从 MainActivity 类将新字符串注入 Fragment/ViewModel

如何解决从 MainActivity 类将新字符串注入 Fragment/ViewModel

使用 android studio 底部导航活动模板，希望每次从底部导航访问主页时更改我的 HomeFragment / Homeviewmodel 页面文本。当我第一次点击它时，它显示的文字应该与第二次显示的文字不同。

我能够创建一个保存文本输出的变量，并且我已经创建了一个应该能够更新该变量的函数，但我无法在 MainActivity 类中调用它。有什么想法吗？

主活动

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val navView: BottomNavigationView = findViewById(R.id.nav_view)
        val navController = findNavController(R.id.nav_host_fragment)
        val appBarConfiguration = AppBarConfiguration(setof(
                R.id.navigation_home,R.id.navigation_dashboard,R.id.navigation_notifications))


        navView.setupWithNavController(navController)
    }

HomeFragment

class HomeFragment : Fragment() {

    private lateinit var homeviewmodel: Homeviewmodel
    var chosenOutput:String = "primary output";

    override fun onCreateView(
            inflater: LayoutInflater,container: ViewGroup?,savedInstanceState: Bundle?
    ): View? {
        //using update here crashes the app
        homeviewmodel =
                viewmodelProvider(this).get(Homeviewmodel::class.java)
        val root = inflater.inflate(R.layout.fragment_home,container,false)
        val textView: TextView = root.findViewById(R.id.text_home)
        homeviewmodel.text.observe(viewLifecycleOwner,Observer {
            textView.text = chosenOutput; //changing 'chosenOutput' to 'it' means that text update must take place within Homeviewmodel instead of fragment.
        })
       
        return root
    }
    fun update(result: String)
    {
        chosenOutput = result; 
    }
}

Homeviewmodel

class Homeviewmodel : viewmodel() {
    var temp:String = "demo"
    //constructor()
    private val _text = mutablelivedata<String>().apply {
        value = temp;
    }
    fun update(result: String) //function may be obsolete if text change can be done in fragment.
    {
        temp = result; 
    }
    val text: LiveData<String> = _text

}

将字符串注入我给出的 Fragment 类中的 viewmodel 或 MainActivity 的正确方法是什么？

解决方法

您可以更改方法并将 chosenOutput 变量存储在 MainActivity 并在每次选择 HomeFragment

时更新它

class MainActivity : AppCompatActivity() 
{
    var chosenOutput = "primary output"
    ...

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val navView: BottomNavigationView = findViewById(R.id.nav_view)
        val navController = findNavController(R.id.nav_host_fragment)
        val appBarConfiguration = AppBarConfiguration(setOf(
                R.id.navigation_home,R.id.navigation_dashboard,R.id.navigation_notifications))

        navView.apply {
            setupWithNavController(navController)

            setOnNavigationItemSelectedListener { menuItem ->
                when (menuItem.itemId) {
                    R.id.navigation_home -> {
                        chosenOutput = "new value" // <-- Update chosen output here
                        // Add this method to call the `onResume` method in the fragment
                        showFragment(HomeFragment.newInstance())
                    }
                    else { }
                }

                true
        }
    }

    private fun showFragment(fragment: Fragment) {
        supportFragmentManager.beginTransaction()
                .replace(R.id.nav_host_fragment,fragment)
                .addToBackStack(null)
                .commit()
    }
}

然后您可以使用此代码在您的 Fragment 中检索更新后的字符串。 请注意，我添加了 companion object 以避免创建 Fragment 的多个实例。

class HomeFragment : Fragment() 
{
    companion object
    {
        fun newInstance() : HomeFragment
        {
            return HomeFragment()
        }
    }

    override fun onResume() {
        super.onResume()

        val newChosenOutput = (activity as MainActivity).chosenOutput
    }
}