Chapter -5 : Modern Android Architecture ‘Android Jetpack’ - Handling background tasks (WorkManager)

        In this Android Jetpack, team at Google release one library specifically designed for scheduling and managing the background tasks. It’s called “WorkManager”. The WorkManager API makes it easy to specify deferrable, asynchronous tasks and when they should run. These APIs let you create a task and hand it off to WorkManager to run immediately or at an appropriate time.

Drawbacks of existing background task APIs

       Earlier we had AlarmManager, JobScheduler, FirebaseJobDispatcher for scheduling the background tasks. But the issues were

• JobScheduler – Available only for API >= 21
• FirebaseJobDispatcher – For backward compatibility

So developer had to understand which method to use when. To overcome these issues we have WorkManager, and it will automatically choose the best method for your task and you do not need to write logic for it. So basically WorkManager is providing an abstraction layer. It gives us a clean interface hiding all the complexities and giving the guaranteed execution of the task.

WorkManager Features

1. It is fully backward compatible so in your code you do not need to write if-else for checking the android version.
2. So it has benefits over the Firebase Job Dispatcher which backwards compatible but not work for device without Google play service.
3. With WorkManager we can check the status of the work.
4. Tasks can be chained as well, for example when one task is finished it can start another.
5. And it provides guaranteed execution with the constraints, we have many constrained available that we will see ahead. 

While scheduling background/long running task like network availability, device in doze mode (Like, if your app is in DOZE mode then it wouldn’t execute, it wouldn’t wake up your phone just to do that work), sync data only when charging so many but WorkManager is taking care of all these constraints and give app guaranteed to be executed even if device get rebooted or app get exited without executing that task.

Technical details of Base classes

Worker - Specifies what tasks we need to perform. By extending this class and perform the work here.
WorkRequest - Represents an individual task. At a minimum, a WorkRequest object specifies which Worker class should perform the task. Every WorkRequest has an auto-generated unique ID; we can use the ID to do things like cancel a queued task or get the task's state. We will be using one of the direct subclasses, OneTimeWorkRequest or 

PeriodicWorkRequest.
WorkManager - Enqueues and manages the work requests. You pass your WorkRequest object to WorkManager to enqueue the task.
WorkInfo - The class contains information about the works. For each WorkRequest we can get a LiveData using WorkManager. The LiveData holds the WorkInfo and by observing it we can determine the Work Informations.

Creating a Worker

         For creating a worker we need to create a class and then in the class we need to extend Worker. So here I am creating a class named NotificationWorker. This class has one abstract method called doWork(). As the name suggests, you need to perform the work you want to do in the background. This method will be called in the background/worker thread. Write a program to perform the task in this method.

public class NotificationWorker extends Worker {
public static final String TASK_DESC = "Desc";
public NotificationWorker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
super(context, workerParams);
}
@NonNull
@Override
public Result doWork() {
//getting the input data
String taskDesc = getInputData().getString(TASK_DESC);
displayNotification("Hello", "Welcome to my world!"+"\n"+taskDesc);
//setting output data
Data data = new Data.Builder()
.putString(TASK_DESC, "The notification is shown.")
.build();
setOutputData(data);
return Result.SUCCESS;
}
private void displayNotification(String title, String task) {
NotificationManager notificationManager = (NotificationManager) getApplicationContext().getSystemService(Context.NOTIFICATION_SERVICE);
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
NotificationChannel channel = new NotificationChannel("truedreamz", "truedreamz", NotificationManager.IMPORTANCE_DEFAULT);
notificationManager.createNotificationChannel(channel);
}
NotificationCompat.Builder notification = new NotificationCompat.Builder(getApplicationContext(), "truedreamz")
.setContentTitle(title)
.setContentText(task)
.setSmallIcon(R.mipmap.ic_launcher);
notificationManager.notify(1, notification.build());
}
}

WorkResult has three values
1) SUCCESS
2) FAILURE
3) RETRY : Means encountered a transient error i.e if device lost network connection in the middle so we will have to retry after little bit some back off.

Performing the Work

Now let’s perform the work that we created. For this first come inside MainActivity.java and write the following code.



OneTimeWorkRequest: Used when we want to perform the work only once. This is because WorkRequest is an abstract class and we have to use the direct subclasses of it.
Finally to enqueue the work we simple used the enqueue() method from WorkManager.

Determining Work Status

So we successfully performed the work. Now let’s learn how we can determine the status of the work.



Now you can run your application to see it is working fine.

Data transfer to/from WorkManager

We can also pass data to our WorkManager class and we can also get back some data after finishing the work.

Sending Data

Data data = new Data.Builder()
.putString(NotificationWorker.TASK_DESC, "The data passed from MainActivity")
.build();
final OneTimeWorkRequest oneTimeWorkRequest=
new OneTimeWorkRequest.Builder(NotificationWorker.class)
.setInputData(data)
.build();

So to receive data we will do the following modification in our NotificationWorker class.
String taskDesc = getInputData().getString(TASK_DESC);

Adding Constraints

Now let’s add some constraints in our work so that it will execute at a specific time. We have many constraints available for example.

• setRequiresCharging(boolean b): If it is set true the work will only be done when the device is charging.
• setRequiresBatteryNotLow(boolean b): Work will be done only when the battery of the device is not low.
• setRequiresDeviceIdle(boolean b): Work will be done only when the device is idle.

Canceling Work

We can also cancel the work if required. For this we have a method cancelWorkById(). It takes the work id as an argument that we can get from our WorkRequest object.

We also have the following methods to cancel the work.

• cancelAllWork(): To cancel all the work.
• cancelAllWorkByTag(): To cancel all works of a specific tag.
• cancelUniqueWork(): To cancel a unique work.

PeriodicWorkRequest

Sometimes it is needed to perform the work periodically for example taking backup to the server. In scenarios like this we can use PeriodicWorkRequest class. Everything else is the same.

public class PeriodicToastWorker extends Worker {
public PeriodicToastWorker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
super(context, workerParams);
}
@NonNull
@Override
public Result doWork() {
//getting the input data
displayNotification("Hello", "Periodic notification!");
return Result.SUCCESS;
}
private void displayNotification(String title, String task) {
NotificationManager notificationManager = (NotificationManager) getApplicationContext().getSystemService(Context.NOTIFICATION_SERVICE);
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
NotificationChannel channel = new NotificationChannel("truedreamz", "truedreamz", NotificationManager.IMPORTANCE_DEFAULT);
notificationManager.createNotificationChannel(channel);
}
NotificationCompat.Builder notification = new NotificationCompat.Builder(getApplicationContext(), "truedreamz")
.setContentTitle(title)
.setContentText(task)
.setSmallIcon(R.mipmap.ic_launcher);
notificationManager.notify(1, notification.build());
}
}



The WorkManager attempts to run your task at the interval you request, subject to the constraints you impose and its other requirements.
I have uploaded the latest source code in GitHub for your reference. Kindly raise your queries in the command section.

References :
https://developer.android.com/topic/libraries/architecture/workmanager/
https://developer.android.com/topic/libraries/architecture/workmanager/basics#java
https://www.simplifiedcoding.net/android-workmanager-tutorial/

Happy coding!!!
Cheers!!!

Chapter -3 : Modern Android Architecture ‘Android Jetpack’ - Infinity feeds using Paging Library

       Android Paging library is a component of android jetpack which makes it easier to load data gradually in our app. The Paging library supports both large bounded list and unbounded lists, such as continuously updating feeds. For instance consider the Instagram App, it shows you a list of posts, and the app has too many posts, but it does not load all the post at once, it shows you some posts, as soon as you reaches the last item of the post list it loads more posts. This is called paging.

      This post provides an overview of how we can use the Paging Library to request and display data that users want to see while consuming system resources more economically and elegantly.

Merits of using Paging

• We only load a small chunk from your large data set, it will consume less bandwidth.
• The app will use less resources resulting in a smooth app and nice user experience.

Pre-Requisites

• Before moving ahead, you should go through these tutorials first, as we are going to use these things. Using Retrofit in Android: Complete Retrofit Playlist from Scratch.
• We will use the Retrofit Library to fetch the data from backend API. The above tutorial discusses about Retrofit and Building API from scratch. So you can check the series. 
• RecyclerView: RecyclerView Tutorial.
• We will load the items in a RecyclerView after fetching it from server.
• Android ViewModel: Android ViewModel Tutorial
• This is another component from Android Jetpack. It helps us to store UI related data in a more efficient way.

In this tutorial I am going to use a readymade News API. In the above API URL we are passing the below parameters.

page: The number of page that we want to fetch.
pagesize: The total number of items that we want in the page.

The above URL will give the following response.

The data is coming from NewsAPI, and it has a very large data set, so you will get may be an infinite number of pages.

Now our task is to fetch from the page 1 and load the next page as soon as the user reaches the end of the list. And to do this we will use the Android Paging Library. Please find folder structure of sample application,

Step 1: Add the Paging library to the app

Go to app level build.gradle file and add the following dependencies.

After adding all the above required dependencies sync your project and you are good to go. Here we have added a lot of dependencies these are:

Retrofit and Gson - For fetching and parsing JSON from URL.
ViewModel - Android architecture component for storing data.
Paging - The paging library.
RecyclerView and CardView - For building the List.
Picasso - For loading image from URL.

Step 2 : Creating Model Class

Now we will create a model class in our project. We need this class to parse the JSON response automatically. Please refer above a really complex nested JSON in the response. So we need many classes to bind the response into respective java class automatically.
Create a file named Post.java and write the following code in it.

Step 3 : Creating Retrofit Singleton Class

Each time when we want to fetch data from a new page, we need the Retrofit object. So creating a singleton instance of Retrofit is a good idea. For the singleton instance I will create a new class named RetrofitFactory.

We have to create RetrofitApi interface to fetch news feed.

Step 4 : Creating Item Data Source

Now here comes the very important thing, the data source of our item from where we will fetch the actual data. And you know that we are using the News API.

For creating a Data Source we have many options, like ItemKeyedDataSource, PageKeyedDataSource, PositionalDataSource. For this tutorial we are going to use PageKeyedDataSource, as in our API we need to pass the page number for each page that we want to fetch. So here the page number becomes the Key of our page.

In this scenario, we would be using a PageKeyedDataSource. The following code shows how we can create PageKeyedDataSource for our NewsFeedDataSource class.

public class NewsFeedDataSource extends PageKeyedDataSource<Long, Post> implements Constants {
private static final String TAG = NewsFeedDataSource.class.getSimpleName();
private InfinityFeedApp appController;
private MutableLiveData networkState;
private MutableLiveData initialLoading;

public NewsFeedDataSource(InfinityFeedApp appController) {
this.appController = appController;
networkState = new MutableLiveData();
initialLoading = new MutableLiveData();
}

public MutableLiveData getNetworkState() {
return networkState;
}

public MutableLiveData getInitialLoading() {
return initialLoading;
}

@Override
public void loadInitial(@NonNull LoadInitialParams<Long> params,
@NonNull final LoadInitialCallback<Long, Post> callback) {
initialLoading.postValue(NetworkState.LOADING);
networkState.postValue(NetworkState.LOADING);
appController.getRetrofitApi().getFeeds(QUERY, API_KEY, 1, params.requestedLoadSize)
.enqueue(new Callback<NewsFeed>() {


@Override
public void onResponse(Call<NewsFeed> call, Response<NewsFeed> response) {
if (response.isSuccessful()) {
callback.onResult(response.body().getPosts(), null, 2l);
initialLoading.postValue(NetworkState.LOADED);
networkState.postValue(NetworkState.LOADED);
} else {
initialLoading.postValue(new NetworkState(NetworkState.Status.FAILED, response.message()));
networkState.postValue(new NetworkState(NetworkState.Status.FAILED, response.message()));
}
}

@Override
public void onFailure(Call<NewsFeed> call, Throwable t) {
String errorMessage = t == null ? "unknown error" : t.getMessage();
networkState.postValue(new NetworkState(NetworkState.Status.FAILED, errorMessage));
}
});
}

@Override
public void loadBefore(@NonNull LoadParams<Long> params,
@NonNull LoadCallback<Long, Post> callback) {
}

@Override
public void loadAfter(@NonNull final LoadParams<Long> params,
@NonNull final LoadCallback<Long, Post> callback) {
Log.i(TAG, "Loading Rang " + params.key + " Count " + params.requestedLoadSize);
networkState.postValue(NetworkState.LOADING);
appController.getRetrofitApi().getFeeds(QUERY, API_KEY, params.key, params.requestedLoadSize).enqueue(new Callback<NewsFeed>() {

@Override
public void onResponse(Call<NewsFeed> call, Response<NewsFeed> response) {
if (response.isSuccessful()) {
long nextKey = (params.key == response.body().getTotalResults()) ? null : params.key + 1;
callback.onResult(response.body().getPosts(), nextKey);
networkState.postValue(NetworkState.LOADED);
} else
networkState.postValue(new NetworkState(NetworkState.Status.FAILED, response.message()));
}

@Override
public void onFailure(Call<NewsFeed> call, Throwable t) {
String errorMessage = t == null ? "unknown error" : t.getMessage();
networkState.postValue(new NetworkState(NetworkState.Status.FAILED, errorMessage));
}
});
}
}

Note: I’m using LiveData to push the network updates to the UI. This is because it’s lifecycle aware and will handle the subscription for us.

• We extended PageKeyedDataSource<Integer, Item> in the above class. Integer here defines the page key. Every time we want a new page from the API we need to pass the page number that we want which is an integer. Item is the Post model class that we will get from the API or that we want to get. 
• Then we defined the size of a page which is 21, the initial page number which is 1. You are free to change these values if you want.

Then we have 3 overridden methods.

• loadInitials(): This method will load the initial data. Or you can say it will be called once to load the initial data, or first page according to this post.
• loadBefore(): This method will load the previous page.
• loadAfter(): This method will load the next page.

Step 5 : Creating Item Data Source Factory

DataSourceFactory is responsible for retrieving the data using the DataSource and PagedList configuration. We are going to use MutableLiveData<> to store our PageKeyedDataSource and for this we have to create NewsFeedDataFactory class by extending DataSource.Factory.

Step 6 : Setup the ViewModel

The view model will be responsible for creating the PagedList along with its configurations and send it to the activity so it can observe the data changes and pass it to the adapter.

PagedList is a wrapper list that holds your data items (in our case the list of news we need to display) and invokes the DataSource to load the elements. It typically consists of a background executor (which fetches the data) and the foreground executor (which updates the UI with the data).

For instance, let’s say we have some data that we add to the DataSource in the background thread. The DataSource invalidates the PagedList and updates its value. Then on the main thread, the PagedList notifies its observers of the new value. Now the PagedListAdapter knows about the new value.

PagedList.Config pagedListConfig =
(new PagedList.Config.Builder())
.setEnablePlaceholders(false)
.setInitialLoadSizeHint(10)
.setPrefetchDistance(10)
.setPageSize(20).build();

1. setEnablePlaceholders(boolean enablePlaceholders) - Enabling placeholders mean we can see placeholders instead of the image since it is not fully loaded. 
2. setInitialLoadSizeHint(int initialLoadSizeHint) - The number of items to load initially. 
3. setPageSize(int pageSize) - The number of items to load in the PagedList. 
4. setPrefetchDistance(int prefetchDistance) — The number of preloads that occur. For instance, if we set this to 10, it will fetch the first 10 pages initially when the screen loads.

Now create a class named ItemViewModel and write the following code.

public class NewsFeedViewModel extends ViewModel {
private Executor executor;
private LiveData<NetworkState> networkState;
private LiveData<PagedList<Post>> feedsLiveData;
private InfinityFeedApp infinityFeedApp;

public NewsFeedViewModel(@NonNull InfinityFeedApp infinityFeedApp) {
this.infinityFeedApp = infinityFeedApp;
init();
}

private void init() {
executor = Executors.newFixedThreadPool(5);
NewsFeedDataFactory newsFeedDataFactory = new NewsFeedDataFactory(infinityFeedApp);
networkState = Transformations.switchMap(newsFeedDataFactory.getFeedDataSourceMutableLiveData(),
dataSource -> dataSource.getNetworkState());

PagedList.Config pagedListConfig =
(new PagedList.Config.Builder())
.setEnablePlaceholders(false)
.setInitialLoadSizeHint(10)
.setPrefetchDistance(10)
.setPageSize(20).build();

feedsLiveData = (new LivePagedListBuilder(newsFeedDataFactory, pagedListConfig))
.setFetchExecutor(executor)
.build();

}

public LiveData<NetworkState> getNetworkState() {
return networkState;
}

public LiveData<PagedList<Post>> getFeedsLiveData() {
return feedsLiveData;
}
}

Step 7 : Creating the PagedListAdapter

PagedListAdapter is an implementation of RecyclerView.Adapter that presents data from a PagedList. It uses DiffUtil as a parameter to calculate data differences and do all the updates for you.

public static DiffUtil.ItemCallback<Post> DIFF_CALLBACK = new DiffUtil.ItemCallback<Post>() {
@Override
public boolean areItemsTheSame(@NonNull Post oldItem, @NonNull Post newItem) {
return oldItem.id == newItem.id;
}
@Override
public boolean areContentsTheSame(@NonNull Post oldItem, @NonNull Post newItem) {
return oldItem.equals(newItem);
}
};

FeedListAdapter class extends PageListAdapter and does not override the usual getItemCount() as this is provided by the PageList object. If we do need to override this method, we need to add super.getItemCount() to the method. 

public class FeedListAdapter extends PagedListAdapter<Post, RecyclerView.ViewHolder> {
private static final int TYPE_PROGRESS = 0;
private static final int TYPE_ITEM = 1;

private Context context;
private NetworkState networkState;

public FeedListAdapter(Context context) {
super(Post.DIFF_CALLBACK);
this.context = context;
}

@NonNull
@Override
public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
LayoutInflater layoutInflater = LayoutInflater.from(parent.getContext());
if(viewType == TYPE_PROGRESS) {
NetworkItemBinding headerBinding = NetworkItemBinding.inflate(layoutInflater, parent, false);
NetworkStateItemViewHolder viewHolder = new NetworkStateItemViewHolder(headerBinding);
return viewHolder;
} else {
FeedItemBinding itemBinding = FeedItemBinding.inflate(layoutInflater, parent, false);
ArticleItemViewHolder viewHolder = new ArticleItemViewHolder(itemBinding);
return viewHolder;
}
}

@Override
public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, int position) {
if(holder instanceof ArticleItemViewHolder) {
((ArticleItemViewHolder)holder).bindTo(getItem(position));
} else {
((NetworkStateItemViewHolder) holder).bindView(networkState);
}
}

private boolean hasExtraRow() {
if (networkState != null && networkState != NetworkState.LOADED) {
return true;
} else {
return false;
}
}

@Override
public int getItemViewType(int position) {
if (hasExtraRow() && position == getItemCount() - 1) {
return TYPE_PROGRESS;
} else {
return TYPE_ITEM;
}
}

public void setNetworkState(NetworkState newNetworkState) {
NetworkState previousState = this.networkState;
boolean previousExtraRow = hasExtraRow();
this.networkState = newNetworkState;
boolean newExtraRow = hasExtraRow();
if (previousExtraRow != newExtraRow) {
if (previousExtraRow) {
notifyItemRemoved(getItemCount());
} else {
notifyItemInserted(getItemCount());
}
} else if (newExtraRow && previousState != newNetworkState) {
notifyItemChanged(getItemCount() - 1);
}
}

public class ArticleItemViewHolder extends RecyclerView.ViewHolder {
private FeedItemBinding binding;
public ArticleItemViewHolder(FeedItemBinding binding) {
super(binding.getRoot());
this.binding = binding;
}

public void bindTo(Post post) {
binding.itemImage.setVisibility(View.VISIBLE);
binding.itemDesc.setVisibility(View.VISIBLE);
String author = post.getAuthor() == null || post.getAuthor().isEmpty() ? context.getString(R.string.author_name) : post.getAuthor();
String titleString = String.format(context.getString(R.string.item_title), author, post.getTitle());
SpannableString spannableString = new SpannableString(titleString);
spannableString.setSpan(new ForegroundColorSpan(ContextCompat.getColor(context.getApplicationContext(), R.color.secondary_text)),
titleString.lastIndexOf(author) + author.length() + 1, titleString.lastIndexOf(post.getTitle()) - 1, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
binding.itemTitle.setText(spannableString);
binding.itemTime.setText(String.format(context.getString(R.string.item_date), Utility.getDate(post.getPublishedAt()), Utility.getTime(post.getPublishedAt()))); binding.itemDesc.setText(post.getDescription());
Picasso.get().load(post.getUrlToImage()).resize(250, 200).into(binding.itemImage);
}
}

public class NetworkStateItemViewHolder extends RecyclerView.ViewHolder {
private NetworkItemBinding binding;
public NetworkStateItemViewHolder(NetworkItemBinding binding) {
super(binding.getRoot());
this.binding = binding;
}

public void bindView(NetworkState networkState) {
if (networkState != null && networkState.getStatus() == NetworkState.Status.RUNNING) {
binding.progressBar.setVisibility(View.VISIBLE);
} else {
binding.progressBar.setVisibility(View.GONE); 

}


if (networkState != null && networkState.getStatus() == NetworkState.Status.FAILED) {
binding.errorMsg.setVisibility(View.VISIBLE);
binding.errorMsg.setText(networkState.getMsg());
} else {
binding.errorMsg.setVisibility(View.GONE);
}
}
}
}

Step 8 : Displaying the Paged List at Activity

The last step is to set up our Activity class with the ViewModel, RecyclerView, PagedListAdapter:





Now you are done and you can try running your application. If everything is fine you will see the below output.

I have uploaded the latest source code in GitHub for your reference. Kindly raise your queries in the command section.

References :

Paging library overview
https://developer.android.com/topic/libraries/architecture/paging/

Paging library UI components and considerations
https://developer.android.com/topic/libraries/architecture/paging/ui

Paging library data components and considerations
https://developer.android.com/topic/libraries/architecture/paging/data

https://proandroiddev.com/8-steps-to-implement-paging-library-in-android-d02500f7fffe
https://www.simplifiedcoding.net/android-paging-library-tutorial/



Happy coding!!!
Cheers!!!

Chapter -2 : Modern Android Architecture ‘Android Jetpack’ - Working with DataBinding

       The Data Binding Library i.e. support library enables us to bind UI components in our layouts to data sources in your app using a declarative format rather than programmatically. In Simple words, Data binding enables a way to combine the UI with business logic allowing the UI values to update automatically without manual intervention.

Benefits

• Reduces lot of boilerplate code in our business logic while sync up the UI when new data is available. 
• Making them simpler and easier to maintain. 
• Improve the app's performance 
• Help prevent memory leaks and null pointer exceptions.

How to use ?

To get started with DataBinding, open the build.gradle located under app and enable dataBinding under android module. Once enabled, Sync the project

Adding Data Binding element to Layout file

The expression language allows us to write expressions that connect variables to the views in the layout. The next step in converting the layout file to a data binding layout file is to add the data element. To enable DataBinding in a layout, the root element should start with <layout> tag. Along with it, <data> and <variable> tags are used.



<layout> – Indicate the compiler that this layout has data binding in it so generate the binding class for this layout.
<import> – Similar to how we interpret imports in normal classes. Import a class in this layout.
<variable> – similar to how we interpret variables in normal classes. Declare a variable 'name' and it’s type for the layout to use.
<data> – Parent tag for <import> and <variable> tags for the “data” that we are going to bind for this layout.

Access data in layout through Layout Expressions

It’s structured like a normal class where our imports and most of our variables are declared at the top and we use them at the bottom. The user variable declared within data describes a property that may be used within this layout. Also, if you want to use expressions inside your layout, you can call attribute properties using the “@{}" syntax.

Project structure for Binding Data

The Data Binding Library provides classes and methods to easily observe data for changes. You don't have to worry about refreshing the UI when the underlying data source changes. You can make your variables or their properties observable. The library allows you to make objects, fields, or collections observable.

public class User extends BaseObservable {
String username;
String email;
String userImage;
String info;
public ObservableField<Long> numberOfFriends = new ObservableField<>();
public ObservableField<Long> numberOfArticles = new ObservableField<>();
public ObservableField<Long> numberOfFavourite = new ObservableField<>();
public User() {
}

@Bindable
public String getUsername() {
return username; 

}

public void setUsername(String username) {
this.username = username;
notifyPropertyChanged(BR.username);
}
@BindingAdapter({"profileImage"})
public static void loadImage(ImageView view, String imageUrl) {
Glide.with(view.getContext())
.load(imageUrl)
.apply(RequestOptions.circleCropTransform())
.into(view);
}
@Bindable
public String getUserImage() {
return userImage;
}
public void setUserImage(String userImage) {
this.userImage = userImage;
notifyPropertyChanged(BR.userImage);
}
@Bindable
public String getInfo() {
return info;
}
public void setInfo(String info) {
this.info = info;
notifyPropertyChanged(BR.info);
}
public ObservableField<Long> getNumberOfFriends() {
return numberOfFriends;
}
public ObservableField<Long> getNumberOfArticles() {
return numberOfArticles;
}
public ObservableField<Long> getNumberOfFavourite() {
return numberOfFavourite;
}
}

• To make User pojo class Observable, extend the class from BaseObservable. For demonstration, both Observable and ObservableField are used in the same class.
• For variables username, email, userImage and info., @Bindable annotation is used and notifyPropertyChanged is called upon setting new data if needed.
• Variables numberOfArticless, numberOfFriends, numberOfFavourites are declared as ObservableFields.
• @BindingAdapter is used to bind profileImage to ImageView in order to load the image from URL using Glide library.

Generating binding class

When you set the root tag of your layout to <layout>, a binding class is generated by the Data Binding library. For instance, activity_main.xml will have a binding class called ActivityMainBinding.
Note : This class holds all the bindings from the layout properties to the layout’s views and knows how to assign values for the binding expressions.



If you are using data binding items in a Fragment, ListView, or RecyclerView adapter, use the inflate() methods of the binding classes or the DataBindingUtil class:




In this chapter, we will make a very simple app that loads images in RecyclerView using data binding library.

DataBinding in RecyclerView

Binding a RecyclerView layout is similar to normal binding except few changes in onCreateViewHolder and onBindViewHolder methods. Create layout named article_row_item.xml. This layout contains an ImageView to render the image in RecyclerView. In this layout, data binding is enabled by keeping the root element as <layout>. The Post model in bound to this layout using <variable> tag.

Generating adapter binding class

Create a class named ArticleAdapter.java under view package. As the layout name is article_row_item.xml, the generated binding class will be ArticleRowItemBinding. In onCreateViewHolder() method, article_row_item layout is inflated with the help of ArticleRowItemBinding class. articleViewHolder.rowItemBinding.setArticle() binds the Post model to each row.

public class ArticleAdapter extends RecyclerView.Adapter<ArticleAdapter.ArticleViewHolder> {
private List<Article> articleList;
private LayoutInflater layoutInflater;
private ArticlesAdapterListener listener;
public ArticleAdapter(List<Article> articleList, ArticlesAdapterListener listener) {
this.articleList = articleList;
this.listener = listener;
}
@NonNull
@Override
public ArticleViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
if (null == layoutInflater) {
layoutInflater = LayoutInflater.from(viewGroup.getContext());
}
ArticleRowItemBinding articleRowItemBinding = DataBindingUtil.inflate(layoutInflater, R.layout.article_row_item, viewGroup, false);
return new ArticleViewHolder(articleRowItemBinding);
}
@Override
public void onBindViewHolder(@NonNull ArticleViewHolder articleViewHolder, final int i) {
articleViewHolder.rowItemBinding.setArticle(articleList.get(i));
articleViewHolder.rowItemBinding.thumbnail.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if (listener != null) {
listener.onArticleClicked(articleList.get(i));
}
}
});
}
@Override
public int getItemCount() {
return articleList.size();
}
public class ArticleViewHolder extends RecyclerView.ViewHolder {
private ArticleRowItemBinding rowItemBinding;
public ArticleViewHolder(final ArticleRowItemBinding binding) {
super(binding.getRoot());
this.rowItemBinding = binding;
}
}
public interface ArticlesAdapterListener {
void onArticleClicked(Article post);

}
}

Loading RecyclerView in MainActivity

As the main activity layout name is activity_main, the generated binding class will be ActivityMainBinding. loadUserData() renders the user information such as username, , articles, friends and favourite count. init() initialize the RecyclerView with sample images data. AppClickHandlers handles the click events of UI elements. Here, all the binding of click events is done via xml layout only. We don’t explicitly assign anything from activity code.

public class MainActivity extends AppCompatActivity implements ArticleAdapter.ArticlesAdapterListener {
private User user;
private ActivityMainBinding binding;
private AppClickHandlers handler;
private RecyclerView recyclerView;
private ArticleAdapter articleAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
handler = new AppClickHandlers(this);
loadUserData();
init();
}

private void loadUserData() {
user = new User();
user.setUsername("Mr. Sherlock Holmes");
user.setUserImage("https://tse4.mm.bing.net/th?id=OIP.gHhJlg-RAvR-XosdERsIMQHaEo&pid=Api&w=1440&h=900&rs=1&p=0");
// updating ObservableField
user.numberOfArticles.set(1500L);
user.numberOfFriends.set(205090L);
user.numberOfFavourite.set(10L);
// display user
binding.setUser(user);
// assign click handlers
binding.setHandlers(handler);
}

private void init() {
recyclerView = binding.recyclerView;
recyclerView.setLayoutManager(new GridLayoutManager(this, 3));
recyclerView.addItemDecoration(new GridItemDecoration(3, ImageUtil.dpToPx(MainActivity.this, 4), true));
recyclerView.setItemAnimator(new DefaultItemAnimator());
recyclerView.setNestedScrollingEnabled(false);
articleAdapter = new ArticleAdapter(getArticles(), this);
recyclerView.setAdapter(articleAdapter);
}

private ArrayList<Article> getArticles() {
ArrayList<Article> articles = new ArrayList<>();
for (int i = 1; i < 10; i++) {
Article article = new Article();
article.setImageUrl("https://tse3.mm.bing.net/th?id=OIP.f1bDVK_DN6xiTaYAM79ucgHaEK&pid=Api");
articles.add(article);
}
return articles;
}

@Override
public void onArticleClicked(Article article) {
Toast.makeText(getApplicationContext(), "Article clicked! " + article.getImageUrl(), Toast.LENGTH_SHORT).show();
}

public class AppClickHandlers {
Context context;
public AppClickHandlers(Context context) {
this.context = context;
}
public void onFriendsClicked(View view) {
Toast.makeText(context, "Friends is clicked!", Toast.LENGTH_SHORT).show();
}
public void onFavouriteClicked(View view) {
Toast.makeText(context, "Favourite is clicked!", Toast.LENGTH_SHORT).show();
}
public void onArticlesClicked(View view) {
Toast.makeText(context, "Article is clicked!", Toast.LENGTH_SHORT).show();
}
}
}

Run and test the app once. Make sure the device is having internet connection as images will be downloaded from network.
I have uploaded the latest source code in GitHub for your reference. Kindly raise your queries in the command section.

References :

https://developer.android.com/topic/libraries/data-binding/
https://www.androidhive.info/android-working-with-databinding/
http://imakeanapp.com/android-jetpack-data-binding/

Happy coding!!!
Cheers!!!

Chapter 1 : Modern Android Architecture ‘Android Jetpack’ – Managing & Tracking Android Lifecycle (Room, LiveData and ViewModel)

        While managing lifecycle, we have to manage the data, orientations or memory leaks all based on lifecycle and we have to do it on our own which is cumbersome sometimes. So, we need these new set of libraries which can do most of our work very easily without going into deep. The Support Library components aren’t the only familiar features in the list of Jetpack components. All of the Architecture Components (Lifecycle, LiveData, Room and ViewModel) are now part of Jetpack, too.

It helps us to solve two critical points:

• Manage our UI components lifecycle
• Persist data over configuration changes

In this chapter, We are going to develop an app, which basically

1. Takes some input from the user(LiveData)
2. Save into the local database(Room) 
3. Show it on the screen(ViewModel)

Let’s get started!!

Note : Use Android Studio 3.0 or later version.

Adding Dependencies (Pre AndroidX)

Room is basically a Database object mapping library use to access the database. Lifecycle, this has some good set of classes like ViewModel and LiveData which we will use to manage the lifecycle of our app. Add these libraries to build.gradle (Module: app) file, at the end of the dependencies block.

dependencies {
// Room components
implementation 'android.arch.persistence.room:runtime:2.0.0'
annotationProcessor 'android.arch.persistence.room:compiler:2.0.0'

// Lifecycle components
implementation 'android.arch.lifecycle:extensions:2.0.0'
annotationProcessor 'android.arch.lifecycle:compiler:2.0.0'
}
Refer here for AndroidX Dependencies.

Room (Local database)

We can query your data without having to deal with cursors or loaders. We can define our database by adding annotations in your Model class. So get rid of third-party ORMs like Sugar, In fact, when the official Android libraries give you an equal, or if not, better solution. The library helps you create a cache of your app's data on a device that's running your app. If user does not have internet connection, Just chill ! No problem! This cache allows users to view a consistent copy of key information within your app.

3 key concepts of Room

@Entity - We just have to annotate “@Entity” to a class and the name of the class becomes table name and, data members becomes the name of the columns. “@Entity” class represent an entity in a table.



Here, we have class StudentModel, and name of table is also same. We had made a column itemName, personName, and addedDate.

@Dao (Data Access Object) - An Interface where we put all our SQL queries. No need to write whole quires now, we just need to make a method and annotate with specific annotations like “@Insert”, “@Delete”, “@Query(SELECT FROM *)”



Here, we have an interface StudentModelDao. To insert the data we annotated “@Insert” to insert method. Room doesn’t gives us annotations which can help us in selecting everything so we have “@Query” to do some custom queries.

@Database - We need to create an abstract class (Room class) which extends RoomDatabase. It is a database layer over the SQLite database; this helps us in all the work which we use to do in SQLiteOpenHelper class. We need only a single instance for the whole app.



Here, we have a Room class AppRoomDatabase in which we declare all our entities and version of the database. getDatabase() method will return the room database instance. If we want to access the database lets deep dive into ViewModel and LiveData.

ViewModel (Data-handling business logic)

ViewModel is the new class provided by lifecycle. It can be termed as the bridge between model and UI but quite intelligent one in the sense that it can be automatically retained in the case of orientation change .

Generally what happens during orientation change?
        For instance we are in one activity and the activity loads some list and add it to the RecyclerView. Now when the orientation change has not been handled then new instance of activity will be created and the loading of list takes place and is then again added to RecyclerView.

How does ViewModel solve this problem?
        If you have used ViewModel along with LiveData class to store data then during orientation change new instance of activity will be created but the data won't be downloaded again. ViewModel will provide the most recent available data.

Note: Don’t think that ViewModel will hold data forever or for every case. If you close or activity is destroyed the ViewModel will also be destroyed or cleared.



StudentListViewModel class must extend the ViewModel class. If the ViewModel needs the application context, then it must extend the AndroidViewModel class. In our ViewModel, we first get an instance of our database using AppRoomDatabase.getDatabase(this.getApplication())

We need to load the list of student items from the database. For that, we should use the query we defined in the DAO class, getAllStudItems(). Next, call the abstract method we created for DAO and then call the query method. 

appDatabase.itemAndStudentModel().getAllStudItems();

The ViewModel will contain all the data needed for our Activity. In our example, we are using something called LiveData.

LiveData

LiveData is an observable data holder. It can only be observed in the context of a lifecycle, more precisely in the context of an Activity or Fragment lifecycle. By passing the reference of an Activity or Fragment, it can understand whether your UI onScreen, offScreen or Destroyed. After passing the UI object to LiveData, whenever the data in the live data changes. It notifies the lifecycle owner with updates and then the UI redraw itself with updates.

MutableLiveData extends LiveData internally and also the two methods of LiveData available,
1. setValue() : Set the value and dispatch the value to all the active observers. It cannot be done in background thread it must be done in the main thread only.
2. postValue() : Post a task to main thread to override value set by setvalue. As setvalue cannot be called from background thread so post value must be used to set value from background thread.

How LiveData solved developer`s major Headaches ?

• No more manual life-cycling handle - Observers just observe relevant data and don’t stop or resume observation. LiveData manages all of this under control.
• Proper configuration changes - If an observer is recreated due to a configuration change, like device rotation, it immediately receives the latest available data.
• Ensures your UI matches the data state -Instead of updating the UI every-time when the data changes, your observer can update the UI every time there’s a change. It receives the latest data upon becoming active again.
• No memory leaks - Observers are bound to Lifecycle objects and clean up after themselves when their associated life cycle destroyed.
• No crashes due to stopped activities - It means if an activity is in the back stack, then it doesn’t receive any LiveData stream.
• Sharing resources - You can extend LiveData object using the singleton pattern to wrap system services so that they can be shared in your app.
• We wrap our list of student items inside LiveData so that the Activity can observe changes in the data and update the UI.

Framing RecyclerView Adapter

Now since we will be displaying a list of student items, we need a RecyclerView. So first, let’s create an adapter for the same.

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.RecyclerViewHolder> {
private List<StudentModel> studentModelList;
private View.OnLongClickListener longClickListener;

public RecyclerViewAdapter(List<StudentModel> studentModelList, View.OnLongClickListener longClickListener) {
this.studentModelList = studentModelList;
this.longClickListener = longClickListener;
}

@Override
public RecyclerViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
return new RecyclerViewHolder(LayoutInflater.from(parent.getContext())
.inflate(R.layout.recycler_item, parent, false));
}

@Override
public void onBindViewHolder(final RecyclerViewHolder holder, int position) {
StudentModel borrowModel = studentModelList.get(position);
holder.itemTextView.setText(borrowModel.getItemName());
holder.nameTextView.setText(borrowModel.getPersonName());
holder.dateTextView.setText(borrowModel.getAddedDate().toLocaleString().substring(0, 11));
holder.itemView.setTag(borrowModel);
holder.itemView.setOnLongClickListener(longClickListener);
}

@Override
public int getItemCount() {
return studentModelList.size();
}

public void addItems(List<StudentModel> borrowModelList) {
this.studentModelList = borrowModelList;
notifyDataSetChanged();
}

static class RecyclerViewHolder extends RecyclerView.ViewHolder {
private TextView itemTextView;
private TextView nameTextView;
private TextView dateTextView;
RecyclerViewHolder(View view) {
super(view);
itemTextView = view.findViewById(R.id.itemTextView);
nameTextView = view.findViewById(R.id.nameTextView);
dateTextView = view.findViewById(R.id.dateTextView);
}
}
}

Creating the Android LifecycleObserver & LifecycleOwner

The basic idea here is that there is a class named LifecycleOwner which emits various lifecycle events (such as ON_CREATE, ON_PAUSE, etc) which you as an Android Developer are already familiar with. Now you can implement an interface name LifecycleObserver which listens to these events and respond accordingly.

public class AppLifeCycleObserver implements LifecycleObserver {
private String LOG_TAG = "AppLifeCycleObserver";
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void onResume() {
Log.i(LOG_TAG, "onResume");
}

@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void onPause() {
Log.i(LOG_TAG, "onPause");
}
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
public void onCreate() {
Log.i(LOG_TAG, "onCreate");
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
public void onStart() {
Log.i(LOG_TAG, "onStart");
}

@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
public void onStop() {
Log.i(LOG_TAG, "onStop");
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
public void onDestroy() {
Log.i(LOG_TAG, "onDestroy");
}
}

Adding the Observer

Observers are added to lifecycle owners via calls to the addObserver() method of owner’s Lifecycle object, a reference to which is obtained via a call to the getLifecycle() method. Edit the HomeActivity.java class file and add code to the onCreate() method to add the observer:

Creating a Lifecycle Owner

By creating a custom lifecycle owner class and demonstrate how to trigger events and modify the lifecycle state from within that class. The class is going to need a LifecycleRegistry instance initialized with a reference to itself, and a getLifecycle() method configured to return the LifecycleRegistry instance. Declare a variable to store the LifecycleRegistry reference, a constructor to initialize the LifecycleRegistry instance and add the getLifecycle() method:




Next, the class will need to notify the registry of lifecycle state changes. By triggering lifecycle events using the handleLifecycleEvent() method. For this example, we will add some methods that simply trigger lifecycle events when called. Finally change within the AppLifecycleOwner class is to add the AppLifeCycleObserver class as an observer.

In HomeActivity.java, create an instance of the AppLifecycleOwner class and to call the startLifecycleOwner() and stopLifecycleOwner() methods.



We must remove observer in onDestory() method of calling activity to avoid memory leaks.

@Override
protected void onDestroy() {
super.onDestroy();
getLifecycle().removeObserver(new AppLifeCycleObserver());
}

I have uploaded the latest source code in GitHub for your reference. Kindly raise your queries in the command section.

References :

https://developer.android.com/jetpack/arch/lifecycle
https://medium.com/mindorks/android-architecture-components-room-viewmodel-and-livedata-50611793e4a9
https://blog.iamsuleiman.com/android-architecture-components-tutorial-room-livedata-viewmodel/
https://codinginfinite.com/android-livedata-example/


Happy coding!!!
Cheers!!!

Chapter 0 : Modern Android Architecture ‘Android Jetpack’ - Lifecycle

       Jetpack is “the next generation of components to accelerate app development”. It’s a set of libraries, tools and architectural guidelines to help making building great Android apps quick and easy. It provides common infrastructure code so you can focus on what makes your app unique.



Activity/Fragment - Depend only on a view model.
View model - Provides the data for fragment or activity, and contains data-handling business logic to communicate with the model.
Repository - Depends on a persistent data model and a remote backend data source and handle data operations.
Room - An object-mapping library that provides local data persistence with minimal boilerplate code.
Retrofit library - To access our backend through REST API.

Lifecycle-manageable components such as activities and fragments perform actions in response to a change in another Android component. The lifecycle methods of activities and fragments leads to a poor organization of the code and to the proliferation of errors in most of the cases.
The lifecycle-aware component helps to move the code of dependent components out of the lifecycle methods of activity & fragments and into the components themselves.

Dependencies

AndroidX

dependencies {
def lifecycle_version = "2.0.0"

// Lifecycles only (no ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" // use kapt for Kotlin
// alternately - if using Java8, use the following instead of lifecycle-compiler
implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
}

Pre-AndroidX

dependencies {
def lifecycle_version = "1.1.1"
// Lifecycles only (no ViewModel or LiveData)
implementation "android.arch.lifecycle:runtime:$lifecycle_version"
annotationProcessor "android.arch.lifecycle:compiler:$lifecycle_version" // use kapt for Kotlin
// alternately - if using Java8, use the following instead of compiler
implementation "android.arch.lifecycle:common-java8:$lifecycle_version"
}

Refer here for updated Dependencies.

The android.arch.lifecycle package can automatically adjust their behavior based on the current lifecycle state of an activity or fragment that has three main classes that we’ll deal with:

• LifeCycle
• LifeCycleOwner
• LifeCycleObserver

Lifecycle

Lifecycle class holds the information about the component (like an activity or a fragment) lifecycle state where allows other objects to observe this state.

The lifecycle events (i.e. Lifecycle.Event.ON_CREATE, Lifecycle.Event.ON_RESUME, Lifecycle.Event.ON_PAUSE and etc) & lifecycle states (INITIALIZED, CREATED, RESUMED and etc) are used to track the lifecycle status for its associated component.




A single state can span multiple lifecycle owner events, so for an activity it’s considered in the CREATED state once it’s created and just before it’s paused, and in the same time the duration between the activity’s onStart() and just before onPause() it’s considered also in the STARTED state.

Since multiple states can interleave for a given point of time, if we want to check for a specific state, we always use the isAtLeast method:

if (lifeCycle.currentState.isAtLeast(Lifecycle.State.STARTED)) { //...}

LifecycleOwner and Lifecycle Observers

If we're trying to manage the lifecycle of activity or Fragment, we must implement LifecycleOwner interface to the activity. The lifecycle of a whole application process is managed by ProcessLifecycleOwner. Fragments and Activities in Support Library 26.1.0 and later already implement the LifecycleOwner interface.

If we have a custom class that we would like to make a LifecycleOwner, we can use the LifecycleRegistry class, but we need to forward events into that class.

LifecycleOwners such as Activity & Fragments are objects with lifecycle like Activity and Fragments. LifecycleObserver (e.g. LiveData) observes LifecycleOwners and are notified of lifecycle changes.

class SampleObserver implements LifecycleObserver{

@OnLifecycleEvent(ON_CREATE)
void startUp(LifecycleOwner source) {
}

@OnLifecycleEvent(ON_ANY)
void onAny(LifecycleOwner source, Event event) {
}

@OnLifecycleEvent(ON_STOP)
void cleanUp(LifecycleOwner source) {
}
}

The best way to handle lifecycle-aware components (activities and fragments) as lean as possible. They should not try to acquire their own data; instead, use a ViewModel to do that, and observe a LiveData object to reflect the changes back to the views.
So, Android Lifecycle-aware components are tracked and managed by Room, LiveData and ViewModel in the next episode.

References:

https://developer.android.com/topic/libraries/architecture/lifecycle
https://medium.com/@MinaSamy/android-architecture-components-lifecycle-433ace1ec05d



Happy Coding !!!