问题

在Activity的onCreate生命周期里执行下列代码,导致 xxx does not have a NavController set on xxx 报错

布局:

   <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:name="androidx.navigation.fragment.NavHostFragment"
        app:defaultNavHost="true"
        app:navGraph="@navigation/snake_settings_nav"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toRightOf="@id/settings"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@id/titleLayout" />

    private NavController mController;

    @Override protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBinding = SnakeActivitySettingsBinding.inflate(getLayoutInflater());
        setContentView(mBinding.getRoot());
        mController = Navigation.findNavController(SettingsActivity.this, R.id.fragment);

    }

问题原因

问题原因就是在onCreate生命周期里FragmentContainerView对应的NavController未构建成功.

解决方法

方式一:

很简单直接onStart生命周期里获取NavController,如下代码:

 @Override
    protected void onStart() {
        super.onStart();
        mController = Navigation.findNavController(SettingsActivity.this, R.id.fragment);
    }

或者,将获取NavController的任务放到view创建测绘完成后在执行

        mBinding.fragment.post(new Runnable() {
            @Override public void run() {
                mController = Navigation.findNavController(SettingsActivity.this, R.id.fragment);
            }
        });

方式二:

使用getSupportFragmentManager获取

       NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.fragment);
        mController = navHostFragment.getNavController();