MainActivity.java

import android.support.annotation.NonNull;
import android.support.design.widget.Snackbar;
import android.view.View;

import com.edablogs.tubelite.R;
import com.edablogs.tubelite.adapter.RecyclerAdapter;
import com.edablogs.tubelite.api.APIService;
import com.edablogs.tubelite.api.YoutubeApi;
import com.edablogs.tubelite.model.youtube.Item;
import com.edablogs.tubelite.model.youtube.ItemLatest;
import com.edablogs.tubelite.model.youtube.LatestModel;
import com.edablogs.tubelite.model.youtube.YoutubeResponse;
import com.edablogs.tubelite.utils.CacheManager;
import com.edablogs.tubelite.utils.CacheUtils;
import com.edablogs.tubelite.utils.Constants;
import com.edablogs.tubelite.utils.M;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.Locale;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class MainActivity extends AppCompatActivity {

	private static final String TAG = "MainActivity";
	private RecyclerView latestList;
	private RecyclerAdapter latestAdapter;
	private CacheManager cacheManager;
	
	@Override
    	protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            latestList = (RecyclerView) findViewById(R.id.recent_list);
	    cacheManager = new CacheManager(this);
		
	    latestList.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false));
	    latestAdapter = new RecyclerAdapter(this,new ArrayList<>());
	    latestList.setAdapter(latestAdapter);
		
	    initFromCache(); //load data from cache if exist.
    	}
	

	private void initFromCache() {
        
            Type type = new TypeToken<ArrayList<ItemLatest>>() {}.getType();
            ArrayList<ItemLatest> itemLatest = (ArrayList<ItemLatest>) cacheManager.readJson(type, CacheUtils.LATEST);
            if (itemLatest != null)
                latestAdapter.replaceWith(itemLatest);
    	}
	
	@Override
    	public void onResume() {
            getLatest();
            super.onResume();
    	}
	
	private void getLatest() {
        if (M.isConnectionAvailable(this)) {
            YoutubeApi youtubeApi = APIService.create(YoutubeApi.class);
            Call<LatestModel> latestVideos = youtubeApi.getRecentVideos("video", "date", Constants.KEY, Constants.PART, "10","");
            latestVideos.enqueue(new Callback<LatestModel>() {
                @Override
                public void onResponse(@NonNull Call<LatestModel> call, @NonNull Response<LatestModel> response) {
                    if (response.body() != null) {
						//here is your json data from api, save this into json file.
                        ArrayList<ItemLatest> items = (ArrayList<ItemLatest>) response.body().getItems();
                        if (items != null){
                            latestAdapter.replaceWith(items);
                            Type type = new TypeToken<ArrayList<ItemLatest>>() {
                            }.getType();
                            cacheManager.writeJson(response.body().getItems(), type, CacheUtils.LATEST);
                        }
                    }
                }

                @Override
                public void onFailure(@NonNull Call<LatestModel> call, @NonNull Throwable t) {
                    //api failed to return data due to network problem or something else, display data from json file.
					M.l(TAG, t);
                    Type type = new TypeToken<ArrayList<ItemLatest>>() {
                    }.getType();
                    ArrayList<ItemLatest> itemLatests = (ArrayList<ItemLatest>) cacheManager.readJson(type, CacheUtils.LATEST);
                    if (itemLatests != null)
                        latestAdapter.replaceWith(itemLatests);
                }
            });
        } else {
			//Internet not available, display data from json file.
            Type type = new TypeToken<ArrayList<ItemLatest>>() {
            }.getType();
            ArrayList<ItemLatest> itemLatests = (ArrayList<ItemLatest>) cacheManager.readJson(type, CacheUtils.LATEST);
            if (itemLatests != null)
                latestAdapter.replaceWith(itemLatests);
            Snackbar.make(rootLayout, R.string.no_internet_connection, Snackbar.LENGTH_SHORT).setAction(R.string.retry, new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    MainActivity.this.getLatest();
                }
            }).show();
        }

    }
}