Challenge: Adding a Custom Gson Deserializer

Android Programming (4th Edition) 24. HTTP and Background Tasks
1

PhotoDeserializer.kt

import com.bignerdranch.android.photogallery.GalleryItem
import com.google.gson.JsonDeserializationContext
import com.google.gson.JsonDeserializer
import com.google.gson.JsonElement
import com.google.gson.JsonObject
import java.lang.reflect.Type

class PhotoDeserializer: JsonDeserializer<PhotoResponse> {
    override fun deserialize(
        json: JsonElement?,
        typeOfT: Type?,
        context: JsonDeserializationContext?
    ): PhotoResponse {
        val jsonObject:JsonObject = json?.asJsonObject!!
        val jsonArray = jsonObject.get("photo").asJsonArray
        val photoResponse:PhotoResponse = PhotoResponse()
        val photos:MutableList<GalleryItem> = mutableListOf()
        jsonArray.forEach {photo ->
            val photoElement = photo.asJsonObject
            val galleryItem:GalleryItem = GalleryItem(
                photoElement.get("title").asString,
                photoElement.get("id").asString,
                photoElement.get("url_s").asString
            )
            photos.add(galleryItem)
        }
        photoResponse.galleryItems = photos
        return photoResponse
    }

}

FlickrFetchr.kt>

    val gson:Gson = GsonBuilder().registerTypeAdapter(PhotoResponse::class.java, PhotoDeserializer()).create()
    val retrofit = Retrofit.Builder()
        .baseUrl("https://api.flickr.com/")
        .addConverterFactory(GsonConverterFactory.create(gson))
        .build()
2 Likes
2

I tried your solution but at the end i get empty list. after debugging i notice that jsonObject.get(“photo”) return a null. Maybe due to casting jsonArray to jsonObject. However, calling jsonObject.getAsJsonArray(“photo”) fix the problem.

val jsonArray = jsonObject.getAsJsonArray(“photo”)

EDIT 2:
another issue which is getting “photo” memeber before “photos” and other thing is using !! which can lead to serious problem like NullPointerException

val jsonObject = json?.asJsonObject?.get(“photos”)?.asJsonObject
val jsonArray = photoJsonObject?.getAsJsonArray(“photo”)?.asJsonArray

3

who’s photoJsonObject?

4

I had a headache updating the code after removing FlickrResponse.kt

Can I see your code?

here is another way in doing the deserializer by the way

package com.bignerdranch.android.photogallery.api

import com.google.gson.*
import java.lang.reflect.Type

class PhotoDeserializer: JsonDeserializer<PhotoResponse> {
    override fun deserialize(
        json: JsonElement?,
        typeOfT: Type?,
        context: JsonDeserializationContext?
    ): PhotoResponse {
        val jsonObject: JsonObject = json?.asJsonObject!!

        val photos = Gson().fromJson(jsonObject, PhotoResponse::class.java)

        return photos
    }
}
5

I did it, but may be you have better solution

class PhotoDeserializer: JsonDeserializer {

lateinit var photos: PhotoResponse

override fun deserialize(
    json: JsonElement?,
    typeOfT: Type?,
    context: JsonDeserializationContext?
): PhotoResponse {

    val jsonObject: JsonObject = json?.asJsonObject!!

    photos = Gson().fromJson(jsonObject, PhotoResponse::class.java)

    return photos
}

class FlickrFetchr{

private val flickrApi: FlickrApi

init {
    val gsonPhotoDeserializer = GsonBuilder()
        .registerTypeAdapter(PhotoResponse::class.java, PhotoDeserializer())
        .create()
    val retrofit = Retrofit.Builder()
        .baseUrl("https://api.flickr.com/")
        .addConverterFactory(GsonConverterFactory.create(gsonPhotoDeserializer))
        .build()
    flickrApi = retrofit.create(FlickrApi::class.java)
}

fun fetchPhotos(): LiveData<List<GalleryItem>>{
    val responseLiveData: MutableLiveData<List<GalleryItem>> = MutableLiveData()
    val flickrRequest: Call<PhotoDeserializer> = flickrApi.fetchPhotos()
  

    flickrRequest.enqueue(object : Callback<PhotoDeserializer> {
        override fun onFailure(call: Call<PhotoDeserializer>, t: Throwable){
            Log.e(TAG, "Failed to fetch photos", t)
        }

        override fun onResponse(
            call: Call<PhotoDeserializer>,
            response: Response<PhotoDeserializer>
        ) {
            Log.d(TAG, "Response received")
            val flickrResponse: PhotoDeserializer? = response.body()
            val photoResponse: PhotoResponse? = flickrResponse?.photos
            var galleryItems: List<GalleryItem> = photoResponse?.galleryItems
                ?: mutableListOf()
            galleryItems = galleryItems.filterNot {
                it.url.isBlank()
            }
            responseLiveData.value = galleryItems
        }
    })
    return  responseLiveData
}

}

6

But the “val photoResponse: PhotoResponse? = flickrResponse?.photos” photos is showing in red??? PhotoDeserializer dosen’t reference any photos variable. Any help please because this is where I am stuck.

7

I just used the simple photoJsonObject = json?.asJsonObject but what I don’t know is if it returned the “photos” object or “photo” JSON array because I declared a lateinit var property named photos in the custom deserialization to reference it from FlickrFetchr

8

For some reason I receives null list of GalleryItem when using your snippet.
It seems to me that Gson does not know how to work if you skip the beginning of a json.

val jsonObject = json?.asJsonObject?.getAsJsonObject("photos")
        return Gson().fromJson(jsonObject, PhotoResponse::class.java)
9

thank you for trying my code. i forgot how i did it that time… i wont do this work this way now…
usually i create an object (data class) and let the code fit in it directly.
in addition that, since we are working in Kotlin, people are moving from retrofit to ktor…