Code example of using Android loaders to make a web request and fetch data.

BaseLoader.java

package com.firstapp.recommend.loaders;

import android.content.Context;
import android.support.v4.content.Loader;

import com.android.volley.DefaultRetryPolicy;
import com.android.volley.RequestQueue;
import com.firstapp.recommend.MyApplication;

import java.util.List;

/**
 * Created by gamerboy on 10/5/15.
 * This class serves as the base loader which any class can extend to do a network call.
 * The response is returned as a list which clients can handle in {@link #newRequest()}.
 * <p/>
 * Clients should override {@link #getUrl()} to set the url, {@link #newRequest()} to 
 * set the {@link GsonRequest} to handle the response and {@link #getUniqueRequestTag()}
 * to return a unique request tag for this request.
 */
public abstract class BaseLoader extends Loader<List> {

    private String requestTAG;
    private List cachedData;
    private RequestQueue requestQueue;
    private GsonRequest gsonRequest;

    /**
     * Stores away the application context associated with context.
     * Since Loaders can be used across multiple activities it's dangerous to
     * store the context directly; always use {@link #getContext()} to retrieve
     * the Loader's Context, don't use the constructor argument directly.
     * The Context returned by {@link #getContext} is safe to use across
     * Activity instances.
     *
     * @param context used to retrieve the application context.
     */
    public BaseLoader(Context context) {
        super(context);
        requestQueue = MyApplication.getRequestQueue();
        requestTAG = getUniqueRequestTag();
    }

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

        //start a new request only if result is null and there is no existing request
        if (cachedData == null) {
            forceLoad();
        } else {
            // request exists, deliver the result.
            super.deliverResult(cachedData);
        }
    }

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

        //cancel all existing requests made for the same call
        cancelRequest();

        gsonRequest = newRequest();
        gsonRequest.setTag(requestTAG);
        gsonRequest.setRetryPolicy(new DefaultRetryPolicy(
                10000,
                DefaultRetryPolicy.DEFAULT_MAX_RETRIES,
                DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));

        requestQueue.add(gsonRequest);
    }

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

    /**
     * {@inheritDoc}
     */
    @Override
    protected void onReset() {
        //cancel request
        cancelRequest();
        cachedData = null;
        super.onReset();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void deliverResult(List data) {
        cachedData = data;
        if (isStarted()) {
            super.deliverResult(data);
        }
    }

    private void cancelRequest() {
        if (requestTAG != null) {
            requestQueue.cancelAll(requestTAG);
        }
    }

    /**
     * @return The url as string.
     */
    public abstract String getUrl();

    /**
     * @return The {@link GsonRequest} to handle the response.
     */
    public abstract GsonRequest newRequest();

    /**
     * Request tag for cancelling a given request. Be careful to not pass the same
     * request tag if loading data at two different places simultaneously. Eg. a viewpager
     * @return A string to uniquely identify this request.
     */
    public abstract String getUniqueRequestTag();
}