错误: requestHandler尚未初始化

Question

这个项目是一个回收器视图片段，从flickr获取图像并显示它。我使用循环和处理程序在回收器片段和下载的缩略图片段之间进行通信...我正在使用Android Studio Bumblebee..。

我遇到了一个请求处理程序…错误也许我没有看到一个具体的问题。但是我检查了代码几次，都没有找到缺少的部分…

我得到了这个错误：

kotlin.UninitializedPropertyAccessException: lateinit property requestHandler has not been initialized at com.bignerdranch.android.photogallery.ThumbnailDownloader.queueThumbnail(ThumbnailDownloader.kt:94)

它指的是这一个…

fun queueThumbnail(target: T, url: String){ //page 510
        Log.i(TAG, "Got a URL: $url")
        requestMap[target] = url //page 521
        requestHandler.obtainMessage(MESSAGE_DOWNLOAD, target)
            .sendToTarget()
    }

此代码正由Recyclerview从另一个片段中调用

override fun onBindViewHolder(holder: PhotoHolder, position: Int) {
            val galleryItem = galleryItems[position]
            //holder.bindTitle(galleryItem.title)
            val placeholder: Drawable = ContextCompat.getDrawable(
                requireContext(),
                R.drawable.hala_atamleh
            )?: ColorDrawable()
            holder.bindTitle(placeholder) //should be bindDrawable but it didn't work

        -->    thumbnailDownloader.queueThumbnail(holder, galleryItem.url) //page 515
        }

据我所知，我是这样初始化的：

@Suppress("UNCHECKED_CAST") //page 522
    @SuppressLint("HandlerLeak")
    override fun onLooperPrepared() {
        requestHandler = object : Handler(Looper.getMainLooper()){
            override fun handleMessage(msg: Message) {
                if (msg.what == MESSAGE_DOWNLOAD){
                    val target = msg.obj as T
                    Log.i(TAG, "Got a request for URL: ${requestMap[target]}")
                    handleRequest(target)
                }
            }
        }
    }

为了得到这个错误，我错过了什么？

文件代码如下：

import android.annotation.SuppressLint
import android.graphics.Bitmap
import android.os.Handler
import android.os.HandlerThread
import android.os.Looper
import android.os.Message
import android.util.Log
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleObserver
import androidx.lifecycle.OnLifecycleEvent
import java.util.concurrent.ConcurrentHashMap

private const val TAG = "ThumbnailDownloader"
private const val MESSAGE_DOWNLOAD = 0

class ThumbnailDownloader<in T>(private val responseHandler: Handler, //page. 524
private val onThumbnailDownloaded : (T, Bitmap) -> Unit)
    : HandlerThread(TAG) /*, LifecycleObserver page 512*/ { //page. 510

    private lateinit var requestHandler: Handler //page516++
    private var hasQuit = false
    private val requestMap = ConcurrentHashMap<T, String>()
    private val flickrFetchr = FlickrFetchr()



    val fragmentLifecycleObserver: LifecycleObserver = //page 527
        object : LifecycleObserver{

            @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
            fun setup() {
                Log.i(TAG, "Starting background thread")
                start() //page 514
                looper
            }

            @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
            fun tearDown() {
                Log.i(TAG, "Destroying background thread")
                quit()
            }
        }

    @Suppress("UNCHECKED_CAST") //page 522
    @SuppressLint("HandlerLeak")
    override fun onLooperPrepared() {
        requestHandler = object : Handler(Looper.getMainLooper()){
            override fun handleMessage(msg: Message) {
                if (msg.what == MESSAGE_DOWNLOAD){
                    val target = msg.obj as T
                    Log.i(TAG, "Got a request for URL: ${requestMap[target]}")
                    handleRequest(target)
                }
            }
        }
    }

    //page 528
    val viewLifecycleObserver : LifecycleObserver =
        object : LifecycleObserver {
            @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
            fun tearDown(){
                Log.i(TAG, "Clearing all requests from Queue")
                requestHandler.removeMessages(MESSAGE_DOWNLOAD)
                requestMap.clear()
            }
        }

    override fun quit(): Boolean {
        hasQuit = true
        return super.quit()
    }


    fun queueThumbnail(target: T, url: String){ //page 510
        Log.i(TAG, "Got a URL: $url")
        requestMap[target] = url //page 521
        requestHandler.obtainMessage(MESSAGE_DOWNLOAD, target)
            .sendToTarget()
    }

    fun clearQueue() {
        requestHandler.removeMessages(MESSAGE_DOWNLOAD)
        requestMap.clear()
    }


    private fun handleRequest(target: T){
        val url = requestMap[target] ?: return
        val bitmap = flickrFetchr.fetchPhoto(url) ?: return

        responseHandler.post(Runnable { //page.526
            if(requestMap[target] != url || hasQuit){
                return@Runnable
            }
            requestMap.remove(target)
            onThumbnailDownloaded(target, bitmap)
        })
    }
}

PHOTOGalleryFragment：

package com.bignerdranch.android.photogallery

import android.graphics.drawable.BitmapDrawable
import android.graphics.drawable.ColorDrawable
import android.graphics.drawable.Drawable
import android.os.Bundle
import android.os.Handler
import android.os.Looper
import android.util.Log
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ImageView
import androidx.core.content.ContextCompat
import androidx.fragment.app.Fragment
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import androidx.recyclerview.widget.GridLayoutManager
import androidx.recyclerview.widget.RecyclerView

private lateinit var photoRecyclerView : RecyclerView
private lateinit var photoGalleryViewModel: PhotoGalleryViewModel
private lateinit var thumbnailDownloader: ThumbnailDownloader<PhotoGalleryFragment.PhotoHolder> //it should be PhotoHolder not Type or Handler
private const val TAG = "PhotoGalleryFragment"

class PhotoGalleryFragment:Fragment() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        retainInstance = true //page 511


        photoGalleryViewModel = //page 494
            ViewModelProvider(this).get(PhotoGalleryViewModel::class.java)


        //thumbnailDownloader = ThumbnailDownloader() //p.513

        val responseHandler = Handler(Looper.getMainLooper())//page 525
        thumbnailDownloader =
            ThumbnailDownloader(responseHandler){ photoHolder, bitmap ->
                val drawable = BitmapDrawable(resources, bitmap)
                photoHolder.bindDrawable(drawable)
            }
        lifecycle.addObserver(thumbnailDownloader.viewLifecycleObserver)
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        viewLifecycleOwner.lifecycle.addObserver( //page. 529
            thumbnailDownloader.viewLifecycleObserver
        )
        val view = inflater.inflate(R.layout.fragment_photo_gallery, container, false)

        photoRecyclerView = view.findViewById(R.id.photo_recycler_view)
        photoRecyclerView.layoutManager = GridLayoutManager(context,3)

        return view
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        photoGalleryViewModel.galleryItemLiveData.observe(
            viewLifecycleOwner,
            Observer { galleryItems ->
            Log.d(TAG,"Have gallery items from ViewModel $galleryItems" )

                photoRecyclerView.adapter = PhotoAdapter(galleryItems)

            }
        )
    }

    class PhotoHolder(private val itemImageView: ImageView)
        : RecyclerView.ViewHolder(itemImageView){ //page 496
         val bindDrawable: (Drawable) -> Unit = itemImageView::setImageDrawable
    }

    private inner class PhotoAdapter(private val galleryItems: List<GalleryItem>)
        :RecyclerView.Adapter<PhotoHolder>(){
        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): PhotoHolder {
        val view = layoutInflater.inflate(
            R.layout.list_item_gallery,
            parent,
            false) as ImageView
            return PhotoHolder(view)
        }

        override fun onBindViewHolder(holder: PhotoHolder, position: Int) {
            val galleryItem = galleryItems[position]
            //holder.bindTitle(galleryItem.title)
            val placeholder: Drawable = ContextCompat.getDrawable(
                requireContext(),
                R.drawable.hala_atamleh
            )?: ColorDrawable()
            holder.bindDrawable(placeholder)

            thumbnailDownloader.queueThumbnail(holder, galleryItem.url) //page 515
        }

        override fun getItemCount(): Int = galleryItems.size

    }


    companion object{
        fun newInstance() = PhotoGalleryFragment()
    }

    override fun onDestroyView() { //page 530
        super.onDestroyView()
        thumbnailDownloader.clearQueue()
        viewLifecycleOwner.lifecycle.removeObserver(
            thumbnailDownloader.viewLifecycleObserver
        )
    }
    override fun onDestroy() {
        super.onDestroy()
        lifecycle.removeObserver(
            thumbnailDownloader.fragmentLifecycleObserver
        )
    }
}

Answer 1

我分两步解决了这个问题

1-使requesthandler可为空...感谢@Tenour04

2-以这种方式制作画廊

package com.bignerdranch.android.photogallery

import android.graphics.drawable.BitmapDrawable
import android.graphics.drawable.ColorDrawable
import android.graphics.drawable.Drawable
import android.os.Bundle
import android.os.Handler
import android.util.Log
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ImageView
import android.widget.TextView
import androidx.core.content.ContextCompat
import androidx.fragment.app.Fragment
import androidx.lifecycle.LifecycleObserver
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import androidx.recyclerview.widget.GridLayoutManager
import androidx.recyclerview.widget.RecyclerView

private const val TAG = "PhotoGalleryFragment"

class PhotoGalleryFragment : Fragment() {

    private lateinit var photoGalleryViewModel: PhotoGalleryViewModel
    private lateinit var photoRecyclerView: RecyclerView
    private lateinit var thumbnailDownloader: ThumbnailDownloader<PhotoHolder>

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

        retainInstance = true

        photoGalleryViewModel =
                ViewModelProviders.of(this).get(PhotoGalleryViewModel::class.java)

        val responseHandler = Handler()
        thumbnailDownloader =
                ThumbnailDownloader(responseHandler) { photoHolder, bitmap ->
                    val drawable = BitmapDrawable(resources, bitmap)
                    photoHolder.bindDrawable(drawable)
                }
        lifecycle.addObserver(thumbnailDownloader.fragmentLifecycleObserver)
    }

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        viewLifecycleOwner.lifecycle.addObserver(
            thumbnailDownloader.viewLifecycleObserver
        )
        val view = inflater.inflate(R.layout.fragment_photo_gallery, container, false)

        photoRecyclerView = view.findViewById(R.id.photo_recycler_view)
        photoRecyclerView.layoutManager = GridLayoutManager(context, 3)

        return view
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        photoGalleryViewModel.galleryItemLiveData.observe(
            viewLifecycleOwner,
            Observer { galleryItems ->
                Log.d(TAG, "Have gallery items from view model $galleryItems")
                photoRecyclerView.adapter = PhotoAdapter(galleryItems)
            })
    }

    override fun onDestroyView() {
        super.onDestroyView()
        thumbnailDownloader.clearQueue()
        viewLifecycleOwner.lifecycle.removeObserver(
            thumbnailDownloader.viewLifecycleObserver
        )
    }

    override fun onDestroy() {
        super.onDestroy()
        lifecycle.removeObserver(
            thumbnailDownloader.fragmentLifecycleObserver
        )
    }

    private class PhotoHolder(private val itemImageView: ImageView) : RecyclerView.ViewHolder(itemImageView) {

        val bindDrawable: (Drawable) -> Unit = itemImageView::setImageDrawable
    }

    private inner class PhotoAdapter(private val galleryItems: List<GalleryItem>) :
        RecyclerView.Adapter<PhotoHolder>() {

        override fun onCreateViewHolder(
            parent: ViewGroup,
            viewType: Int
        ): PhotoHolder {
            val view = layoutInflater.inflate(
                R.layout.list_item_gallery,
                parent,
                false
            ) as ImageView
            return PhotoHolder(view)
        }

        override fun getItemCount(): Int = galleryItems.size

        override fun onBindViewHolder(holder: PhotoHolder, position: Int) {
            val galleryItem = galleryItems[position]
            val placeholder: Drawable = ContextCompat.getDrawable(
                requireContext(),
                R.drawable.bill_up_close
            ) ?: ColorDrawable()
            holder.bindDrawable(placeholder)
            thumbnailDownloader.queueThumbnail(holder, galleryItem.url)
        }
    }

    companion object {
        fun newInstance() = PhotoGalleryFragment()
    }
}