这是我的解决方案:

在MyActivity.java:

public interface OnBackClickListener {
        boolean onBackClick();
    }

    private OnBackClickListener onBackClickListener;

public void setOnBackClickListener(OnBackClickListener onBackClickListener) {
        this.onBackClickListener = onBackClickListener;
    }

@Override
    public void onBackPressed() {
        if (onBackClickListener != null && onBackClickListener.onBackClick()) {
            return;
        }
        super.onBackPressed();
    }

在片段中:

((MyActivity) getActivity()).setOnBackClickListener(new MyActivity.OnBackClickListener() {
    @Override
    public boolean onBackClick() {
        if (condition) {
            return false;
        }

        // some codes

        return true;
    }
});

更新:OnBackPressedDispatcher应该被使用。

指南如何使用可在developer.android.com/guide/navigation/navigation-custom-back

你可以在activity中注册fragment来处理背按:

interface BackPressRegistrar {
    fun registerHandler(handler: BackPressHandler)
    fun unregisterHandler(handler: BackPressHandler)
}

interface BackPressHandler {
    fun onBackPressed(): Boolean
}

用法:

在片段:

private val backPressHandler = object : BackPressHandler {
    override fun onBackPressed(): Boolean {
        showClosingWarning()
        return false
    }
}

override fun onResume() {
    super.onResume()
    (activity as? BackPressRegistrar)?.registerHandler(backPressHandler)
}

override fun onStop() {
    (activity as? BackPressRegistrar)?.unregisterHandler(backPressHandler)
    super.onStop()
}

在活动:

class MainActivity : AppCompatActivity(), BackPressRegistrar {


    private var registeredHandler: BackPressHandler? = null
    override fun registerHandler(handler: BackPressHandler) { registeredHandler = handler }
    override fun unregisterHandler(handler: BackPressHandler) { registeredHandler = null }

    override fun onBackPressed() {
        if (registeredHandler?.onBackPressed() != false) super.onBackPressed()
    }
}

public class MyActivity extends Activity {

    protected OnBackPressedListener onBackPressedListener;

    public interface OnBackPressedListener {
        void doBack();
    }

    public void setOnBackPressedListener(OnBackPressedListener onBackPressedListener) {
        this.onBackPressedListener = onBackPressedListener;
    }

    @Override
    public void onBackPressed() {
        if (onBackPressedListener != null)
            onBackPressedListener.doBack();
        else
            super.onBackPressed();
    } 

    @Override
    protected void onDestroy() {
        onBackPressedListener = null;
        super.onDestroy();
    }
}

在你的片段中添加以下内容，不要忘记实现mainactivity的接口。

public class MyFragment extends Framgent implements MyActivity.OnBackPressedListener {
    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
         ((MyActivity) getActivity()).setOnBackPressedListener(this);
    }

@Override
public void doBack() {
    //BackPressed in activity will call this;
}

}

在kotlin的情况下使用这个onAttach回调

        override fun onAttach(context: Context) {
          super.onAttach(context)
           val callback: OnBackPressedCallback = object : 
                 OnBackPressedCallback(true) {
                 override fun handleOnBackPressed() {
                 // your onbackpressed code 


            }
          }
      requireActivity().onBackPressedDispatcher.addCallback(this, callback)
      }

不要实现ft.addToBackStack()方法，这样当你按下返回按钮时，你的活动就会结束。

proAddAccount = new ProfileAddAccount();
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, proAddAccount);
//fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();

使用导航组件，你可以这样做:

Java

public class MyFragment extends Fragment {

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // This callback will only be called when MyFragment is at least Started.
    OnBackPressedCallback callback = new OnBackPressedCallback(true /* enabled by default */) {
        @Override
        public void handleOnBackPressed() {
            // Handle the back button event
        }
    });
    requireActivity().getOnBackPressedDispatcher().addCallback(this, callback);

    // The callback can be enabled or disabled here or in handleOnBackPressed()
}
...
}

科特林

class MyFragment : Fragment() {

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    // This callback will only be called when MyFragment is at least Started.
    val callback = requireActivity().onBackPressedDispatcher.addCallback(this) {
        // Handle the back button event
    }

    // The callback can be enabled or disabled here or in the lambda
}
...
}