fragment中的navigation.findnavcontroller（）无法正常工作

Question

我正在尝试在某些条件下使用导航来使用导航对相应片段的相应片段进行Develope单活动应用程序（请参阅下面的代码片段）。但是，当我使用条件 isloggedin == true 启动应用程序时，出现一个例外：

2022-05-01 17:42:22.888 14974-14974/com.example.shercofaqapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.shercofaqapp, PID: 14974
java.lang.IllegalStateException: Fragment LoginFragment{d69320f} (2bebfc01-ce4e-435e-be4f-821b5d142eb8 id=0x7f0a0096) did not return a View from onCreateView() or this was called before onCreateView().
    at androidx.fragment.app.Fragment.requireView(Fragment.java:2008)
    at com.example.shercofaqapp.view.LoginFragment.onCreateView(LoginFragment.kt:47)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2995)
    at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:523)
    at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:261)
    at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:113)
    at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1374)
    at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2841)
    at androidx.fragment.app.FragmentManager.dispatchViewCreated(FragmentManager.java:2777)
    at androidx.fragment.app.Fragment.performViewCreated(Fragment.java:3020)
    at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:551)
    at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:261)
    at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:113)
    at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1374)
    at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2841)
    at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:2784)
    at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:262)
    at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:478)
    at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:246)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1425)
    at android.app.Activity.performStart(Activity.java:7825)
    at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3294)
    at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
    at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
    at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7356)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

这是我的代码的片段：

package com.example.shercofaqapp.view

import android.content.Context
import android.os.Bundle
import android.text.TextUtils
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.Toast
import androidx.databinding.DataBindingUtil
import androidx.navigation.Navigation
import com.example.shercofaqapp.R
import com.example.shercofaqapp.databinding.FragmentLoginBinding
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.FirebaseUser
import kotlin.properties.Delegates

class LoginFragment : Fragment() {

    lateinit var binding: FragmentLoginBinding
    private var isLoggedIn by Delegates.notNull<Boolean>()

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {

        val sharedPref = requireActivity()
            .getSharedPreferences("MyPreferences", Context.MODE_PRIVATE)
        val editor = sharedPref.edit()
        var isLoggingIn = false


        binding = DataBindingUtil.inflate(
            inflater, R.layout.fragment_login, container, false)

        binding.apply {
            //Log in / sign up logic
            isLoggedIn = sharedPref.getBoolean("isLoggedIn", false)
            if (isLoggedIn) {
                Navigation.findNavController(requireView())
                    .navigate(R.id.action_loginFragment_to_garageFragment)
            }
        }
        return binding.root
    }
}

问题是：在 oncreateview（）返回之前，如何获取导航 ？

我尝试使用 this 在我的情况下，请了解我应该传递到 .findnavcontroller（...）的参数。

PS NavHostFragment是主要的：

<?xml version="1.0" encoding="utf-8"?>
<layout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/containerView"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph" />

</layout>

Answer 1

根据导航到目标文档，您应该使用< fragment 的kotlin扩展名，这是一个便利包装器，围绕调用

binding.apply {
    //Log in / sign up logic
    isLoggedIn = sharedPref.getBoolean("isLoggedIn", false)
    if (isLoggedIn) {
        // Make sure to:
        // 1. Add a dependency on navigation-fragment-ktx in your build.gradle
        // 2. import androidx.navigation.fragment.findNavController
        findNavController()
            .navigate(R.id.action_loginFragment_to_garageFragment)
    }
}

Answer 2

findnavcontroller（需求反应（））
。

​

​

Navigation.findNavController(getView()).navigate(R.id.action_loginFragment_to_garageFragment)