Android 按back时如何更改片段的标题？

我有3个片段，主页、菜单和订单，它们可以通过

bottomnavigtioview

项和它显示的标题加载 一旦从主页导航到订单，如果您想从订单返回主页，标题仍然是“订单”

@凌驾

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    context = this;

     navigation =  findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);


    // attaching bottom sheet behaviour - hide / show on scroll
    CoordinatorLayout.LayoutParams layoutParams = (CoordinatorLayout.LayoutParams) navigation.getLayoutParams();
    layoutParams.setBehavior(new BottomNavigationBehavior());

    toolbar.setTitle(getResources().getString(R.string.title_home));
    Fragment f = HomeFragment.newInstance();
    getSupportFragmentManager().beginTransaction()
            .replace(R.id.frame_container, f)
            .commit();
}`

我也试过这个，但不起作用

`

fragmentmanger.addonbackbackchangedListener->public void onbackbackbackchanged（）{

---

不要在

onNavigationItemSelected

方法中调用

toolbar.setTitle

，而是将其移动到每个片段的

onCreateView

。当您导航回某个片段时，

onCreateView

将再次执行。

在

onResume

中调用它时，对我有效，就像在每个片段中这样：

如果（getActivity（）！=null）{
（（MainActivity）getActivity（））.setActionBarTitle
（context.getResources（）.getString（R.string.YOUR_TITLE））；

这是

main活动

public void setActionBarTitle（字符串标题）{
工具栏.setTitle（标题）；
}

我在fragment.HomeFragment.onCreateView（HomeFragment.java:112）的空对象引用上得到了nullpointerexception'void android.app.ActionBar.setTitle（int）'

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    context = this;

     navigation =  findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);


    // attaching bottom sheet behaviour - hide / show on scroll
    CoordinatorLayout.LayoutParams layoutParams = (CoordinatorLayout.LayoutParams) navigation.getLayoutParams();
    layoutParams.setBehavior(new BottomNavigationBehavior());

    toolbar.setTitle(getResources().getString(R.string.title_home));
    Fragment f = HomeFragment.newInstance();
    getSupportFragmentManager().beginTransaction()
            .replace(R.id.frame_container, f)
            .commit();
}`

        }`