如何使用 Retrofit/Moshi 解析嵌套的 JSON 对象

Question

我使用这个 CodeLabs 教程来学习如何从 Google Books API 发出 HTTP 请求 https://codelabs.developers.google.com/codelabs/kotlin-android-training-internet-data/#4

现在，我正在尝试访问 Google Books API 吐出的嵌套 JSON 对象 即

"items": [{
"kind": "books#volume",
"id": "mOsbHQAACAAJ",
"volumeInfo" : {
"description": "Young wizard Harry Potter finds himself back at the miserable Hogwarts School of Witchcraft and Wizardry. He doesn't realize the difficulty of the task that awaits him. Harry must pull out all the stops in order to find his missing friend. No Canadian Rights for the Harry Potter Series HARRY POTTER and all related characters and elements are trademarks of and (c) Warner Bros. Entertainment Inc. Harry Potter publishing rights (c) J. K. Rowling. (s05)",
"imageLinks": {
"smallThumbnail": "http://books.google.com/books/content?id=mOsbHQAACAAJ&printsec=frontcover&img=1&zoom=5&source=gbs_api",
"thumbnail": "http://books.google.com/books/content?id=mOsbHQAACAAJ&printsec=frontcover&img=1&zoom=1&source=gbs_api"
}
},

我只想要描述和缩略图属性。

我的API服务接口是

package com.example.customapp.network
import com.squareup.moshi.Moshi
import com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactory
import retrofit2.Call
import retrofit2.Retrofit
import retrofit2.converter.moshi.MoshiConverterFactory
import retrofit2.http.GET

//Code from https://codelabs.developers.google.com/codelabs/kotlin-android-training-internet-data/#3

private const val BASE_URL = "https://www.googleapis.com/books/v1/"

private val moshi = Moshi.Builder()
    .add(KotlinJsonAdapterFactory())
    .build()

private val retrofit = Retrofit.Builder()
    .addConverterFactory(MoshiConverterFactory.create(moshi))
    .baseUrl(BASE_URL)
    .build()

interface BookApiService {
    //Get annotation specifies the endpoint for this web service method.
    //when getProperties() method is invoked, Retrofit appends the endpoint 'book' to the base URL
    //And creates a Call object. The Call object is used to start the request.
    @GET("volumes?q='harry+potter")
    suspend fun getProperties(): BookProperty
}

object BookApi {
    val retrofitService: BookApiService by lazy {
        retrofit.create(BookApiService::class.java)
    }
}
}

我的 BookProperty.kt 是

data class BookProperty(@field:Json(name = "items" ) val bookDetail: List<BookDetail>)
data class BookDetail(@field:Json(name = "volumeInfo") val volumeInfo: VolumeInfo)
data class VolumeInfo(@field:Json(name = "description") val description: String, @field:Json(name= "imageLinks") val imageLink: ImageLink)
data class ImageLink(@field:Json(name = "thumbnail") val thumbnail: String)

我正在从我的 ViewModel 调用 API

val readAllData: LiveData<List<BookItem>>
    private val repository: BookRepository
    private val _response = MutableLiveData<String>()

    val response: LiveData<String>
        get() = _response

    init {
        val bookDao = BookDatabase.getDatabase(application).bookDao()
        repository = BookRepository(bookDao)
        readAllData = repository.readAllData
    }

    fun addBook(book: BookItem) {
        viewModelScope.launch(Dispatchers.IO) {
            repository.addBook(book)
        }
    }

    fun updateBook(book: BookItem) {
        viewModelScope.launch(Dispatchers.IO) {
            repository.updateBook(book)
        }
    }

    fun getBookDetailProperties() {
        viewModelScope.launch {
            try {
                //calling get properties from the BookApi service creates and starts the network call
                //on a background thread
                var listResult = BookApi.retrofitService.getProperties()
                _response.value = "${
                    listResult.bookDetail[0].volumeInfo.description} book properties received"
            } catch (e: Exception) {
                _response.value = "Failure: ${e.message}"
            }
        }
    }

每次我在我的 CRUD 应用程序上更新一个项目时，即当我点击一个按钮时，我都会尝试发出一个 HTTP 请求，但我似乎无法得到任何响应。这是我发起 API 调用的 UpdateFragment。

class UpdateFragment : Fragment() {
    //Read up on delegation
    //https://codelabs.developers.google.com/codelabs/kotlin-bootcamp-classes/#7

    //UpdateFragmentArgs is a class that is automatically generated
    //when we created an argument for our Update Fragment in the nav graph
    //UpdateFragmentArgs will contain our current book
    //we can also use bundle
    private val args by navArgs<UpdateFragmentArgs>()

    private lateinit var mBookViewModel: BookViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        val view = inflater.inflate(R.layout.fragment_update, container, false)

        //So the keyboard doesn't push the EditText fields up
        this.activity?.window?.setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_ADJUST_PAN)

        Glide
            .with(this)
            .load(args.currentBook.image)
            .into(view.bookImageDetail)

        mBookViewModel = ViewModelProvider(this).get(BookViewModel::class.java)


        view.apply {
            updateInputName.setText(args.currentBook.title)
            updateInputAuthor.setText(args.currentBook.author)
            updateBookDesc.text = args.currentBook.desc
            updateRatingBar.rating = args.currentBook.rating.toFloat()
            updateBookCompleted.isChecked = args.currentBook.finished
            updateBookCompleted.text =
                if (updateBookCompleted.isChecked) getString(R.string.book_completed) else getString(
                    R.string.book_not_completed
                )
            updateDateCreated.text = getString(R.string.date_created, args.currentBook.dateCreated)
        }

        view.updateBtn.setOnClickListener {
            updateItem()
        }

        view.updateBookCompleted.setOnCheckedChangeListener { _, isChecked ->
            if (isChecked) {
                view.updateBookCompleted.text = getString(R.string.book_completed)
            } else {
                view.updateBookCompleted.text = getString(R.string.book_not_completed)
            }
        }

        return view
    }

    private fun updateItem() {
        val bookName = updateInputName.text.toString()
        val bookAuthor = updateInputAuthor.text.toString()
        val bookRating = updateRatingBar.rating.toDouble()
        val bookFinished = updateBookCompleted.isChecked

        if (inputCheck(bookName, bookAuthor)) {
            
            //***Initiate API call here ****
            mBookViewModel.getBookDetailProperties()

            //Get description and image from API
            mBookViewModel.response.observe(viewLifecycleOwner, {
                println("Get resp " + it)
            })

            //Create book object
            val updatedBook = BookItem(
                args.currentBook.id,
                bookName,
                bookAuthor,
                args.currentBook.desc,
                args.currentBook.image,
                bookRating,
                args.currentBook.dateCreated,
                bookFinished
            )

            //update current book
            mBookViewModel.updateBook(updatedBook)

            Toast.makeText(requireContext(), "Updated book successfully!", Toast.LENGTH_SHORT)
                .show()

            //navigate back
            findNavController().navigate(R.id.action_updateFragment_to_listFragment)
        }
    }

    private fun inputCheck(bookName: String, authorName: String): Boolean {
        return !(TextUtils.isEmpty(bookName) && TextUtils.isEmpty(authorName))
    }

}

问题是我无法从 API 调用中获得任何响应 - 我不确定这是否是因为 JSON 中的嵌套对象。请帮我解释一下，我还是 Kotlin 编程的新手。

Answer 1

我找到了没有得到任何回复的原因。 在我的 UpdateFragment 中，我正在这样做：

            //Get description and image from API
            mBookViewModel.response.observe(viewLifecycleOwner, {
                println("Get resp " + it)
            })

            //Create book object
            val updatedBook = BookItem(
                args.currentBook.id,
                bookName,
                bookAuthor,
                args.currentBook.desc,
                args.currentBook.image,
                bookRating,
                args.currentBook.dateCreated,
                bookFinished
            )

            //update current book
            mBookViewModel.updateBook(updatedBook)

            Toast.makeText(requireContext(), "Updated book successfully!", Toast.LENGTH_SHORT)
                .show()

            //navigate back
            findNavController().navigate(R.id.action_updateFragment_to_listFragment)

在我观察到 HTTP 响应的任何变化之前，我正在导航回另一个片段。这会导致观察者停止观察任何变化，因此我无法得到响应。我只需要将我的代码放在回调中，这样我就可以对收到的数据做一些事情。像这样：

            //Get description and image from API
            mBookViewModel.response.observe(viewLifecycleOwner, {
                println("Get resp " + it)
          

            //Create book object
            val updatedBook = BookItem(
                args.currentBook.id,
                bookName,
                bookAuthor,
                args.currentBook.desc,
                args.currentBook.image,
                bookRating,
                args.currentBook.dateCreated,
                bookFinished
            )

            //update current book
            mBookViewModel.updateBook(updatedBook)

            Toast.makeText(requireContext(), "Updated book successfully!", Toast.LENGTH_SHORT)
                .show()

            //navigate back
            findNavController().navigate(R.id.action_updateFragment_to_listFragment)
 })

希望这对刚开始学习 LiveData 和使用 HTTP 请求的人有所帮助。