的代码：

Question

我在泡菜中，无法理解发生了什么。我的应用程序使用OnBackPressed关闭窗口和主屏幕检查，简而言之“如果在家，否则，请使用退出菜单”

@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();
    }
}

我不想发生的事情是紧逼让我直接回到我的“大怪兽”片段。我希望这能跟随后台。

我在“大怪物”页面上有大约30个按钮，可以将您带入怪物的信息，但是立即按下会显示“你确定要退出”菜单，而不是回到大怪物片段：< / p>

Large Monsters

Pressing Back button from a Monster fragment

显然，它并没有跟随后台。与任何其他屏幕相同，如果我去任何其他片段，它会直接回到Nav_Large_Monsters而不是跟随后台。

我假设我的问题是：

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

但如果我删除它，应用程序将立即要求退出，而不是回家，仍然没有跟随后台。

如果我将onBackPressed返回到其默认代码，它将跟随backstack，但它不会加载一个主片段并以空白片段开头。

的尝试：

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

这也行不通。不加载主片段。

以前我有一个showHome（）方法，这个DID可以使用backStack并使用一个home片段，但在将我的Fragments更改为v4.support.app.Fragment后，我不得不改变我的Home Activity如何处理我的onNavigationItemSelected。它不喜欢我的showHome方法并且一直在调用我使用旧的FragmentManager而不是v4.support.app.FragmentManager（此方法导致我的应用在更改为v4后立即崩溃）：< / p>
```
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，这似乎没有做任何有用的事情：

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，检查Backstack是否为＆gt; 0但是这也没有帮助，虽然我删除了那个字符串并且不确定我在哪里找到它。也许我错误地构建了这个字符串。

任何帮助都会很棒！谢谢！

的代码：

Home.java（MainActivity，我刚在这个项目中将其命名为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

如果你要覆盖onBackPressed，你必须自己处理后退的撤消。这就是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();
    }
}

如果你跳过它，你就不会得到片段弹出行为。所以只需更新你的onBackPressed以包含一个片段管理器的弹出窗口 - 如果成功，你可以提前返回，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();
    }
}

<强>更新

您还需要更新displayView方法，因为现在它始终推送堆栈上的内容。所以你的家庭＆＃34;片段实际上是堆栈中的第一件事，并会弹出让你留下一个空白屏幕。所以，如果你正在展示＆＃34; home＆＃34;片段，不在片段管理器上调用addToBackStack。

此外，您不需要明确设置＆＃34; home＆＃34;按下后面的片段，因为弹出后台堆栈应该做正确的事情（参见上面的更新示例）。

希望有所帮助！

OnBackPressed不跟随backstack;直接进入家庭片段

的尝试：

的代码：

1 个答案: