java - 考虑到其创建顺序，如何管理片段和 Activity ？

Question

我想做什么

我想在Android应用程序中从Fragment调用Activity来同时实现Fragment和Activity之间的转换。

我已成功从片段文件调用 Activity 文件的一个特定方法，但在从片段调用整个 Activity 时遇到问题。

SampleFragment - MainActivity's fragmentMethod() is called once  the button is clicked on the page
Sample1Fragment - FormActivity is called once the button is clicked on the page
FormActivity - FormActivity2 is called once the button is clicked on the page

以上与我之前的问题相同:How can I fix the code to call Activity from Fragment in Android App?

错误和问题

已成功构建项目并在模拟器上运行应用程序。然而，当我点击Sample1Fragment上的FormActivity按钮时，屏幕突然关闭了。

我无法解决此问题。

Sample1Fragment.kt 上以下部分的错误消息

Modifier 'override' is not applicable to 'local function'

Sample1Fragment.kt

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        override fun onViewCreated(view: View, savedInstanceState: Bundle) {
            btnClick2.setOnClickListener(object:View.OnClickListener{

                // here I would like to move to FormActivity
                override fun onClick(v: View?) {
                    activity?.startActivity(Intent(context, FormActivity::class.java))
                }

            })
        }

当前代码

MainActivity.kt

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // make the list of fragment
        val fragmentList = arrayListOf<Fragment>(
            SampleFragment(),
            Sample1Fragment()
        )

        // create instance of adapter
        val adapter = SamplePagerAdapter(supportFragmentManager, fragmentList)
        /// set adapter
        viewPager.adapter = adapter
    }



    public fun fragmentMethod() {
        Toast.makeText(this@MainActivity, "Method called From Fragment", Toast.LENGTH_LONG).show();
    }
}

SampleFragment.kt

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.fragment_sample.*

class SampleFragment : Fragment() {


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        arguments?.let {

        }
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_sample, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        btnClick.setOnClickListener(object:View.OnClickListener{
            override fun onClick(v: View?) {
                (activity as MainActivity).fragmentMethod()
            }

        })
    }



}

Sample1Fragment.kt

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.fragment_sample1.*
import android.content.Intent

class Sample1Fragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_sample1, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        override fun onViewCreated(view: View, savedInstanceState: Bundle) {
            btnClick2.setOnClickListener(object:View.OnClickListener{

                // here I would like to move to FormActivity
                override fun onClick(v: View?) {
                    activity?.startActivity(Intent(context, FormActivity::class.java))
                }

            })
        }

    }

}

开发环境

Android Studio 3.6.1

我尝试做什么

它尝试使用答案中推荐的 SingleLiveEvent，但我还没有找到在我当前的代码 ListViewModel.kt 和 上实现它的适当方法>Sample1Fragment.kt.

引用:LiveData with SnackBar, Navigation and other events (the SingleLiveEvent case)

ListViewModel.kt

import android.app.usage.UsageEvents
import android.os.Build
import androidx.annotation.RequiresApi
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class ListViewModel : ViewModel() {
    private val _navigateToDetails = MutableLiveData<UsageEvents.Event<String>>()

    val navigateToDetails : LiveData<UsageEvents.Event<String>>
        get() = _navigateToDetails


    @RequiresApi(Build.VERSION_CODES.LOLLIPOP)
    fun userClicksOnButton(itemId: String) {
        _navigateToDetails.value =
            UsageEvents.Event(itemId)  // Trigger the event by setting a new Event as a new value
    }
}

这些是我面临的错误消息

Sample1Fragment.kt

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import kotlinx.android.synthetic.main.fragment_sample1.*
import android.content.Intent


class Sample1Fragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_sample1, container, false)
    }

    myViewModel.navigateToDetails.observe(this, Observer {
        it.getContentIfNotHandled()?.let { // Only proceed if the event has never been handled
        fun onViewCreated(view: View, savedInstanceState: Bundle) {
            btnClick2.setOnClickListener(object:View.OnClickListener{

                // here I would like to move to FormActivity
                override fun onClick(v: View?) {
                    activity?.startActivity(Intent(context, FormActivity::class.java))
                }

            })
        }
    })

    }

}

这是我面临的错误消息

Answer 1

在 Sample1Fragment.kt 中，您正在重写 onViewCreated() 内部 onActivityCreated() 方法。
这就是为什么你会得到

Modifier 'override' is not applicable to 'local function'

关于导航:有几种可能的方法可以在不同的 Activity 和片段之间导航:

1. navigation ，这通常是最容易实现的
2. callbacks ，当您在 Android Studio 中创建 Fragment 时已预设
3. view model ，在片段及其 Activity 之间共享。在这种情况下，我建议使用 SingleLiveEvent触发事件。