使用注释通过嵌套事件在 Moshi 中序列化 Null

我正在尝试添加自定义注释，以便在调用时将模型中的特定值序列化为 nulltoJSON来自莫西的方法。我有一些基于此的工作response https://stackoverflow.com/questions/52254876/moshi-custom-qualifier-annotation-to-serialise-null-on-one-property-only但当我有一个嵌套对象时，它对我来说就不够了。

@JsonClass(generateAdapter = true)
data class EventWrapper(
    @SerializeNulls val event: Event?,
    @SerializeNulls val queries: Queries? = null) {

    @JsonClass(generateAdapter = true)
    data class Queries(val stub: String?)

    @JsonClass(generateAdapter = true)
    data class Event(
       val action: String?,
       val itemAction: String)
}

如果我将 null 传递给event or queries它们被序列化为：

{
    'event': null,
    'query': null
}

问题是，当事件不为空时，其中的字段如果为空（例如操作），我不想序列化。我首选的结果是这样的：

{
    'event': {
        'itemAction': "test" 
    },
    'query': null
}

但我得到的是：

{
    'event': {
        'action': null,
        'itemAction': "test" 
    },
    'query': null
}

以下是基于链接响应的自定义适配器的代码：

@Retention(RetentionPolicy.RUNTIME)
 @JsonQualifier
 annotation class SerializeNulls {
     companion object {
         var JSON_ADAPTER_FACTORY: JsonAdapter.Factory = object : JsonAdapter.Factory {

             @RequiresApi(api = Build.VERSION_CODES.P)
             override fun create(type: Type, annotations: Set<Annotation?>, moshi: Moshi): JsonAdapter<*>? {
                 val nextAnnotations = Types.nextAnnotations(annotations, SerializeNulls::class.java)

                 return if (nextAnnotations == null) {
                     null
                 } else {
                     moshi.nextAdapter<Any>(this, type, nextAnnotations).serializeNulls()
                 }
        }
    }
}

@Su-Au Hwang 在线程中的回答对我有用。这比为我们的模型创建自定义适配器更好。谢谢。

这是与 Kotlin 相同的代码：

1. 创建一个新适配器NullIfNullJsonAdapter

   class NullIfNullJsonAdapter<T>(val delegate: JsonAdapter<T>) : JsonAdapter<T>() {
   
           override fun fromJson(reader: JsonReader): T? {
               return delegate.fromJson(reader)
           }
   
           override fun toJson(writer: JsonWriter, value: T?) {
               if (value == null) {
                   val serializeNulls: Boolean = writer.serializeNulls
                   writer.serializeNulls = true
                   try {
                       delegate.toJson(writer, value)
                   } finally {
                       writer.serializeNulls = serializeNulls
                   }
               } else {
                   delegate.toJson(writer, value)
               }
           }
   
           override fun toString(): String {
               return "$delegate.serializeNulls()"
           }
       }
   

2. Create JsonQualifier您可以使用它来进行注释

    @Retention(AnnotationRetention.RUNTIME)
    @JsonQualifier
    annotation class SerializeNull {
        companion object {
            object Factory : JsonAdapter.Factory {
                override fun create(type: Type, annotations: MutableSet<out Annotation>, moshi: Moshi): JsonAdapter<*>? {
                    val nextAnnotations = Types.nextAnnotations(annotations, SerializeNull::class.java)
   
                    return if (nextAnnotations == null) {
                        null
                    } else {
                        NullIfNullJsonAdapter<Any>(moshi.nextAdapter(this, type, nextAnnotations))
                    }
                }
            }
        }
    }
   

3. 将工厂添加到 moshi 构建器

    val moshi = Moshi.Builder()
                .add(SerializeNull.Companion.Factory)
                .build()
   

4. 用注释属性@SerializeNull（例如主持人）

    @JsonClass(generateAdapter = true)
    data class Event(
         val name: String,
         @SerializeNull val host: Host?,
         val venue: String?,
     )
   

一些测试

 @JsonClass(generateAdapter = true)
 data class Host(val firstName: String, val lastName: String?)

 @JsonClass(generateAdapter = true)
 data class Event(
     val name: String,
     @SerializeNull val host: Host?,
     val venue: String?,
 )


 @Test
 fun `everything is added to json if object has everything`() {
     val moshi = Moshi.Builder()
         .add(SerializeNull.Companion.Factory)
         .build()
     val jsonAdapter: JsonAdapter<Event> = moshi.adapter()
     val event = Event(
         name = "Birthday Party",
         host = Host(firstName = "Harsh", lastName = "Bhakta"),
         venue = "House"
     )
     val json = jsonAdapter.toJson(event)
     assertEquals("""{"name":"Birthday Party","host":{"firstName":"Harsh","lastName":"Bhakta"},"venue":"House"}""", json)
 }

 @Test
 fun `check host  is null in json since host is annotated with serializeNull and venue is excluded because it's not annotated`() {
     val moshi = Moshi.Builder()
         .add(SerializeNull.Companion.Factory)
         .build()
     val jsonAdapter: JsonAdapter<Event> = moshi.adapter()
     val event = Event(
         name = "Birthday Party",
         host = null,
         venue = null
     )
     val json = jsonAdapter.toJson(event)
     assertEquals("""{"name":"Birthday Party","host":null}""", json)
 }

 @Test
 fun `check nested property (host lastname) is not included if it's null because it's not annotated with SerializeNull`() {
     val moshi = Moshi.Builder()
         .add(SerializeNull.Companion.Factory)
         .build()
     val jsonAdapter: JsonAdapter<Event> = moshi.adapter()
     val event = Event(
         name = "Birthday Party",
         host = Host(firstName = "Harsh", lastName = null),
         venue = null
     )
     val json = jsonAdapter.toJson(event)
     assertEquals("""{"name":"Birthday Party","host":{"firstName":"Harsh"}}""", json)
 }