OnBackPressed 不跟随 backstack；直接转到主页片段

Question

我很困惑，不明白发生了什么。我的应用程序使用带有关闭 window 和主屏幕检查的 OnBackPressed，简而言之 "if home, else, use exit menu"

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else if (!viewIsAtHome) {
        displayView(R.id.nav_large_monsters);
    } else {
        new AlertDialog.Builder(this)
                .setMessage("Are you sure you want to exit?")
                .setCancelable(false)
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        Home.this.finish();
                    }
                })
                .setNegativeButton("No", null)
                .show();
    }
}

正在发生但我不想发生的事情是 按下后退会直接将我带回我的 "Large Monsters" 片段。我想让它跟在后台。

我在大型怪物页面上有大约 30 个按钮可以带你进入怪物的信息，但是按下后退会立即弹出 "Are you sure you want to quit" 菜单，而不是返回到大型怪物碎片：

Large Monsters

Pressing Back button from a Monster fragment

很明显它没有跟随后退堆栈。与任何其他屏幕一样，如果我转到任何其他片段，它会直接带我回到 Nav_Large_Monsters 而不是跟随后退堆栈。

我假设我的问题是这样的：

    } else if (!viewIsAtHome) {
        displayView(R.id.nav_large_monsters);

但是如果我删除它，应用程序会立即要求退出，而不是return home，仍然没有跟随backstack。

如果我 return 将 onBackPressed 返回到其默认代码，它将跟随 backstack，但它不会加载主页片段并以空白片段开始。

已尝试：

我试图删除 onBackPressed 中的“else if (!viewIsAtHome)”部分，并在我的 onNavigationItemSelected 方法中实现了 "default"
```
default:
fragment = new Large_Monsters();
break;
```

这也行不通。不加载主页片段。

以前我有一个 showHome() 方法，这个 DID 可以使用 backStack 和 home 片段，但是在将我的片段更改为 v4.support.app.Fragment 之后，我有改变我的主页 Activity 处理我的 onNavigationItemSelected 的方式。它 不喜欢 我的 showHome 方法，并一直说我使用的是旧的 FragmentManager 而不是 v4.support.app.FragmentManager（此方法导致我的应用程序在更改为 v4 后立即崩溃） :
```
private void showHome() {
fragment = new Large_Monsters();
if (fragment != null) {
    android.app.FragmentManager fragMgmt = getFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.content_frame, fragment).commit();
}
```
}

我还尝试添加一个关于弹出 backStack 的 If 语句，但这似乎没有做任何有成效的事情：

if (!fragment.getFragmentManager().popBackStackImmediate()) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
        finishAfterTransition();
    }
}

我还尝试添加一个 If 语句来检查 Backstack 是否 > 0，但这也没有帮助，尽管我删除了那个字符串并且不确定我在哪里找到它。也许我构建的那个字符串不正确。

任何帮助都会很棒！谢谢！

代码：

Home.java（MainActivity，我只是在这个项目上将它命名为 Home）

public class Home extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {
private Fragment fragment = null;
private FragmentManager fragmentManager;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    DatabaseHelper myDbHelper = new DatabaseHelper(Home.this);


    try {
        myDbHelper.createDataBase();
    } catch (IOException ioe) {
        throw new Error("Unable to create database");
    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
    navigationView.setItemIconTintList(null);
//        showHome();

    displayView(R.id.nav_large_monsters);

}

private boolean viewIsAtHome;

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else if (!viewIsAtHome) {
        displayView(R.id.nav_large_monsters);
    } else {
        new AlertDialog.Builder(this)
                .setMessage("Are you sure you want to exit?")
                .setCancelable(false)
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        Home.this.finish();
                    }
                })
                .setNegativeButton("No", null)
                .show();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.search_menu, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();
    return super.onOptionsItemSelected(item);
}

//    private void showHome() {
//        fragment = new Large_Monsters();
//        if (fragment != null) {
//            android.app.FragmentManager fragMgmt = getFragmentManager();
//            fragmentManager.beginTransaction().replace(R.id.content_frame, fragment).commit();
//        }
//    }


@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    displayView(item.getItemId());
    return true;

}

public void displayView(int viewId) {

    Fragment fragment = null;
    String title = getString(R.string.app_name);

    switch (viewId) {
        case R.id.nav_large_monsters:
            fragment = new Large_Monsters();
            title  = "Large Monsters";
            viewIsAtHome = true;
            break;

        case R.id.nav_small_monsters:
            fragment = new Small_Monsters();
            title = "Small Monsters";
            viewIsAtHome = false;
            break;

        case R.id.nav_weapons:
            fragment = new Weapons();
            title = "Weapons";
            viewIsAtHome = false;
            break;

        case R.id.nav_armors:
            fragment = new Armors_Low_High();
            title = "Armor Sets";
            viewIsAtHome = false;
            break;

        case R.id.nav_charms:
            fragment = new Charms();
            title = "Charms";
            viewIsAtHome = false;
            break;

        case R.id.nav_items:
            fragment = new Items();
            title = "Items";
            viewIsAtHome = false;
            break;

        case R.id.palico_armor:
            fragment = new Palico_Armor();
            title = "Palico Armor";
            viewIsAtHome = false;
            break;

        case R.id.palico_gadgets:
            fragment = new Palico_Gadgets();
            title = "Palico Gadgets";
            viewIsAtHome = false;
            break;

        case R.id.palico_weps:
            fragment = new Palico_Weapons();
            title = "Palico Weapons";
            viewIsAtHome = false;
            break;

        case R.id.palico_helms:
            fragment = new Palico_Helms();
            title = "Palico Helmets";
            viewIsAtHome = false;
            break;

        default:
            fragment = new Large_Monsters();
            break;

    }

    if (fragment != null) {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction()
                .replace(R.id.content_frame, fragment, fragment.getTag())
                .addToBackStack(null);
                ft.commit();
    }

    // set the toolbar title
    if (getSupportActionBar() != null) {
        getSupportActionBar().setTitle(title);
    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);

}

}

Answer 1

如果您要覆盖 onBackPressed，您必须自己处理回溯的撤消。这是 FragmentActivity 的作用：

/**
 * Take care of popping the fragment back stack or finishing the activity
 * as appropriate.
 */
@Override
public void onBackPressed() {
    FragmentManager fragmentManager = mFragments.getSupportFragmentManager();
    final boolean isStateSaved = fragmentManager.isStateSaved();
    if (isStateSaved && Build.VERSION.SDK_INT <= Build.VERSION_CODES.N_MR1) {
        // Older versions will throw an exception from the framework
        // FragmentManager.popBackStackImmediate(), so we'll just
        // return here. The Activity is likely already on its way out
        // since the fragmentManager has already been saved.
        return;
    }
    if (isStateSaved || !fragmentManager.popBackStackImmediate()) {
        super.onBackPressed();
    }
}

如果您跳过它，您将不会出现片段弹出行为。因此，只需更新您的 onBackPressed 以包含片段管理器的弹出 - 如果成功，您可以提前 return 因为系统会根据需要弹出片段。

@Override
public void onBackPressed() {
    // Pressing back popped the back stack, nothing else to do
    FragmentManager fragmentManager = getSupportFragmentManager();

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else if (!fragmentManager.isStateSaved() && fragmentManager.popBackStackImmediate()) {
        return;
    } else {
        new AlertDialog.Builder(this)
                .setMessage("Are you sure you want to exit?")
                .setCancelable(false)
                .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        Home.this.finish();
                    }
                })
                .setNegativeButton("No", null)
                .show();
    }
}

更新

您还需要更新您的 displayView 方法，因为现在它总是将某些东西压入堆栈。因此，您的 "home" 片段实际上将成为堆栈中的第一件事，并且会弹出并留下一个空白屏幕。因此，如果您要显示 "home" 片段，不要在片段管理器上调用 addToBackStack。

此外，您不需要在按下时显式设置 "home" 片段，因为弹出 backstack 应该做正确的事情（参见上面更新的示例）。

希望对您有所帮助！

OnBackPressed 不跟随 backstack；直接转到主页片段

OnBackPressed not following backstack; goes directly to home Fragment

java

android

android-fragments

onbackpressed

已尝试：

代码：