这样比暴露在这里更安全

supportFragmentManager.fragments.lastOrNull()?.let { currentFragment ->
               
      //Do something here
 }

In the main activity, the onAttachFragment(Fragment fragment) method is called when a new fragment is attached to the activity. In this method, you can get the instance of the current fragment. However, the onAttachFragment(Fragment fragment) method is not called when a fragment is popped off the back stack, ie, when the back button is pressed to get the top fragment on top of the stack. I am still looking for a callback method that is triggered in the main activity when a fragment becomes visible inside the activity.

也许最简单的方法是:

public MyFragment getVisibleFragment(){
    FragmentManager fragmentManager = MainActivity.this.getSupportFragmentManager();
    List<Fragment> fragments = fragmentManager.getFragments();
    for(Fragment fragment : fragments){
        if(fragment != null && fragment.getUserVisibleHint())
            return (MyFragment)fragment;
    }
    return null;
}

这对我很有效

嗯，这个问题得到了很多观点和关注，但仍然没有得到遏制 从我的端最简单的解决方案-使用getFragments()。

            List fragments = getSupportFragmentManager().getFragments();
            mCurrentFragment = fragments.get(fragments.size() - 1);

我最近不得不这样做，这里没有一个答案真正适合这种情况。

如果你确信只有一个片段是可见的(全屏)，那么真的要找到backstack顶部的内容。例如，作为Fragment的Kotlin:

import androidx.fragment.app.Fragment

fun Fragment.setVisibilityChangeListener(clazz: Class<out Fragment>, listener: (Boolean) -> Unit) {
    fragmentManager?.run {
        addOnBackStackChangedListener {
            val topFragmentTag = getBackStackEntryAt(backStackEntryCount - 1).name
            val topFragment = findFragmentByTag(topFragmentTag)
            listener(topFragment != null && topFragment::class.java == clazz)
        }
    }
}

像这样使用它:

class MyFragment: Fragment {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        setVisibilityChangeListener(this::class.java) { visible -> 
            // Do stuff
        }
    }
}

您可以添加一个类变量selectedFragment，并且每次更改片段都更新该变量。

public Fragment selectedFragment;
public void changeFragment(Fragment newFragment){
    FragmentManager fragMgr = getSupportFragmentManager();
    FragmentTransaction fragTrans = fragMgr.beginTransaction();
    fragTrans.replace(android.R.id.content, newFragment);
    fragTrans.addToBackStack(null);
    fragTrans.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
    fragTrans.commit();
    //here you update the variable
    selectedFragment = newFragment;
}

然后你可以在任何你想要的地方使用selectedFragment