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.

为这个FragmentManager返回当前活动的主导航片段。

public @Nullable Fragment getPrimaryNavigationFragment()      
Fragment fragment = fragmentManager.getPrimaryNavigationFragment();  
    

我的方法是基于try / catch，如下所示:

MyFragment viewer = null;
    if(getFragmentManager().findFragmentByTag(MY_TAG_FRAGMENT) instanceOf MyFragment){
    viewer = (MyFragment) getFragmentManager().findFragmentByTag(MY_TAG_FRAGMENT);
}

但也许有更好的办法……

有点奇怪，但我看了FragmentManager$FragmentManagerImpl和以下工作为我:

public static Fragment getActiveFragment(Activity activity, int index)
{
    Bundle bundle = new Bundle();
    String key = "i";
    bundle.putInt(key,index);
    Fragment fragment=null;
    try
    {
        fragment = activity.getFragmentManager().getFragment(bundle, key);
    } catch(Exception e){}
    return fragment;
}

要获得第一个活动片段，请使用0作为索引

你也可以很容易地在logcat中使用URL，它将重定向到当前片段源代码的源代码。首先，你需要在主机活动中添加一个OnBackStackChangedListener，比如-

activity.getChildFragmentManager().addOnBackStackChangedListener(backStackListener);

OnBackStackChangedListener的实现是-

    public FragmentManager.OnBackStackChangedListener backStackListener = () -> {

    String simpleName = "";
    String stackName = getStackTopName().trim();

    if (Validator.isValid(stackName) && stackName.length() > 0) {

      simpleName = stackName.substring(Objects.requireNonNull(stackName).lastIndexOf('.') + 1).trim();

      List<Fragment >
       fragmentList = getChildFragmentManager().getFragments();
      Fragment myCurrentFragment;

      for (int i = 0; i < fragmentList.size(); i++) {
       myCurrentFragment= fragmentList.get(i);
       if (myCurrentFragment.getClass().getSimpleName().equals(simpleName)) {
        //Now you get the current displaying fragment assigned in myCurrentFragment.
        break;
       }
       myFragment = null;
      }
     }


     //The code below is for the source code redirectable logcat which would be optional for you.
     StackTraceElement stackTraceElement = new StackTraceElement(simpleName, "", simpleName + ".java", 50);
     String fileName = stackTraceElement.getFileName();
     if (fileName == null) fileName = "";
     final String info = "Current Fragment is:" + "(" + fileName + ":" +
     stackTraceElement.getLineNumber() + ")";
     Log.d("now", info + "\n\n");
    };

getStackTopName()方法是-

public String getStackTopName() {
    FragmentManager.BackStackEntry backEntry = null;
    FragmentManager fragmentManager = getChildFragmentManager();
    if (fragmentManager != null) {
        if (getChildFragmentManager().getBackStackEntryCount() > 0)
            backEntry = getChildFragmentManager().getBackStackEntryAt(
                    getChildFragmentManager().getBackStackEntryCount() - 1
            );
    }
    return backEntry != null ? backEntry.getName() : null;
}

当您在事务中添加片段时，您应该使用标记。

fragTrans.replace(android.R.id.content, myFragment, "MY_FRAGMENT");

.．.稍后，如果你想检查片段是否可见:

MyFragment myFragment = (MyFragment)getSupportFragmentManager().findFragmentByTag("MY_FRAGMENT");
if (myFragment != null && myFragment.isVisible()) {
   // add your code here
}

参见http://developer.android.com/reference/android/app/Fragment.html