从服务器通过API传输数据完成后，如何使用远程数据构建recyclerview

Question

我在片段中构建了一个回收视图。 要在 recyclerview 中填充的数据存储在远程服务器中，并通过 JSON 进行解析。

我正在使用 Retrofit 将数据从服务器传输到应用程序。 注意：我是 Android 开发初学者，第一次通过看教程使用 Retrofit。

到目前为止，我已经理解了以下内容，如果我错了，请纠正我由于 Retrofit 库使用另一个线程而不是主线程来从服务器获取响应，数据完全传输需要几秒钟，但直到那时 recyclerview 已经构建带有空的 ArrayList。

所以，我的问题是，如何保持 recyclerView 从构建到数据传输完成？ 或如何在数据传输完成后更新空的 ArrayList 并从该数据填充 recyclerview？ 或者你可以告诉我什么是达到预期结果的最好和最有效的方法。

我已经尝试过adapter.notifyDataSetChanged() ，但是我没有在正确的地方使用它，或者我没有以正确的方式使用它，因为它不起作用。 你可以在代码中看到它。

库片段：-

package com.diginfoexpert.mybooks.Fragment;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.diginfoexpert.mybooks.Activity.CategoryActvity;
import com.diginfoexpert.mybooks.Adapter.LibraryCategoriesCardAdapter;
import com.diginfoexpert.mybooks.Model.Category;
import com.diginfoexpert.mybooks.Model.LibraryAPI;
import com.diginfoexpert.mybooks.Model.LibraryCategories;
import com.diginfoexpert.mybooks.R;

import java.util.ArrayList;

import Controllers.JSONApiHolder;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.Retrofit.Builder;
import retrofit2.converter.gson.GsonConverterFactory;

public class LibraryFragment extends Fragment implements LibraryCategoriesCardAdapter.CategoryClickListener {

    private JSONApiHolder jsonApiHolder;
    ArrayList<Category> categories = new ArrayList<>();
    LibraryCategoriesCardAdapter adapter;
    RecyclerView recyclerView;


    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_library, container, false);
        Log.d("RETROFIT", "onCreateView: Library Fragment Started");

        recyclerView = view.findViewById(R.id.library_category_recycler_view);

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("http://myurl.in/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        jsonApiHolder = retrofit.create(JSONApiHolder.class);

        getCategories();

        buildRecyclerView();

        return view;
    }

    private void buildRecyclerView() {
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
        adapter = new LibraryCategoriesCardAdapter(buildCategoryList(),this);
        recyclerView.setLayoutManager(linearLayoutManager);
        recyclerView.setAdapter(adapter);
    }

    private void getCategories() {
        Call<LibraryAPI> call = jsonApiHolder.getLibrary();

        call.enqueue(new Callback<LibraryAPI>() {
            @Override
            public void onResponse(Call<LibraryAPI> call, Response<LibraryAPI> response) {
                if(!response.isSuccessful()){
                    Log.d("RETROFIT", "onResponse: callback failed");
                    return;
                }

                else{
                    Log.d("RETROFIT", "onResponse: callback successful");
                    LibraryAPI libraryAPI = response.body();
                    categories = libraryAPI.getCategories();
                    adapter.notifyDataSetChanged();

                    for (Category category :categories){
                        Log.d("RETROFIT", "onResponse: "+category.getTitle());
                    }

                }
            }

            @Override
            public void onFailure(Call<LibraryAPI> call, Throwable t) {
                Log.d("RETROFIT", "onFailure: response failed : " +t.getMessage());
            }
        });
    }

    private ArrayList<Category> buildCategoryList() {
        ArrayList<Category> mList = new ArrayList<>();
//        mList.add(new Category("Art"));
//        mList.add(new Category("Action"));
//        mList.add(new Category("Thrill"));
//        mList.add(new Category("Fiction"));
//        mList.add(new Category("Romance"));
//        mList.add(new Category("Crime"));
//        mList.add(new Category("Classical"));
//        mList.add(new Category("Financial"));
//        mList.add(new Category("Business"));
        Log.d("RETROFIT", "Size of categories ArrayList "+categories.size());
        return categories;
    }

    @Override
    public void onCategoryClick() {
        Intent intent = new Intent(getActivity(), CategoryActvity.class);
        startActivity(intent);
    }
}

调试日志：-

2020-03-25 14:26:37.536 1703-1703/com.myurl.mybooks D/RETROFIT: onCreateView: Library Fragment Started
2020-03-25 14:26:37.630 1703-1703/com.myurl.mybooks D/RETROFIT: Size of categories ArrayList 0
2020-03-25 14:26:42.171 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: callback successful
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC Mains Syllabus
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER 1 PART "A" History
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER 1 PART "B" Geography
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER-2 (PART "A") Polity
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER 2 (PART "B") Economics & Social
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER 3 Science
2020-03-25 14:26:42.172 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC MAINS PAPER 4 Ethics
2020-03-25 14:26:42.173 1703-1703/com.myurl.mybooks D/RETROFIT: onResponse: MPPSC HINDI

Answer 1

这是您修改后的代码部分

public void onResponse(Call<LibraryAPI> call, Response<LibraryAPI> response) {
                if(!response.isSuccessful()){
                    Log.d("RETROFIT", "onResponse: callback failed");
                    return;
                }

                else{
                    Log.d("RETROFIT", "onResponse: callback successful");
                    LibraryAPI libraryAPI = response.body();
                    categories = libraryAPI.getCategories();
                    adapter.notifyDataSetChanged();

                    for (Category category :categories){
                      categories.add(category) // Added by me  
                      Log.d("RETROFIT", "onResponse: "+category.getTitle());
                    }
                    adapter.notifyDataSetChanged() // Added by me
                }
            }

Answer 2

过了一段时间我找到了出路。 我在 main 方法中构建了回收器视图，这是主线程。 当我们在改造中对网络请求进行排队时，它在不同的线程上工作，因此直到数据传输完成，回收器视图已经构建在主线程中，没有可用的数据。

因此，我没有在主线程中构建回收器视图，而是创建了一个方法来构建它。 在应用程序得到服务器响应后，我调用了该方法。

private void buildRecyclerView() {
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getActivity());
        adapter = new LibraryCategoriesCardAdapter(buildCategoryList(),this);
        recyclerView.setLayoutManager(linearLayoutManager);
        recyclerView.setAdapter(adapter);
    }



private void getCategories() {
    Call<LibraryAPI> call = jsonApiHolder.getLibrary();

    call.enqueue(new Callback<LibraryAPI>() {
        @Override
        public void onResponse(Call<LibraryAPI> call, Response<LibraryAPI> response) {
            if(!response.isSuccessful()){
                Log.d("RETROFIT", "onResponse: callback failed");
                return;
            }

            else{
                Log.d("RETROFIT", "onResponse: callback successful");
                LibraryAPI libraryAPI = response.body();
                categories = libraryAPI.getCategories();
                buildRecyclerView();

                for (Category category :categories){
                    Log.d("RETROFIT", "onResponse: "+category.getTitle());
                }

            }
        }

        @Override
        public void onFailure(Call<LibraryAPI> call, Throwable t) {
            Log.d("RETROFIT", "onFailure: response failed : " +t.getMessage());
        }
    });
}