我是这样做的，对我很管用

简单的界面

FragmentOnBackClickInterface.java

public interface FragmentOnBackClickInterface {
    void onClick();
}

示例实现

MyFragment.java

public class MyFragment extends Fragment implements FragmentOnBackClickInterface {

// other stuff

public void onClick() {
       // what you want to call onBackPressed?
}

然后在activity中重写onBackPressed

    @Override
public void onBackPressed() {
    int count = getSupportFragmentManager().getBackStackEntryCount();
    List<Fragment> frags = getSupportFragmentManager().getFragments();
    Fragment lastFrag = getLastNotNull(frags);
    //nothing else in back stack || nothing in back stack is instance of our interface
    if (count == 0 || !(lastFrag instanceof FragmentOnBackClickInterface)) {
        super.onBackPressed();
    } else {
        ((FragmentOnBackClickInterface) lastFrag).onClick();
    }
}

private Fragment getLastNotNull(List<Fragment> list){
    for (int i= list.size()-1;i>=0;i--){
        Fragment frag = list.get(i);
        if (frag != null){
            return frag;
        }
    }
    return null;
}

以下是我对这个问题的解决方案:

活动A:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) 
{
    super.onActivityResult(requestCode, resultCode, data);

    if(requestCode == REQUEST_CODE)
    {
        if(resultCode == Activity.RESULT_OK)
        {
            tvTitle.setText(data.getExtras().getString("title", ""));
        }
    }
}

活动B:

@Override
public void onBackPressed() 
{
    setResult(Activity.RESULT_OK, getIntent());

    super.onBackPressed();
}

活动b保存片段。

在片段:

private void setText(String text)
    {
        Intent intent = new Intent();
        intent.putExtra("title", text);
        getActivity().setIntent(intent);
    }

通过这种方式，活动A中的意图对象“data”将从片段中获取字符串

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()
    }
}

谷歌发布了一个新的API来处理碎片中的onBackPressed:

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

            }
        })

这是我的答案，处理碎片按下后退按钮。 我有底部导航与五个项目(A, B, C, D,E)附加到活动时，点击一个项目，我附加的片段到活动。 让我们假设点击项目(A)，我附加片段(A)，然后如果点击项目(B)，我附加片段(B)，以此类推。

点击项目的流程。A - C > B - > - > D - > E

返回按钮按下的流程。D E - > - > - > C > B

 @Override
        public void onBackPressed() {
            int count = getSupportFragmentManager().getBackStackEntryCount();
            if (count == 0) {
                super.onBackPressed();
            } else {
                int index = ((getSupportFragmentManager().getBackStackEntryCount()) -1);
                getSupportFragmentManager().popBackStack();
                FragmentManager.BackStackEntry backEntry = getSupportFragmentManager().getBackStackEntryAt(index);
                int stackId = backEntry.getId();
                bottomNavigationView.getMenu().getItem(stackId).setChecked(true);
            }
        }

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)
      }