如果你使用androidx.appcompat:appcompat:1.1.0或以上，那么你可以添加一个OnBackPressedCallback到你的片段，如下所示

requireActivity()
    .onBackPressedDispatcher
    .addCallback(this, object : OnBackPressedCallback(true) {
        override fun handleOnBackPressed() {
            Log.d(TAG, "Fragment back pressed invoked")
            // Do custom work here    

            // if you want onBackPressed() to be called as normal afterwards
            if (isEnabled) {
                isEnabled = false
                requireActivity().onBackPressed()
            }
        }
    }
)

参见https://developer.android.com/guide/navigation/navigation-custom-back

新的和更好的方法:以下片段中的代码段将帮助您捕获反按事件。

JAVA

@Override
public void onAttach(@NonNull Context context) {
    super.onAttach(context);

    OnBackPressedCallback callback = new OnBackPressedCallback(true) {
        @Override
        public void handleOnBackPressed() {
            Toast.makeText(mContext, "back pressed", Toast.LENGTH_SHORT).show();
            
            // And when you want to go back based on your condition
            if (yourCondition) {
                this.setEnabled(false);
                requireActivity().onBackPressed();
            }
        }
    };

    requireActivity().getOnBackPressedDispatcher().addCallback(this, callback);
}

科特林

activity?.onBackPressedDispatcher?.addCallback(viewLifecycleOwner, object : OnBackPressedCallback(true) {
    override fun handleOnBackPressed() {

    }
})

像这样执行 Fragment_1 -> Fragment_2 -> Fragment_3

    Button btn = (Button) rootView.findViewById(R.id.your_button_id);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            Fragment_2 nextFrag= new Fragment_2();
            getActivity().getSupportFragmentManager().beginTransaction()
                    .replace(R.id.content_frame, nextFrag,getTag())
                    .addToBackStack(null)
                    .commit();

        }
    });

Fragment_3 -> Fragment_2 -> Fragment_1

Step_1:在Base Activity中创建一个可公开访问的字符串

step2:每当一个新的Fragment被激活时，在Base Activity中改变String的值

Step_3:然后添加onBackPressed()方法，并将字符串值传递给另一个方法，其中fagments可以被替换

在基础活动中

public static String currentFragment=null;

@Override
public void onBackPressed() 
{
        displayPreviousFragment(currentFragment);
}

public void displayPreviousFragment(String currentFragment)
{
    //creating fragment object
    Fragment fragment = null;

    //initializing the fragment object which is selected
    switch (currentFragment)
    {
        case "Fragment_2"    :   fragment = new Fargment_1();     break;
        case "Fragment_3"    :   fragment = new Fragment_2();     break;
    }

    //replacing the fragment
    if (fragment != null) {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.replace(R.id.frame_to_replace_fragment, fragment);
        ft.commit();
    }
}

在Fragment_2 在OnCreateView方法内

BaseActivity.currentFragment="Fragment_2";

在Fragment_3 在OnCreateView方法内

BaseActivity.currentFragment="Fragment_3";

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
        }
    }
}

if片段和使用NavController简单调用

findNavController().popBackStack()

简单地在onKeyUp()中做:

@Override
public boolean onKeyUp(int keyCode, KeyEvent event) {

    if (keyCode == KeyEvent.KEYCODE_BACK) {
        // do something
        return true;  // return true if back handled, false otherwise
    }

    return super.onKeyUp(keyCode, event);
}