final FragmentManager fm=this.getSupportFragmentManager();
final Fragment fragment=fm.findFragmentByTag("MY_FRAGMENT");

if(fragment != null && fragment.isVisible()){
      Log.i("TAG","my fragment is visible");
}
else{
      Log.i("TAG","my fragment is not visible");
}

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.

如果你在使用Kotlin:

var fragment = supportFragmentManager.findFragmentById(R.id.fragment_container)

R.id.fragment_container是片段在其活动中呈现的id

或者如果你想要一个更好的解决方案:

supportFragmentManager.findFragmentById(R.id.content_main)?.let {
    // the fragment exists

    if (it is FooFragment) {
        // The presented fragment is FooFragment type
    }
}

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
        }
    }
}

  SupportFragmentManager.BeginTransaction().Replace(Resource.Id.patientFrameHome, test, "Test").CommitAllowingStateLoss();  

var fragment = SupportFragmentManager.FindFragmentByTag("Test") as V4Fragment;

  if (fragment == null && fragment.IsVisiable is true)
{
}

在你的活动中，在创建之前初始化你的片段

    MyFragment myFragment = new MyFragment(); // add this
 @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
........

然后调用该方法来查看您的片段

openFragment(this.myFragment);

这是方法

(R.id.frame_container)是xml文件中的片段容器id (框架布局)

 private void openFragment(final Fragment fragment)   {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.replace(R.id.frame_container, fragment);
        transaction.commit();

    }

那么在你的活动中，Override方法应该是这样的

    public void onBackPressed() {
            if (myFragment.isVisible()) {
                myFragment.onBackPressed(this);
                return;
            }
            super.onBackPressed();
        }

然后在你的片段中放入这个方法

public  void onBackPressed(Activity activity) {
    Toast.makeText(activity, "Back Pressed inside Fragment", Toast.LENGTH_SHORT).show();
}