Volley Library adaptation for Loader pattern.

JsonLoader.java

import android.os.Handler;
import android.os.Message;
import android.support.v4.content.Loader;

import com.android.volley.NoConnectionError;
import com.android.volley.Response;
import com.android.volley.RetryPolicy;
import com.android.volley.TimeoutError;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.JsonRequest;
import com.artfulbits.annotations.NonNull;
import com.artfulbits.io.InternetUtils;
import com.artfulbits.utils.LogEx;
import com.artfulbits.utils.ValidUtils;
import com.artfulbits.workout.TheApp;
import com.artfulbits.workout.utils.VolleyUtils;
import com.artfulbits.workout.web.handlers.AncestorsFactory;
import com.artfulbits.workout.web.responses.Ancestor;
import com.artfulbits.workout.web.responses.Error;

import org.json.JSONObject;

import java.util.logging.Logger;

/**
 * Basic loader that converts server output to Ancestor business object.
 *
 * @param <T> the type of result Business Object, POJO class.
 */
public class JsonLoader<T extends Ancestor>
    extends Loader<Ancestor>
    implements Response.ErrorListener, Response.Listener<JSONObject>, Handler.Callback, WebApiRequest.Converter {
  /* [ CONSTANTS ] ================================================================================================= */

  /** Our own class Logger instance. */
  private final static Logger _log = LogEx.getLogger(JsonLoader.class);

  /** Message ID used for response results delivery to UI thread. */
  private static final int MSG_QUERY_DONE = -1;
  /** indicate normal state of the response. */
  public static final int STATE_OK = 0;
  /** indicate error state of the response. */
  public static final int STATE_ERROR = -1;

	/* [ MEMBERS ] =================================================================================================== */

  /** Request to the server side. */
  private JsonRequest<JSONObject> mRequest;
  /** Main thread attached handler. Used for delivering server response to the UI thread. */
  private final Handler mHandler = new Handler(this);
  /** Reference on Business Objects Layer Factory, used for parsing/converting JSON to BO. */
  private final AncestorsFactory mParserFactory;
  /** Reference on parsed results. */
  private Ancestor mParsedResults;

  /* [ CONSTRUCTORS ] ============================================================================================== */

  public JsonLoader(@NonNull final AncestorsFactory factory) {
    super(TheApp.context());

    ValidUtils.isNull(factory, "Instance of BOL Factory required.");
    mParserFactory = factory;
  }

  /* [ GETTER / SETTER METHODS ] =================================================================================== */

  public Builder requestBuilder() {
    return new Builder();
  }

  /* package */ T getLastResult() {
    return (T) mParsedResults;
  }

  /* package */ JsonRequest<JSONObject> getRequest() {
    return mRequest;
  }

  /* package */ void setRequest(final JsonRequest<JSONObject> request) {
    ValidUtils.isNull(request, "Expected instance of the JsonRequest class.");

    mRequest = request;
  }

  /* [ Interface Handler.Callback ] ================================================================================ */

  /** {@inheritDoc} */
  @Override
  public boolean handleMessage(final Message msg) {
    // WARNING: deliver Result should be always called from UI thread
    if (MSG_QUERY_DONE == msg.what) {
      deliverResult((Ancestor) msg.obj);
      return true;
    }

    return false;
  }

  /* [ Interface Response.ErrorListener ] ========================================================================== */

  /** {@inheritDoc} */
  @Override
  public void onErrorResponse(final VolleyError error) {
    // dump error message to Logs
    _log.severe(LogEx.dump(error));

    // this is often mean a network type error.
    final Message message = mHandler.obtainMessage(MSG_QUERY_DONE, STATE_ERROR, STATE_ERROR, new Error(error));

    // request execution in UI thread
    mHandler.sendMessage(message);

    if (error instanceof NoConnectionError || error instanceof TimeoutError) {
      _log.severe("No Internet Connection? loader: " + hashCode());

      // notify application about connectivity issues
      InternetUtils.forceCheck(TheApp.context());
    }
  }

  /* [ Interface WebApiRequest.Converter ] ========================================================================= */

  /** {@inheritDoc} */
  @Override
  public void onConvertJson(final JSONObject response) {
    ValidUtils.isUiThread("Data Processing should stay in background thread.");
    _log.info("onResponse processing for url: " + mRequest.getOriginUrl());

    Ancestor parsed = null;

    try {
      parsed = parseJson(response);
    } catch (final Throwable ignored) {
      _log.severe(LogEx.dump(ignored));

      parsed = new Error(ignored);
    }

    // parsing error, method should always return instance of Ancestor
    mParsedResults = ((null == parsed) ? new Error() : parsed);
  }

	/* [ Interface Response.Listener ] =============================================================================== */

  /** {@inheritDoc} */
  @Override
  public void onResponse(final JSONObject response) {
    final int state = (mParsedResults instanceof Error) ? STATE_ERROR : STATE_OK;
    final Message message = mHandler.obtainMessage(MSG_QUERY_DONE, state, state, mParsedResults);

    // request execution in UI thread
    mHandler.sendMessage(message);

    _log.info("Done - successful response. loader: " + hashCode());
  }

	/* [ IMPLEMENTATION & HELPERS ] ================================================================================== */

  /** {@inheritDoc} */
  @Override
  protected void onForceLoad() {
    super.onForceLoad();

    ValidUtils.isNull(mRequest, "Expected instance of the JsonRequest class.");

    // recover web request state from CANCELED via reflection (better to modify library of course)
    try {
      VolleyUtils.resetCancelState(mRequest);
    } catch (final Throwable ignored) {
      _log.severe(LogEx.dump(ignored));
    }

    _log.info("[loader] scheduled web request: " + mRequest.getUrl());

    // add request into execution queue
    TheApp.forUI().add(mRequest);
  }

  /** {@inheritDoc} */
  @Override
  protected void onReset() {
    super.onReset();

    onStopLoading();

    // do cleanup
    mParsedResults = null;
  }

  /* {@inheritDoc} */
  @Override
  protected void onAbandon() {
    // in case of started loader, we have to request stop of all background things
    if (isStarted()) {
      stopLoading();
    }

    super.onAbandon();
  }

  /** {@inheritDoc} */
  @Override
  protected void onStartLoading() {
    super.onStartLoading();

    // deliver result if we already have it
    if (null != mParsedResults) {
      deliverResult(mParsedResults);
    }

    // If the data has changed since the last time it was loaded
    // or is not currently available, start a load.
    if (takeContentChanged() || null == mParsedResults) {
      forceLoad();
    }
  }

  /** {@inheritDoc} */
  @Override
  protected void onStopLoading() {
    super.onStopLoading();

    // WARNING: cancel state of the Request cannot be reset. we should override implementation for making
    // possible state recovery. Cancel execution of the request.
    mRequest.cancel();
  }

  /** Do background parsing of the server response. */
  private Ancestor parseJson(final JSONObject response) throws Exception {
    ValidUtils.isUiThread("Json Parsing should stay in background thread.");

    return mParserFactory.parse(response);
  }

  /* [ NESTED DECLARATIONS ] ======================================================================================= */

  /** Simple builder, that hides Web request creation. */
  public final class Builder {
    private String mUrl;
    private int mHttpMethod;
    private RetryPolicy mRetryPolicy;
    private String mTag;
    private boolean mShouldCache;

    public Builder setUrl(final String url) {
      mUrl = url;
      return this;
    }

    public Builder setHttpMethod(final int type) {
      mHttpMethod = type;
      return this;
    }

    public Builder setRetryPolicy(final RetryPolicy policy) {
      mRetryPolicy = policy;
      return this;
    }

    public Builder setTag(final String tag) {
      mTag = tag;
      return this;
    }

    public Builder setShouldCache(final boolean cache) {
      mShouldCache = cache;
      return this;
    }

    /** Compose Web API request, configure it and assign to current loader. */
    public void build() {
      final WebApiRequest<T> request = new WebApiRequest<>(mHttpMethod, mUrl, JsonLoader.this);

      if (null != mRetryPolicy) {
        request.setRetryPolicy(mRetryPolicy);
      }

      if (null != mTag) {
        request.setTag(mTag);
      }

      request.setShouldCache(mShouldCache);

      // configure owner by newly created request
      setRequest(request);
    }
  }
}

proguard-rules.txt

# --------------------------------------------------------------------------------------
# Volley Library
# --------------------------------------------------------------------------------------

-keep class com.android.volley.** { *; }
-keep interface com.android.volley.** { *; }

-dontwarn com.android.volley.**

ReflectionUtils.java

package com.artfulbits.utils;

import com.artfulbits.annotations.NonNull;

import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

/** Common reflection methods. */
public final class ReflectionUtils {
  /* [ CONSTRUCTORS ] ============================================================================================== */

  /** hidden constructor. */
  private ReflectionUtils() {
    throw new AssertionError();
  }

  /* [ STATIC METHODS ] ============================================================================================ */

  /**
   * Extract all fields of the provided class type.
   *
   * @param type the class type
   * @return the list of found fields.
   */
  public static List<Field> allFields(@NonNull final Class<?> type) {
    final List<Field> fields = new LinkedList<Field>();

    return allFields(fields, type);
  }

  /**
   * Extract all fields in recursive manner.
   *
   * @param fields the extracted fields
   * @param type   the type to examine
   * @return the extracted fields
   */
  private static List<Field> allFields(@NonNull final List<Field> fields, @NonNull final Class<?> type) {
    fields.addAll(Arrays.asList(type.getDeclaredFields()));

    if (type.getSuperclass() != null) {
      allFields(fields, type.getSuperclass());
    }

    return fields;
  }

  /**
   * Find field by name in list of fields.
   *
   * @param fields the list of fields to process
   * @param name   the name to find.
   * @return found field instance, otherwise {@code null}.
   */
  public static Field find(@NonNull final List<Field> fields, @NonNull final String name) {
    for (Field f : fields) {
      if (name.equals(f.getName())) {
        return f;
      }
    }

    return null;
  }
}

VolleyUtils.java

import com.android.volley.Request;
import com.artfulbits.annotations.NonNull;

import java.lang.reflect.Field;

import static com.artfulbits.utils.ReflectionUtils.allFields;
import static com.artfulbits.utils.ReflectionUtils.find;

/** Volley library hacking utils. */
public final class VolleyUtils {
  /* [ CONSTANTS ] ================================================================================================= */

  /** multi-threading access lock. */
  private final static Object sLock = new Object();

  /* [ STATIC MEMBERS ] ============================================================================================ */

  /** Cached value. */
  private static Field sCanceledField;

  /* [ STATIC METHODS ] ============================================================================================ */

  /**
   * Reset cancel state of the provided instance.
   *
   * @param <T>     the type parameter
   * @param request the request instance
   * @throws IllegalAccessException the illegal access exception
   */
  public static <T> void resetCancelState(@NonNull final Request<T> request) throws IllegalAccessException {
    if (request.isCanceled()) {
      getCanceledField().setBoolean(request, false);
    }
  }

  /**
   * Gets canceled field by reflection and cache it for future calls. Reflection is a time consuming operation.
   *
   * @return the canceled field instance.
   */
  private static Field getCanceledField() {
    if (null == sCanceledField) {
      synchronized (sLock) {
        if (null == sCanceledField) {
          final Field field = find(allFields(Request.class), "mCanceled");
          field.setAccessible(true);

          sCanceledField = field;
        }
      }
    }

    return sCanceledField;
  }
}

WebApiRequest.java

import com.android.volley.NetworkResponse;
import com.android.volley.Response;
import com.android.volley.toolbox.JsonObjectRequest;
import com.artfulbits.annotations.NonNull;
import com.artfulbits.annotations.Nullable;

import org.json.JSONObject;

/**
 * JSON Object request, deeply integrated with JsonLoader class.
 *
 * @see <a href="http://tiku.io/questions/4747168/android-volley-does-not-work-offline-with-cache">Offline Cache</a>
 */
/* package */ class WebApiRequest<T extends Ancestor> extends JsonObjectRequest {
  /* [ MEMBERS ] =================================================================================================== */

  /** Reference on data processing routine. */
  private Converter mConverter;
  /** Reference on owner of this request. */
  private final JsonLoader<T> mOwner;

	/* [ CONSTRUCTORS ] ============================================================================================== */

  public WebApiRequest(final int method, final String url, @NonNull final JsonLoader<T> owner) {
    super(method, url, null, owner /*Listener<JSONObject>*/, owner /*ErrorListener*/);
    mOwner = owner;

    // Say Volley that JSON response should be cached
    setShouldCache(true);

    // register data parsing
    setDataConverter(owner /*DataProcessingListener*/);
  }

	/* [ GETTER / SETTER METHODS ] =================================================================================== */

  /**
   * Gets loader with easy type definition over generics.
   *
   * @return the loader instance.
   */
  @NonNull
  public JsonLoader<T> getLoader() {
    return mOwner;
  }

  /**
   * Sets data processing converter.
   *
   * @param converter the converter instance
   * @return this instance
   */
  @NonNull
  public WebApiRequest setDataConverter(@Nullable final Converter converter) {
    mConverter = converter;

    return this;
  }

	/* [ IMPLEMENTATION & HELPERS ] ================================================================================== */

  /** {@inheritDoc} */
  @Override
  protected Response<JSONObject> parseNetworkResponse(@NonNull final NetworkResponse response) {
    // NOTE: excellent place for adding Cache header modifiers, Allows to override
    // server cache policy in easy way.

    final Response<JSONObject> result = super.parseNetworkResponse(response);

    // if we have something for parsing, then forward it to 'callback'
    if (null != mConverter && result.isSuccess()) {
      mConverter.onConvertJson(result.result);
    }

    return result;
  }

	/* [ NESTED DECLARATIONS ] ======================================================================================= */

  /** Callback interface that give a chance to convert JSON to Business Object and store it. */
  public interface Converter {
    /** Parse server side response. */
    void onConvertJson(final JSONObject response);
  }
}