LiveDataCallAdapterFactory.java

package com.anikinkirill.foodrecipes.util;

import androidx.lifecycle.LiveData;

import com.anikinkirill.foodrecipes.responses.ApiResponse;

import java.lang.annotation.Annotation;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

import retrofit2.CallAdapter;
import retrofit2.Retrofit;

public class LiveDataCallAdapterFactory extends CallAdapter.Factory {

    /**
     * This method performs a number of checks and then returns the Response type for the Retrofit requests
     * (@bodyType is the ResponseType. It can be RecipeResponse or RecipeSearchResponse)
     *
     * CHECK #1) returnType returns LIVEDATA
     * CHECK #2) Type LiveData<T> is of ApiResponse.class
     * CHECK #3) Make sure ApiResponse is parameeterized. AKA: ApiResponse<T> exists.
     *
     *
     * @param returnType
     * @param annotations
     * @param retrofit
     * @return
     */
    @Override
    public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {

        // Check #1
        // Make sure the CallAdapter is returning a type of LiveData
        if(CallAdapter.Factory.getRawType(returnType) != LiveData.class){
            return null;
        }

        // Check #2
        // Type that LiveData is wrapping
        Type observableType = CallAdapter.Factory.getParameterUpperBound(0, (ParameterizedType) returnType);
        // Check if it's of Type ApiResponse
        Type rawObservableType = CallAdapter.Factory.getRawType(observableType);
        if(rawObservableType != ApiResponse.class){
            throw new IllegalArgumentException("type must be a defined resource");
        }

        // Check #3
        // Check if ApiResponse is parameterized. AKA: Does ApiResponse<T> exist? (must wrap around T)
        // FYI: T is either RecipeResponse or RecipeSearchResponse in this app. But T can be anything theoretically.
        if(!(observableType instanceof ParameterizedType)){
            throw new IllegalArgumentException("resource must be parameterized");
        }

        // get the Response type. (RecipeSearchResponse or RecipeResponse)
        Type bodyType = CallAdapter.Factory.getParameterUpperBound(0, (ParameterizedType) observableType);
        return new LiveDataCallAdapter<Type>(bodyType);
    }
}