航架构组件，使用带有单个 activity 的底部导航视图。如何让片段只打开一次？即使导致这个片段的按钮被点击了几次？如何让片段只被添加到返回堆栈一次？我创建了一个测试项目来试试

xml navigation_test

<?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/test_nav"
    app:startDestination="@id/firstFragment">
    <fragment
        android:id="@+id/firstFragment"
        android:name="ru.artem_nr.navigation_test.FirstFragment"
        android:label="fragment_first"
        tools:layout="@layout/fragment_first" >
        <action
            android:id="@+id/action_firstFragment_to_secondFrag"
            app:destination="@id/navigation"
            app:launchSingleTop="true"
            app:popUpTo="@+id/secondFrag"
            app:popUpToInclusive="false" />
    </fragment>
    <navigation android:id="@+id/navigation"
        app:startDestination="@id/secondFrag">
        <fragment
            android:id="@+id/secondFrag"
            android:name="ru.artem_nr.navigation_test.SecondFrag"
            android:label="fragment_second"
            tools:layout="@layout/fragment_second" />
    </navigation>
</navigation>

main_activity

public class MainActivity 扩展了 AppCompatActivity 实现 NavigationView.OnNavigationItemSelectedListener {

public Toolbar toolbar;
public DrawerLayout drawerLayout;
public NavController navController;
public NavigationView navigationView;

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

    setupNavigation();
}

private void setupNavigation() {

    toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setDisplayShowHomeEnabled(true);

    drawerLayout = findViewById(R.id.drawer_layout);
    navigationView = findViewById(R.id.nav_view);
    navController = Navigation.findNavController(this, R.id.garden_nav_fragment);
    NavigationUI.setupActionBarWithNavController(this, navController, drawerLayout);
    NavigationUI.setupWithNavController(navigationView, navController);
    navigationView.setNavigationItemSelectedListener((NavigationView.OnNavigationItemSelectedListener) this);
}

@Override
public boolean onSupportNavigateUp() {
    return (boolean) NavigationUI.navigateUp(navController, drawerLayout);
}

@Override
public void onBackPressed() {
    if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
        drawerLayout.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
    menuItem.setChecked(true);

    drawerLayout.closeDrawers();

    int id = menuItem.getItemId();

    switch (id) {
        case R.id.nav_home:
            navController.navigate(R.id.navigation);
            break;
    }
    return true;
}

}

碎片 - 只是一片空白

Answer 1

您需要将片段声明为 "SingleTop"。

导航图编辑器视图右侧的菜单上有一个选项可以执行此操作，位于动画选项下方。

或者，您可以通过 NavOptions.Builder class 以编程方式设置它，使用下面 link 中详述的 setLaunchSingleTop(true) 方法：

https://developer.android.com/reference/kotlin/androidx/navigation/NavOptions.Builder#setLaunchSingleTop(kotlin.Boolean)

所以您的 select 方法应该如下所示：

public boolean onNavigationItemSelected(@NonNull MenuItem menuItem){
NavOptions.Builder builder = new NavOptions.Builder()
.setLaunchSingleTop(true);

NavOptions options = builder.build();
navController.navigate(item.getItemId(), null, options);

Answer 2

根据 this issue，BottomNavigationView 提供 OnNavigationItemReselectedListener 优先于 NavigationUI 设置的 OnNavigationItemSelectedListener:

bottomNavigationView.setOnNavigationItemReselectedListener(
    new BottomNavigationView.OnNavigationReselectedListener() {
        @Override
        public void onNavigationItemReselected(MenuItem item) {
            // By doing nothing, we ignore reselection events
        }
    });

NavigationView 不提供与 this other issue, so you'd want to copy the default behavior 相同的界面（请注意，它调用 NavigationUI.onNavDestinationSelected() 而不是直接调用 navigate() ，这与您的代码不同）和添加您自己的 isChecked() 检查以防止重新选择。

关于 android 导航架构组件的问题

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android-architecture-navigation