Loaders, Cursor Loaders And Threads

Idan Felix, 16/3/2015

Agenda

Loaders CursorLoader

What WhatWhy WhyHow Howand how does it all fit in?

First things first

Elections day tomorrow, Don’t forget to vote.

Last week: How to talk to databases

2 weeks ago: Activity lifecycle

Source:

developer.android.com/training/basics/activity-lifecycle/starting.html

Where would you load your data?

onCreate? onStart? onResume?

What happens when the phone rotates?how to get fresh data as soon as it’s ready?

How to get the data? AsyncTask?

a Loader is an Android Specific pattern,

Designed so that async data retrieval is…Easy to implement PROPERLY.

So, What are Loaders?

Available since Android 3.0 (Honeycomb, API level 11 and support libs),to Activities and Fragments,

Also use that Observer Pattern we’ve seen last week,

Are cached between configuration changes.

So, What are Loaders?

- Recommended way to load dataBUT NOT ONLY!

- Works best with Cursors,BUT NOT ONLY!

Why to use loaders?

API Breakdown

Loader Abstract class that performs async

loading of data.

It can monitor its data-source while

active.

API Breakdown

More commonly, we’ll see these:

a loader implementation that provies an AsyncTask to do to loading,

an AsyncTaskLoader that is used to query data from a ContentProvider and returns a Cursor.

Loader

Async Task

Loader

Cursor

Loader

API Breakdown

This is where the magic is:LoaderManager helps an activity manage long-running operations with the activity lifecylce.

Loader

Async Task

Loader

Cursor

Loader

LoaderManager

API Breakdown

The callbacks are used to abstract the specific loader from the loader manager.3 Methods: onCreateLoader (int id, Bundle args),

onLoadFinished(Loader<D> loader, D data),

onLoaderReset(Loader<D> loader)

Loader

Async Task

Loader

Cursor

Loader

LoaderManager

LoaderCallbacks <D>

How to use loaders?

Step 1: Create a unique ID for your loader.

Step 2: LoaderManager.Loadercallbacks<D>

Step 3: init the loader with the id.

How to use loaders? - Step 1

Have a constant int, unique for the loader in the activity.

private static final int LOADER_ID = 31415;

How to use loaders? - Step 2

Implement the LoaderManager.LoaderCallbacks<D>.

Our demo will use a Long, so:implements

LoaderManager.LoaderCallbacks<Long>

How to use loaders? - Step 3

in the Activity’s onCreate, or in the Fragment’s onActivityCreate,

init the loader as such:

getLoaderManager().initLoader( LOADER_ID,

Parameter,

LoaderCallback);

in the Activity’s onCreate, or in the Fragment’s onActivityCreate,

init the loader as such:

getLoaderManager().initLoader( LOADER_ID,

Parameter,

LoaderCallback);

How to use loaders?

If the activity has multiple loaders,

each should have its own unique ID.

This enables the LoaderManager to re-

use the Loader.

How to use loaders?

in the Activity’s onCreate, or in the Fragment’s onActivityCreate,

init the loader as such:

getLoaderManager().initLoader( LOADER_ID,

Parameter,

LoaderCallback);

The parameter is passed to the loader.

How to use loaders?

in the Activity’s onCreate, or in the Fragment’s onActivityCreate,

init the loader as such:

getLoaderManager().initLoader( LOADER_ID,

Parameter,

LoaderCallback);

the manager uses this instance of

LoaderManager.LoaderCallbacks to

manage the loader’s lifecycle.

Loader Initialization - 1

when LM.initLoader is called, if the loader does not exist, the callback’s onCreateLoader method is called.

That’s where you create the loader.

Loader Initialization - 2

when LM.initLoader is called, if the loader does exist, the callback’s onCreateLoader method is not called - but the new Listener replaces the old listener.

That’s how you get away with Configuration Changes.

Demo

We’ll see 2 activities.

in onCreate, one starts a 5 second async task, and the other starts a 5 second loader.

We’ll rotate the device after ~2 seconds in each.Check out the LogCat too - to see just the 1 creation.

Yonatan’s Demo

adb shell dumpsys com.example.felixidan.loadersdemo

Story time: AsyncTaskLoader - Fail

How I thought it would look:public class DemoLoader extends AsyncTaskLoader<Long> {

public DemoLoader(Context context) {

super(context);

}

@Override

public Long loadInBackground() {

try {

Thread.sleep(5000);

} catch (InterruptedException e) {}

return new Long(SystemClock.upTimeMillies());

}

}

This code moved to

“TheBrain”.

Story time: AsyncTaskLoader - Fail

However, it was not that simple -the DemoLoader class is about 100 lines long

(and the “real” data loading is a one-liner).

Why?

Story time: AsyncTaskLoader - Epif.

I did not consider the responsibilities of the loader:

Getting

Data

Keeping

Data

Observing

Data source

Lifecycle

Quiz Time

Is it OK to have 2 loaders with the same ID?→ Yes, if they’re not on the same Activity.

Can an activity have multiple loaders?→ Yes, but they have to have a different ID.

What types can a loader return?→ Everything, as long as it’s an Object.

Quiz Time

How does this mechanism handles configuration changes?→ The loader manager keeps the loader, but replaces

thecallbacks object - so the most recent activity getsnotified when the loader is done.

What happens when the data is changed?→ The Loader is notified (by the Observer) and pushes

new information to the onLoaderFinished method.

Quiz Time

When to init the loader? (to call LoaderManager.init)→ In onCreate(), or onActivityCreated()

Yeah, but when exactly to init the loader?→ TRICKY…→ If the loader already has an answer for you, then

the onLoaderFinished callback will fire - so you should init the loader after you’re ready for this callback.

Quiz Time

What to do when a loader needs to change?→ Call RestartLoader. The loader will get an

onReset(),and the activity will get an onLoaderReset() call.You should release everything there, and let a newloader do the entire cycle for you.

How to restart a loader

The loader manager has a method for that:

getLoaderManager().restartLoader( LOADER_ID,

Parameter,

LoaderCallback);

Any Questions?

So, What’s a cursor loader?

a CursorLoader is an AsyncTaskLoader, designed to handle Cursors -

Which is what our ContentProvider provides,with its SQLite database and the SQLiteOpenHelper

How to use a Cursor Loader

1)Create a Loader ID constant. Simple int.

1)Fill-in Loader callbacks - for Cursor

1)Init a loader with LoaderManager

How to use a Cursor Loader - Step 1

private static final int

WORDS_LOADER_ID=1337;

private WordsAdapter adapter;

// initialize in onCreate()

How to use a Cursor Loader - Step 2

Implement the loader callbacks, keep the adapter.

@Override

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

adapter.swapCursor(data);

}

@Override

public void onLoaderReset(Loader<Cursor> loader) {

adapter.swapCursor(null);

}

How to use a Cursor Loader - Step 3

Step 3 - Initialize as usual

getLoaderManager()

.initLoader(WORDS_LOADER_ID,

null,

this);

Cursor Loader - What’s the benifit?

Super easy to implement (יחסית)

All the benefits of loaders -Minimum lines of code.

Implement the Adapter

See code,

public View newView(Context context, Cursor cursor, ViewGroup parent)public void bindView(View view, Context context, Cursor cursor)

Let’s go over the agenda

In our previous sessions, We talked about:

Handlers and AsyncTasks (2)The activity lifecycle (4a)Databases and Cursors (4b)

Let’s go over the agenda

Loaders are object that abstract data retrieval and observation, and simplify its implementation regarding:

Activity LifecycleData ObservationCache

And is implemented in 3 simple steps.

Let’s go over the agenda

CursorLoaders are loaders that are specific to accessing ContentProviders, and make the best-practices implementation really easy.

Any Questions?

References - This lecture

Slideshttp://goo.gl/aXMfI0

GitHubhttps://github.com/felixidan/Session4C

References - Loaders

Loaders API Guidehttp://developer.android.com/guide/components/loaders.html

Implementing Loaders @ Android Design Patternshttp://www.androiddesignpatterns.com/2012/08/implementing-loaders.html

AsyncTaskLoader documentationhttp://developer.android.com/reference/android/content/AsyncTaskLoader.html

Thank you,

We’ll stick around for any questions you might have, or any issues you’ve stumbled upon when implementing Sunhine or your app.

Drive home safely.

Thank you,

We’ll stick around for any questions you might have, or any issues you’ve stumbled upon when implementing Sunhine or your app.

Drive home safely.