最好的解决方案,

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;

public class BaseActivity extends AppCompatActivity {
    @Override
    public void onBackPressed() {

        FragmentManager fm = getSupportFragmentManager();
        for (Fragment frag : fm.getFragments()) {
            if (frag == null) {
                super.onBackPressed();
                finish();
                return;
            }
            if (frag.isVisible()) {
                FragmentManager childFm = frag.getChildFragmentManager();
                if (childFm.getFragments() == null) {
                    super.onBackPressed();
                    finish();
                    return;
                }
                if (childFm.getBackStackEntryCount() > 0) {
                    childFm.popBackStack();
                    return;
                }
                else {

                    fm.popBackStack();
                    if (fm.getFragments().size() <= 1) {
                        finish();
                    }
                    return;
                }

            }
        }
    }
}

在我看来，最好的解决办法是:

JAVA解决方案

创建简单的界面:

public interface IOnBackPressed {
    /**
     * If you return true the back press will not be taken into account, otherwise the activity will act naturally
     * @return true if your processing has priority if not false
     */
    boolean onBackPressed();
}

在你的活动中

public class MyActivity extends Activity {
    @Override public void onBackPressed() {
    Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.main_container);
       if (!(fragment instanceof IOnBackPressed) || !((IOnBackPressed) fragment).onBackPressed()) {
          super.onBackPressed();
       }
    } ...
}

最后在你的片段中:

public class MyFragment extends Fragment implements IOnBackPressed{
   @Override
   public boolean onBackPressed() {
       if (myCondition) {
            //action not popBackStack
            return true; 
        } else {
            return false;
        }
    }
}

芬兰湾的科特林解决方案

1 -创建接口

interface IOnBackPressed {
    fun onBackPressed(): Boolean
}

2 -准备你的活动

class MyActivity : AppCompatActivity() {
    override fun onBackPressed() {
        val fragment =
            this.supportFragmentManager.findFragmentById(R.id.main_container)
        (fragment as? IOnBackPressed)?.onBackPressed()?.not()?.let {
            super.onBackPressed()
        }
    }
}

3 -实现在你的目标片段

class MyFragment : Fragment(), IOnBackPressed {
    override fun onBackPressed(): Boolean {
        return if (myCondition) {
            //action not popBackStack
            true
        } else {
            false
        }
    }
}

试试这个，如果你真的想在Fragment中启用onBackPressed()。 在浪费了一个小时的时间后，我根据我以前的经验，做出了这个完全符合需求的解决方案。

你只需要关注私有int STATUS_FRAGMENT=0的值;这就满足了片段中addToBackStack()的需求。

import android.view.MenuItem;
import android.view.View;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;

import com.example.growfast.NavigationItemsFolder.CoreFragments.Cart;
import com.example.growfast.NavigationItemsFolder.CoreFragments.HelpDesk;
import com.example.growfast.NavigationItemsFolder.CoreFragments.Home;
import com.example.growfast.NavigationItemsFolder.CoreFragments.ProfileDetails;
import com.example.growfast.R;
import com.google.android.material.bottomnavigation.BottomNavigationView;

public class BusinessManagement extends AppCompatActivity {

    public BottomNavigationView bottomNavigationView;
    private int STATUS_FRAGMENT=0;

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

    }

    private void setBottomNavigationMenu() {
        bottomNavigationView = findViewById(R.id.navigation);
        bottomNavigationView.setVisibility(View.VISIBLE);

        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {

            Fragment fragment = null;

            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {

                switch (item.getItemId()) {

                    case R.id.action_home:
                        fragment = new Home();
                        break;

                    case R.id.action_profile:
                        fragment = new ProfileDetails();
                        break;
                    case R.id.action_cart:
                        fragment = new Cart();
                        break;
                    case R.id.action_favourites_menu:
                        fragment = new HelpDesk();
                        break;

                }
                return loadFromFragment(fragment);

            }
        });
        bottomNavigationView.setSelectedItemId(R.id.action_home);
    }

    private boolean loadFromFragment(Fragment fragment) {
        if (fragment != null) {
            getSupportFragmentManager().beginTransaction().replace(R.id.my_container, fragment)
                    .commit();
            STATUS_FRAGMENT=1;
            return true;
        }
        return false;
    }

    @Override
    public void onBackPressed() {
        if (STATUS_FRAGMENT==1) {
            bottomNavigationView.setSelectedItemId(R.id.action_home);
            STATUS_FRAGMENT=0;
            bottomNavigationView.setVisibility(View.VISIBLE);
        }
        else{
            super.onBackPressed();
        }
    }


}```

只需遵循以下步骤:

在添加片段时，

fragmentTransaction.add(R.id.fragment_container, detail_fragment, "Fragment_tag").addToBackStack(null).commit();

然后在主活动中，重写onBackPressed()

if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
    getSupportFragmentManager().popBackStack();
} else {
    finish();
}

要处理应用程序中的后退按钮，

Fragment f = getActivity().getSupportFragmentManager().findFragmentByTag("Fragment_tag");
if (f instanceof FragmentName) {
    if (f != null) 
        getActivity().getSupportFragmentManager().beginTransaction().remove(f).commit()               
}

就是这样!

你可以像这样使用父活动的onBackPressedDispatcher:

val backpress = requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, true) {
            // here dispatcher works for any action when back pressed
}

你也可以从fragment中启用/禁用backpress按钮，如下所示:

backpress.isEnabled = true/false

这只是一个小代码，可以做到这一点:

 getActivity().onBackPressed();

希望它能帮助到别人:)