每次当你显示fragment时，你必须把它标签放入backstack:

FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.setTransition(FragmentTransaction.TRANSIT_ENTER_MASK);       
ft.add(R.id.primaryLayout, fragment, tag);
ft.addToBackStack(tag);
ft.commit();        

然后当你需要获取当前片段时，你可以使用这个方法:

public BaseFragment getActiveFragment() {
    if (getSupportFragmentManager().getBackStackEntryCount() == 0) {
        return null;
    }
    String tag = getSupportFragmentManager().getBackStackEntryAt(getSupportFragmentManager().getBackStackEntryCount() - 1).getName();
    return (BaseFragment) getSupportFragmentManager().findFragmentByTag(tag);
}

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();
}

以下是我的解决方案，我发现它适用于低片段场景

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

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.

如果你有嵌套片段，如viewpagers内部viewpagers等 你想要得到所有嵌套的片段。

感谢Matt Mombrea的好意，回答一个稍微调整的版本。

private List<Fragment> getVisibleFragments(List<Fragment> searchFragments, List<Fragment> visibleFragments) {
    if (searchFragments != null && searchFragments.size() > 0) {
        for (Fragment fragment : searchFragments) {
            List<Fragment> nestedFragments = new ArrayList<>();
            List<Fragment> childFMFragments = fragment.getChildFragmentManager().getFragments();
            List<Fragment> fmFragments = fragment.getFragmentManager().getFragments();
            fmFragments.retainAll(childFMFragments);
            nestedFragments.addAll(childFMFragments);
            nestedFragments.addAll(fmFragments);
            getVisibleFragments(nestedFragments, visibleFragments);
            if (fragment != null && fragment.isVisible()) {
                visibleFragments.add(fragment);
            }
        }
    }
    return visibleFragments;
}

用法如下:

List<Fragment> allVisibleFragments = getVisibleFragments(searchFragments, visibleFragments);

例如:

List<Fragment> visibleFragments = new ArrayList<>();
List<Fragment> searchFragments = MainActivity.this.getSupportFragmentManager().getFragments();
Toast.makeText(this, ""+getVisibleFragments(searchFragments, visibleFragments), Toast.LENGTH_LONG).show();

如果你从父活动中获得Fragment的当前实例，你可以

findFragmentByID(R.id.container);

这实际上是视图中填充的fragment的当前实例。我也有同样的问题。我不得不加载相同的片段两次，保持一个备份。

下面的方法不起作用。它只是得到一个带有标签的片段。不要在这个方法上浪费时间。我相信它有它的用途，但获得同一片段的最新版本不是其中之一。

findFragmentByTag()