AsyncTaskLoader basic example. (Android)

Question

I am using a Loader in my application and based on the result I get from the query I perform on COntacts using this Loader I perform some calculations and store them back in

Answer 1

Here's step by step tutorial to implement AsyncTaskLoader. or check out this same article on Medium

1. Implement LoaderManager.LoaderCallbacks on MainActivity and create a static int to uniquely identify your loader and create a String key to pass string url to your loader

   public class MainActivity extends AppCompatActivity 
                implements LoaderManager.LoaderCallbacks{
       public static final int OPERATION_SEARCH_LOADER = 22;
       public static final String OPERATION_QUERY_URL_EXTRA = "query";
   //...}
   

2. Override onCreateLoader,onLoadFinishedand onLoaderReset functions inside MainActivity

   @Override
   public Loader onCreateLoader(int id, final Bundle args) {
       //Here we will initiate AsyncTaskLoader
       return null;
   }
   
   @Override
   public void onLoadFinished(Loader loader, String operationResult) {
       //Think of this as AsyncTask onPostExecute method, the result from onCreateLoader will be available in operationResult variable and here you can update UI with the data fetched.
       Log.d("MAINACTIVITY","result : "+ operationResult);
   }
   
   @Override
   public void onLoaderReset(Loader loader) {
       //Don't bother about it, Android Studio will override it for you
   }
   

3. inside onCreateLoader() return a new AsyncTaskLoader as an anonymous inner class with this as the constructor's parameter and override loadInBackground & onStartLoading inside anonymous inner class

   @Override
   public Loader onCreateLoader(int id, final Bundle args) {
       return new AsyncTaskLoader(this) {
           @Override
           public String loadInBackground() {
               //Think of this as AsyncTask doInBackground() method, here you will actually initiate Network call
               return null;
           }
   
           @Override
           protected void onStartLoading() {
              //Think of this as AsyncTask onPreExecute() method,start your progress bar,and at the end call forceLoad(); 
              forceLoad();
           }
       };
   }
   

4. Inside loadInBackground make a network call using HTTPUrlConnection or OKHttp or anything that you use.

    @Override
       public String loadInBackground() {
           String url = args.getString(OPERATION_QUERY_URL_EXTRA);//This is a url in string form 
           if (url!=null&&"".equals(url)) {
               return null;//if url is null, return
           }
           String operationResult="";
           try {
               operationResult = NetworkUtils.getResponseFromHttpUrl(url);//This just create a HTTPUrlConnection and return result in strings
           } catch (IOException e) {
               e.printStackTrace();
           }
           return operationResult;
       }
   

5. Inside onCreate initialize the loader with OPERATION_SEARCH_LOADER as the ID, null for the bundle, and this for the context

   getSupportLoaderManager().initLoader(OPERATION_SEARCH_LOADER, null, this);
   

6. Now call this method, whenever and wherever you want to trigger the loader

   private void makeOperationSearchQuery(String url) {
   
       // Create a bundle called queryBundle
       Bundle queryBundle = new Bundle();
       // Use putString with OPERATION_QUERY_URL_EXTRA as the key and the String value of the URL as the value
       queryBundle.putString(OPERATION_QUERY_URL_EXTRA,url);
       // Call getSupportLoaderManager and store it in a LoaderManager variable
       LoaderManager loaderManager = getSupportLoaderManager();
       // Get our Loader by calling getLoader and passing the ID we specified
       Loader loader = loaderManager.getLoader(OPERATION_SEARCH_LOADER);
       // If the Loader was null, initialize it. Else, restart it.
       if(loader==null){
           loaderManager.initLoader(OPERATION_SEARCH_LOADER, queryBundle, this);
       }else{
           loaderManager.restartLoader(OPERATION_SEARCH_LOADER, queryBundle, this);
       }
   }
   

Walla, you are done, just to remind you NetworkUtils.getResponseFromHttpUrl(url); is my custom function which take string convert it into URL which in turn used to create HTTPUrlConnection