Loader API in Android

Before starting Loader I will recommend you all to get a brief about Fragments which are introduced in Android API level 11 (i.e. HoneyComb).

Loader is introduced in Android 3.0, loaders make it easy to asynchronously load data in an activity or fragment. Loaders have these characteristics:

·      They are available to every Activity and Fragment.

·      They provide asynchronous loading of data.

·      They monitor the source of their data and deliver new results when the content changes.

·      They automatically reconnect to the last loader's cursor when being recreated after a configuration change. Thus, they don't need to re-query their data.

Dealing with Loader you may came across to the classes/interfaces LoaderManager, LoaderManager.LoaderCallbacks, Loader, AsyncTaskLoader, CursorLoader.

The LoaderManager manages one or more Loader instances within an Activity or Fragment. There is only one LoaderManager per activity or fragment.

Starting a Loader¶

Loader should be initialized within the activity's onCreate() method, or within the fragment's onActivityCreated() method using initLoader() method.

The initLoader() method takes the following parameters:

·      A unique ID that identifies the loader. In this example, the ID is 0.

·      Optional arguments to supply to the loader at construction (null in this example).

·      A LoaderManager.LoaderCallbacks implementation, which the LoaderManager calls to report loader events. In this example, the local class implements the LoaderManager.LoaderCallbacks interface, so it passes a reference to itself, this.

// Prepare the loader.  Either re-connect with an existing one,

// or start a new one.

getLoaderManager().initLoader(0, null, this);

Restarting a Loader¶

When you use initLoader(), as shown above, it uses an existing loader with the specified ID if there is one. If there isn't, it creates one. But sometimes you want to discard your old data and start over.

To discard your old data, you use restartLoader().

 getLoaderManager().restartLoader(0, null, this);

Using the LoaderManager Callbacks¶

LoaderManager.LoaderCallbacks is a callback interface that lets a client interact with the LoaderManager.

Loaders, in particular CursorLoader, are expected to retain their data after being stopped. This allows applications to keep their data across the activity or fragment's onStop() and onStart() methods, so that when users return to an application, they don't have to wait for the data to reload. You use the LoaderManager.LoaderCallbacks methods when to know when to create a new loader, and to tell the application when it is time to stop using a loader's data.

LoaderManager.LoaderCallbacks includes these methods:

·      onCreateLoader() — Instantiate and return a new Loader for the given ID.

·      onLoadFinished() — Called when a previously created loader has finished its load.

·      onLoaderReset() — Called when a previously created loader is being reset, thus making its data unavailable.

These methods are described in more detail in the following sections.

onCreateLoader

When you attempt to access a loader (for example, through initLoader()), it checks to see whether the loader specified by the ID exists. If it doesn't, it triggers the LoaderManager.LoaderCallbacks method onCreateLoader(). This is where you create a new loader. Typically this will be a CursorLoader, but you can implement your own Loader subclass.

onLoadFinished

This method is called when a previously created loader has finished its load. This method is guaranteed to be called prior to the release of the last data that was supplied for this loader. At this point you should remove all use of the old data (since it will be released soon), but should not do your own release of the data since its loader owns it and will take care of that.

onLoaderReset

This method is called when a previously created loader is being reset, thus making its data unavailable. This callback lets you find out when the data is about to be released so you can remove your reference to it.

I will explain you how to use Loader in your app.And as I have mentioned you should have a little idea about the Fragments.

Step 1 : Create an android project and Activity¶

Create an Activity name FragmentListCursorLoader

Write following code in your onCreate() method

FragmentManager fm = getFragmentManager();

        // Create the list fragment and add it as our sole content.

        if (fm.findFragmentById(android.R.id.content) == null) {

            CursorLoaderListFragment list = new CursorLoaderListFragment();

            fm.beginTransaction().add(android.R.id.content, list).commit();

        }

Where getFargmentManager is the method defined in Activity Class.

Step 2 : Create CursorLoaderListFragment class¶

CursorLoaderListFragment is the class which will retrieve Contacts from Android Device and will display on the screen using Loader.

CursorLoaderListFragment has extended ListFragment and two interfaces named CursorLoaderListFragment and LoaderManager.LoaderCallbacks<Cursor>

Step 3 : Write onActivityCreateMethod :¶

        // Give some text to display if there is no data.  In a real

        // application this would come from a resource.

        setEmptyText("No phone numbers");

        // We have a menu item to show in action bar.

        setHasOptionsMenu(true);

        // Prepare the loader.  Either re-connect with an existing one,

        // or start a new one.

        getLoaderManager().initLoader(0, null, this);

Now in this method there are three lines which I need to explain you.
1) setEmptyText will display the content which is passed in argument only when there is no data to diplay.
2) setHasOptionMenu will indicate whether to show the optionMenu content in the Activity or not.
3) getLoaderManager().initLoader(0, null, this); will init the loader. We need to give the unique Id to the initLoader, for our example it is 0.

Step 4 : What Loader will do.¶

public Loader<Cursor> onCreateLoader(int id, Bundle args) {

            // This is called when a new Loader needs to be created.  This

            // sample only has one Loader, so we don't care about the ID.

            // First, pick the base URI to use depending on whether we are

            // currently filtering.

            Uri baseUri;

            if (mCurFilter != null) {

                baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,

                        Uri.encode(mCurFilter));

            } else {

                baseUri = Contacts.CONTENT_URI;

            }

            // Now create and return a CursorLoader that will take care of

            // creating a Cursor for the data being displayed.

            String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("

                    + Contacts.HAS_PHONE_NUMBER + "=1) AND ("

                    + Contacts.DISPLAY_NAME + " != '' ))";

            return new CursorLoader(getActivity(), baseUri,

                    CONTACTS_SUMMARY_PROJECTION, select, null,

                    Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");

        }

As you can see the task which a Loader should perform is written in onCreateLoader method.
It needs...

·      uri — The URI for the content to retrieve.

·      projection — A list of which columns to return. Passing null will return all columns, which is inefficient.

·      selection — A filter declaring which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). Passing null will return all rows for the given URI.

·      selectionArgs — You may include ?s in the selection, which will be replaced by the values from selectionArgs, in the order that they appear in the selection. The values will be bound as Strings.

·      sortOrder — How to order the rows, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). Passing null will use the default sort order, which may be unordered.

Step 5 : Time to close the Loader

The loader will release the data once it knows the application is no longer using it. For example, if the data is a cursor from a CursorLoader, you should not call close() on it yourself. If the cursor is being placed in a CursorAdapter, you should use the swapCursor() method so that the old Cursor is not closed. For example:

public void onLoadFinished(Loader<Cursor> loader, Cursor data) {

    // Swap the new cursor in.  (The framework will take care of closing the

    // old cursor once we return.)

    mAdapter.swapCursor(data);

}

Step 6 : Reset the Loader

This callback lets you find out when the data is about to be released so you can remove your reference to it.

public void onLoaderReset(Loader<Cursor> loader) {

    // This is called when the last Cursor provided to onLoadFinished()

    // above is about to be closed.  We need to make sure we are no

    // longer using it.

    mAdapter.swapCursor(null);

}

You will also find method to create Menu and request search query also..

 @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {

        // Place an action bar item for searching.

        MenuItem item = menu.add("Search");

        item.setIcon(android.R.drawable.ic_menu_search);

        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);

        SearchView sv = new SearchView(getActivity());

        sv.setOnQueryTextListener(this);

        item.setActionView(sv);

    }

public boolean onQueryTextChange(String newText) {

        // Called when the action bar search text has changed.  Update

        // the search filter, and restart the loader to do a new query

        // with this filter.

        mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;

        getLoaderManager().restartLoader(0, null, this);

        return true;

    }

I have attached demo project which has two different Activities. You can see it one by one for better understanding of the Loader.