考虑到片段的创建顺序，我如何管理片段和 activity？

Question

我想做什么

我想从 Fragment 调用 Activity 以在 Android 应用程序中同时实现片段和活动之间的转换。

我已经成功地从片段文件调用 activity 文件的一个特定方法，但我无法从片段调用整个 activity。

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

以上同我之前的问题：

错误和问题

在模拟器上构建项目和运行应用程序成功。但是在Sample1Fragment.

上点击调用FormActivity的按钮时突然关机了

我无法解决这个问题。

Sample1Fragment.kt

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

我尝试做的事情

它尝试使用在答案中推荐的 SingleLiveEvent，但我还没有找到在我当前的代码 ListViewModel.kt 和 Sample1Fragment.kt 上实现它的合适方法。

参考：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，您正在覆盖 onViewCreated() inside onActivityCreated() 方法。
这就是为什么你得到

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

关于导航： 有几种可能的方式如何在不同的活动和片段之间导航：

1. navigation，通常最容易实现
2. callbacks，在 Android Studio
中创建片段时已经预设
3. view model, shared between fragment and it's activity. In this case, I suggest using SingleLiveEvent 触发事件。