Kotlin 片段：活动在 onActivityCreated 之外返回 null

Question

我正在尝试在片段中使用startActionMode 。

startActionMode在onActivityCreated方法中工作正常。 但在onActivityCreated方法之外，活动返回 null ，因此activity?.startActionMode(mActionModeCallback)不起作用。

package com.akshat.music
import android.app.Activity
import android.content.ContentUris
import android.content.Context
import android.net.Uri
import android.os.Bundle
import android.provider.MediaStore
import android.util.Log
import android.view.*
import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentActivity
import androidx.recyclerview.widget.LinearLayoutManager
import kotlinx.android.synthetic.main.fragment_songs.*
import java.io.File


class SongsFragment : Fragment(), SongListAdapter.OnItemClickListener{

private val allSongs = ArrayList<Songs>()
var activityRef: FragmentActivity? = null


override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)
    activityRef = activity;
    if (activityRef == null)
        Log.d("MIKE", " $activityRef is empty");
    else
        Log.d("MIKE", " $activityRef is not empty");
    recyclerView.apply {
        layoutManager = LinearLayoutManager(activity)
        adapter = SongListAdapter(allSongs, SongsFragment().apply { this@SongsFragment })
    }
    activity?.startActionMode(mActionModeCallback) // works

}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    getAudioDirectories()
}

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    return inflater!!.inflate(R.layout.fragment_songs, container, false);
}

override fun onItemClicked(song: Songs, position: Int) {
    if (activityRef == null)
        Log.d("MIKE", " $activityRef is empty");
    else
        Log.d("MIKE", " $activityRef is not empty");
    activity?.startActionMode(mActionModeCallback)  // doesn't work
    activityRef?.startActionMode(mActionModeCallback) // activityRef is null
}

private val mActionModeCallback: ActionMode.Callback =
    object : ActionMode.Callback {
        override fun onCreateActionMode(
            actionMode: ActionMode,
            menu: Menu
        ): Boolean {
            val menuInflater = actionMode.menuInflater
            menuInflater.inflate(R.menu.contextual_action_bar, menu)
            return true
        }

        override fun onPrepareActionMode(
            actionMode: ActionMode,
            menu: Menu
        ): Boolean {
            return false
        }

        override fun onActionItemClicked(
            actionMode: ActionMode,
            menuItem: MenuItem
        ): Boolean {
            return false
        }

        override fun onDestroyActionMode(actionMode: ActionMode) {
            var actionMode: ActionMode? = actionMode
            actionMode = null
        }
    }

private fun getAudioDirectories() {
    /////////////
    }
}
}

如何获取对onActivityCreated方法之外的活动的引用？ 谢谢

Answer 1

您可以将上下文转换为 Activity 并从那里调用它，例如

override fun onItemClicked(song: Songs, position: Int) {
    ...
    (requireContext() as Activity).startActionMode(mActionModeCallback)
}

Answer 2

您应该能够从片段内的任何位置获取对活动的引用。 在某些情况下，活动可以是 null 例如在onDetach()或OnCreateView()内部，因此活动并不总是需要一个值。

尝试使用 Stachu 提到的 requireContext() ：

    (requireContext() as Activity).startActionMode(mActionModeCallback)