[英]Retrofit, Gson and an array of heterogeneous objects
我正在使用Retrofit
向我們的服務器執行 REST 請求。 其中一個請求返回一個對象array
,這些對象一旦在POJO
反序列化,就會從abstract class Event
擴展而來。 Event 有方法getEventType()
返回一個String
,這個字符串是鍵"EventType"
的值,我將始終在數組中的JSONObject
s中。
這就是JSON
樣子(我們現在有 7 種類型的對象):
[
{
"EventType":"typeA",
"Data":"data"
},
{
"EventType":"typeB",
"OtherData":3
}
]
我正在嘗試使用Retrofit
和GSON
API
在異步調用中反序列化此JSON
,以使用Callback<List<Event>>
作為調用的參數,但我仍然找不到方法。
您可以為這種情況編寫自定義 Gson TypeAdapterFactory
。 事情是確定事件的類型,然后為該類型使用默認的TypeAdapter
。 這正是我所做的:
public class EventTypeAdapterFactory implements TypeAdapterFactory {
private static final String TAG = EventTypeAdapterFactory.class.getSimpleName();
private Map<EventType, TypeAdapter<? extends Event>> ADAPTERS = new ArrayMap<>();
private TypeAdapter<Event> baseTypeAdapter;
private TypeAdapter<JsonElement> elementAdapter;
private TypeAdapter<EventType> eventTypeAdapter;
@Override public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
if (!Event.class.isAssignableFrom(type.getRawType())) return null;
ADAPTERS.put(EventType.TYPE_A, gson.getDelegateAdapter(this, TypeToken.get(TypeAEvent.class)));
ADAPTERS.put(EventType.TYPE_B, gson.getDelegateAdapter(this, TypeToken.get(TypeBEvent.class)));
baseTypeAdapter = gson.getDelegateAdapter(this, TypeToken.get(Event.class));
elementAdapter = gson.getAdapter(JsonElement.class);
eventTypeAdapter = gson.getAdapter(EventType.class);
return (TypeAdapter<T>) new EventTypeAdapter().nullSafe();
}
private class EventTypeAdapter extends TypeAdapter<Event> {
@Override public void write(JsonWriter out, Event value) throws IOException {
EventType eventType = value.getType();
TypeAdapter<? extends Event> adapter = eventType == null ? baseTypeAdapter : ADAPTERS.get(eventType);
if (value instanceof TypeAEvent) {
writeWrap(adapter, out, (TypeAEvent) value, TypeAEvent.class);
} else if (value instanceof TypeBEvent) {
writeWrap(adapter, out, (TypeBEvent) value, TypeBEvent.class);
} else {
writeWrap(adapter, out, value, Event.class);
}
}
private <T extends Event> void writeWrap(TypeAdapter<? extends Event> adapter,
JsonWriter out, T value, Class<T> dummyForT) throws IOException {
((TypeAdapter<T>)adapter).write(out, value);
}
@Override public Event read(JsonReader in) throws IOException {
JsonObject objectJson = elementAdapter.read(in).getAsJsonObject();
JsonElement typeJson = objectJson.get("EventType");
EventType eventType = eventTypeAdapter.fromJsonTree(typeJson);
if (eventType == null) {
Log.w(TAG, "Unsupported EventType: " + typeJson);
}
TypeAdapter<? extends Event> adapter = eventType == null ? baseTypeAdapter : ADAPTERS.get(eventType);
return adapter.fromJsonTree(objectJson);
}
}
}
// EventType enum, change to reflect your values.
enum EventType {
TYPE_A, TYPE_B;
}
// Base Event type and its successors.
class Event {
@SerializedName("EventType")
private EventType type;
public EventType getType() {
return type;
}
}
class TypeAEvent extends Event {
@SerializedName("Data")
public String data;
}
class TypeBEvent extends Event {
@SerializedName("OtherData")
public int otherData;
}
我不確定,因為我沒有測試過這個,但是如果你寫一個這樣的自定義反序列化器:
private class MyEventDeserialiser implements JsonDeserializer<Event> {
@Override
public Event deserialize(JsonElement json, Type type, JsonDeserializationContext context) throws JsonParseException {
JsonArray jArray = (JsonArray) json;
for (int i=1; i<jArray.size(); i++) {
JsonObject obj = jArray.get(i).getAsJsonObject();
String eventType = String.valueOf(obj.get("EventType"));
//check here which type it is
Event event = null;
if(eventType.equals("TypeA")) {
Event event = context.deserialize(obj, TypeA.class);
}
...
return event;
}
}
}
然后在用於 Retrofit 的 Gson 反序列化器上設置這個,它可能會起作用。
您可能必須將事件列表封裝在另一個類中,例如
public class EventResponse {
List<Event> events;
}
然后在您的界面中使用它作為參數,但我不確定。
與 colriot 的回答非常相似。 我做了一些修改,以便將一個類嵌入到 json 中,並且只有在它被序列化時 - 該映射有點難看。
它也不那么健壯,因為我很高興在空值上失敗。
public final class ModelTypeAdapter extends TypeAdapter<Model> {
private static final String MODEL_CLASS_PROPERTY_NAME = "modelClass";
private final Gson gson;
private final TypeAdapterFactory containerFactory;
@SuppressWarnings("rawtypes")
private final TypeAdapter<Class> classTypeAdapter;
private final TypeAdapter<JsonElement> jsonElementAdapter;
public ModelTypeAdapter(final Gson gson, final TypeAdapterFactory containerFactory) {
this.gson = gson;
this.containerFactory = containerFactory;
this.classTypeAdapter = gson.getAdapter(Class.class);
this.jsonElementAdapter = gson.getAdapter(JsonElement.class);
}
@Override
public final void write(JsonWriter out, Model value) throws IOException {
doWrite(out, value);
}
private final <M extends Model> void doWrite(JsonWriter out, M value) throws IOException {
@SuppressWarnings("unchecked")
final Class<M> modelClass = (Class<M>) value.getClass();
final TypeAdapter<M> delegateAdapter = gson.getDelegateAdapter(containerFactory, TypeToken.get(modelClass));
final JsonObject jsonObject = delegateAdapter.toJsonTree(value).getAsJsonObject();
jsonObject.add(MODEL_CLASS_PROPERTY_NAME, classTypeAdapter.toJsonTree(modelClass));
jsonElementAdapter.write(out, jsonObject);
}
@Override
public final Model read(JsonReader in) throws IOException {
final JsonObject jsonObject = jsonElementAdapter.read(in).getAsJsonObject();
@SuppressWarnings("unchecked")
final Class<? extends Model> modelClass = classTypeAdapter.fromJsonTree(jsonObject.get(MODEL_CLASS_PROPERTY_NAME));
jsonObject.remove(MODEL_CLASS_PROPERTY_NAME);
final TypeAdapter<? extends Model> delegateAdapter = gson.getDelegateAdapter(containerFactory, TypeToken.get(modelClass));
return delegateAdapter.fromJsonTree(jsonObject);
}
}
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