Room with LiveData, ViewModel - Android Architecture Components - Kotlin

-

Introduction:


We have already learnt about Room in last post, Room-Kotlin. In this post, lets learn about how to use ViewModel and LiveData with Room to improve usablity. Using LiveData with Room, allows views to be notified about data changes automatically.

LiveData:

LiveData is nothing but observable data holder class. It allows us to observer changes in data across multiple components of app. It is also aware of and respects lifecycle of Activities/Fragments. It also ensures that LiveData only updates app component observers that are in an active lifecycle state.

The following image depicts the Lifecycle and States:

Lifecycle and its states
Here, CREATED, STARTED, RESUMED - Active States
          INTIALIZED, DESTROYED - InActive States

So, LiveData will remove the updates automatically while in InActive States, i.e., INTIALIZED and DESTROYED.

ViewModel:

ViewModel is an entity that is free of Activity/Fragment's lifecycle. So it allows us to store and manage UI-related data in lifecycle conscious way. For example, it can retain its state/data even when the configuration changed. It doesn't contain any code related to UI. 

Implementing ViewModel:

Every ViewModel class should extends ViewModel class. If it needs to use the ApplicationContext, then it should extends AndroidViewModel.

class MyViewModel : ViewModel() {
    var users: MutableLiveData<List<User>>? =null

    fun getUsers(): LiveData<List<User>> {
        if (users == null) {
            users = MutableLiveData<List<User>>()
            loadUsers()
        }
        return users
    }

    private fun loadUsers() {
        // Do an asyncronous operation to fetch users.
    }
}

The created ViewModel can be accessed in Activity like below:


class MyActivity : AppCompatActivity() {
    public override fun onCreate(savedInstanceState: Bundle?) {
        // Create a ViewModel the first time the system calls an activity's onCreate() method.
        // Re-created activities receive the same MyViewModel instance created by the first activity.

        val model = ViewModelProviders.of(this).get(MyViewModel::class.java)
        model.getUsers().observe(this, { users ->
            // update UI
        })
    }
}

Code:

build.gradle:

Add the following plugin in app module's build.gradle.
         apply plugin: 'kotlin-kapt'
Also, add the following in dependencies in the same:
         compile 'com.android.support:recyclerview-v7:27.0.2'
         compile 'com.android.support:cardview-v7:27.0.2'
         compile "android.arch.persistence.room:runtime:1.0.0"
         kapt "android.arch.persistence.room:compiler:1.0.0"
         compile "android.arch.lifecycle:extensions:1.0.0"
         kapt "android.arch.lifecycle:compiler:1.0.0"

Book.kt (Entity - Class)


@Entity
data class Book(var bookName : String, var author : String,
                var genre : String){
    @PrimaryKey(autoGenerate = true)
    var id : Long? = null
}

BookDao.kt (Dao - Interface)


@Dao
interface BookDao {
    @Query("SELECT * FROM Book")
    fun getBookInfo() : LiveData<MutableList<Book>>

    @Insert
    fun addBook(book : Book)

    @Update(onConflict = REPLACE)
    fun updateBook( book: Book)

    @Delete
    fun deleteBook( book: Book?)
}

MyDatabase.kt (Database - Class)

Every Database operation calls should be in Background thread. Otherwise, the app will crash. 

@Database(entities = arrayOf(Book::class), version = 1)
abstract class MyDatabase : RoomDatabase() {
    abstract fun bookDao(): BookDao

    companion object {
        var INSTANCE: MyDatabase? = null

        fun getInstance(context: Context): MyDatabase {

            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context.applicationContext, MyDatabase::class.java, "Books.db").build()
            }
            return INSTANCE as MyDatabase;
        }

        @SuppressLint("StaticFieldLeak")
        fun insertData(mydata: MyDatabase, book: Book) {
            object : AsyncTask<Void, Void, Void>() {
                override fun doInBackground(vararg voids: Void): Void? {
                    mydata.bookDao().addBook(book)
                    return null
                }
            }.execute()
        }

        @SuppressLint("StaticFieldLeak")
        fun getData(mydata: MyDatabase): LiveData<MutableList<Book>> {
            lateinit var lists: LiveData<MutableList<Book>>

            return object : AsyncTask<Void, Void, LiveData<MutableList<Book>>>() {
                override fun doInBackground(vararg voids: Void): LiveData<MutableList<Book>>? {
                    lists = mydata.bookDao().getBookInfo()
                    return lists
                }
            }.execute().get()
        }

        @SuppressLint("StaticFieldLeak")
        fun deleteData(mydata: MyDatabase, book: Book?) {
            object : AsyncTask<Void, Void, Void>() {
                override fun doInBackground(vararg voids: Void): Void? {
                    mydata.bookDao().deleteBook(book)
                    return null
                }
            }.execute()
        }
    }
}

BookViewModel.kt (ViewModel - Class)

Here, we are using AndroidViewModel. It will handle the data fetching from Database and it have to be get and return as LiveData for observing.

class BookViewModel(application: Application) : AndroidViewModel(application) {
    var list: LiveData<MutableList<Book>>

    init {
       list =  MyDatabase.getData(MyDatabase.getInstance(this.getApplication()))
    }

    fun fetchAllData() : LiveData<MutableList<Book>> = list
}

RecyclerAdapter.kt:

Adapter class for RecyclerView to display the list of data fetched from Database.

class RecyclerAdapter(val context: Context, var data : MutableList<Book>?) : RecyclerView.Adapter<RecyclerAdapter.Holder>() {

    override fun onBindViewHolder(holder: Holder?, position: Int) {
        holder?.bindItems(data?.get(position))
    }

    override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): Holder {
        val v = LayoutInflater.from(context).inflate(R.layout.recycler_item, parent, false)
        return Holder(v)
    }

    override fun getItemCount(): Int = data?.size?:0

    class Holder(itemView: View?) : RecyclerView.ViewHolder(itemView){

        fun bindItems(book: Book?){
            itemView.name.text = book?.bookName
            itemView.author.text = book?.author
            itemView.genre.text = book?.genre
            itemView.setOnLongClickListener(object :View.OnLongClickListener{
                override fun onLongClick(v: View?): Boolean {
                    MyDatabase.deleteData(MyDatabase.getInstance(itemView.context), book)
                    return true
                }
            })
        }
    }

    fun addItems(t: MutableList<Book>?) {
        data = t
        notifyDataSetChanged()
    }
}

DbActivity.kt 

ViewModel class should be registered in this Activity. Data should be fetched and observed using LiveData. If the changes detected in Database, it will invoke onChanged(), there the adapter can be notified of those changes.

class DbActivity : AppCompatActivity() {

    lateinit var adapter: RecyclerAdapter
    lateinit var viewModel: BookViewModel
    var list: MutableList<Book>? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.db_layout)

        recyclerView.layoutManager = LinearLayoutManager(this)
        adapter = RecyclerAdapter(this, list)
        recyclerView.adapter = adapter

        viewModel = ViewModelProviders.of(this).get(BookViewModel::class.java)
        viewModel.fetchAllData().observe(this, object : Observer<MutableList<Book>> {
            override fun onChanged(t: MutableList<Book>?) {
                Log.v("OnChanged","OnChanged!!")
                adapter.addItems(t)
            }
        })
        
        add.setOnClickListener {
            openDialog()
        }

    }

    private fun openDialog() {
        val dialog = Dialog(this)
        dialog.requestWindowFeature(Window.FEATURE_NO_TITLE)
        dialog.setContentView(R.layout.dialog)
        val lp: WindowManager.LayoutParams = WindowManager.LayoutParams().apply {
            copyFrom(dialog.window.attributes)
            width = WindowManager.LayoutParams.MATCH_PARENT
            height = WindowManager.LayoutParams.WRAP_CONTENT
        }

        val submit = dialog.findViewById<View>(R.id.submit) as TextView
        val name = dialog.findViewById<View>(R.id.name) as EditText
        val author = dialog.findViewById<View>(R.id.author) as EditText
        val genre = dialog.findViewById<View>(R.id.genre) as EditText

        submit.setOnClickListener {
            when {
                name.length() == 0 || author.length() == 0 || genre.length() == 0 ->
                    Toast.makeText(this@DbActivity, "Please fill all the fields"
                            , Toast.LENGTH_SHORT).show()

                else -> {
                    val book = Book(name.text.toString(), author.text.toString(), genre.text.toString())
                    MyDatabase.insertData(MyDatabase.getInstance(this), book)
                    dialog.dismiss()
                }
            }
        }
        dialog.show()
        dialog.getWindow().setAttributes(lp)
    }
}

Run Application:


Comments

Post a Comment

Popular posts from this blog

SOAP Client using ksoap2 in Android - Kotlin

-
Image
Introduction: SOAP stands for Simple Object Access Protocol. It is XML-based protocol that allows programs that run on different operating systems (like Windows, Linux, etc) to communicate using HTTP and XML. Pros: Security - SOAP provides its own security known as WS Security . Platform Independent - SOAP web services can be written in any programming language and executed in any platform Cons: Slow - It uses XML format that must be parsed, to be read. It defines many standards to be followed while developing. So its slow and consumes more bandwidth and resource. WSDL dependent - It uses WSDL mechanism and doesn't have any other mechanism to discover the service. Things to know: The following are the things we have to know before we proceed further:                     URL - It is the URL of WSDL file.                     NAMESPACE - It is targetNamespace in WSDL.                                         METHOD_NAME - It is the met
Read more

Using Camera in Android - Kotlin

-
Image
Nowadays, most of our Android app require access to Camera to capture picture or Video. We can do that in two ways.                                 (i) Using built-in Camera,                                 (ii) Using custom Camera. Using Built-in Camera In this post, Let's see how to use the built-in camera to capture images and save it. Its a simple way to capture pics. We can use the mobile's built-in camera app using Intent.  val REQUEST_IMAGE_CAPTURE = 1 private fun takePictureIntent() { val takePictureIntent = Intent(MediaStore . ACTION_IMAGE_CAPTURE) if (takePictureIntent . resolveActivity(packageManager) != null) { startActivityForResult(takePictureIntent, REQUEST_IMAGE_CAPTURE) } } After the picture has been captured, the result will received on onActivityResult method. There, we can get the data, in Bitmap form. override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
Read more

Stripe Integration in Android - Kotlin

-
Image
Stripe is one of the simplest payment gateway to be used. It enables us to process payments in minutes. Let's see how to integrate Stripe in Android using Kotlin. Operational Flow:             1. App should provide order details to Stripe server and the order total will be returned to App.             2. Then the user will enter the payment/card details. That will be sent to Stripe server and the result of the request will be sent back to the App. Create Stripe Account: (a) Go to  https://dashboard.stripe.com/register and enter all your details and create account.   (b) Click API in side tab. Make sure that the Account is in Test mode. And make a note of          Publishable Key and Secret Key for use. Code: build.gradle:   compile 'com.stripe:stripe-android:5.1.0' Layout files: main_layout.xml: <?xml version="1.0" encoding="utf-8"?> <android.support.constraint.ConstraintLayout xmlns:an
Read more

Shimmer Layout like Facebook in Android - Kotlin

-
Image
While loading data from API, we used to show boring loaders. But in Facebook's app, one loading screen will be displayed. That one looks interesting know. We can also do that shimmer effect in our Andorid App using Shimmer library by Facebook. Let's start. Implementation: build.gradle Include the following in app-level gradle file. implementation 'com.facebook.shimmer:shimmer:0.1.0' colors.xml Include the following color in values -> colors.xml <color name= "placeholder_color" > #dddddd </color> Design: 1. First we have to design placeholder layout that is similar to your RecyclerView's adapter layout. placeholder.xml <?xml version="1.0" encoding="utf-8"?> <android.support.constraint.ConstraintLayout xmlns:android= "http://schemas.android.com/apk/res/android" xmlns:app= "http://schemas.android.com/apk/res-auto" android:layout_width
Read more

Exploring Databinding Library in Android - Kotlin

-
Image
Introduction: Data binding library is a support library, which allows us to bind data sources directly to UI components of layout file. Binding components this way, reduces boilerplate code in our business logic where we usually update UI when new data is available. It also improves app's performance and helps to prevent memory leak and NullPointerException. textView.text = user.name can be written simply in layout as follows, and no need to write in Activity class.                                                             <TextView                                                                   ...                                                                   android:text="@{user.name}" /> You can find the project in Github . Prerequisite: build.gradle First, we have enable dataBinding in app-level build.gradle                            android {                                     dataBinding {             
Read more

Exploring Android Slices - JetPack

-
Image
Introduction: Slices are a new way of embedding our app's content in other surfaces, like Google Search and later in other places like Google Assistant. Slices are UI templates, which gives look and feel at home in whatever surface, they get hosts in. Also they are rich, interactive and dynamically updatable. Before jumping into that, first migrate your project to AndroidX, (Android Studio 3.2 & above), by navigating as,                                         Refactor --> Migrate  to AndroidX  Setup: build.gradle implementation 'androidx.slice:slice-core:1.0.0' implementation 'androidx.slice:slice-builders-ktx:1.0.0-alpha6' implementation 'androidx.slice:slice-builders:1.0.0' Creating Slice Provider: Slice Provider is a component that let us display the slices we are creating, on outside our application. We can create the Slice Provider by navigating,                                       File -> New ->
Read more

AppIntro (OnBoard) Screen - Kotlin

-
Image
Introduction: App Intro/Onboard screen is mainly to display the key features of our app to users while launching it, in a sweet way. Let's do that simply with TabLayout, ViewPager and its PageTransformer.  Design: slide_page.xml  Desgin this layout with ViewPager and TabLayout to begin with. <?xml version= "1.0" encoding= "utf-8"? > <RelativeLayout xmlns:android= "http://schemas.android.com/apk/res/android" xmlns:app= "http://schemas.android.com/apk/res-auto" android: id = "@+id/introLayout" android:layout_width= "match_parent" android:layout_height= "match_parent" > <android.support.v4.view.ViewPager android: id = "@+id/viewPager" android:layout_width= "match_parent" android:layout_height= "match_parent" android:layout_above= "@+id/tabLayout" /> <android.support.
Read more

Exploring Android Navigation Architecture Component - MVVM - Kotlin

-
Image
Navigation Architecture Component is released as a part of JetPack and androidx package. Its aim is to simplify the implementation of Navigation in our Android App. It defines a set of principles to build an in-app navigation with consistent and predictable user experience. Its main concept is to use single-activity architecture. It also has support for both Activity and Fragment. Principle of Navigation: The following are the principles of Navigation. App should have fixed starting destination A stack is used to represent the navigation state of app The 'Up' button never exits the app Up and Back are equivalent within your app's task Deep linking to a destination or navigating to the same destination should yield the same stack You can find the detailed explanation, here . Components of Navigation: (i) Navigation Graph - It's nothing but a map/blueprint of our in-app navigation. It has a list of all fragments and activities. We can also
Read more

RecyclerView with different number of columns using SpanSizeLookup

-
Image
We already know how to design RecyclerView. But how to create RecyclerView with different columns depending on its row? Like below image? Yeah, we can do that, by using two different layouts with different viewType on Adapter. Another way is do some logic to do so. But, it's cumbersome. Luckily, we have another simple way to do that using SpanSizeLookUp . Let's see how to implement that in this post. Implementation: build.gradle Include the following in app-level gradle file. implementation 'com.android.support:recyclerview-v7:27.0.2' implementation 'com.android.support.constraint:constraint-layout:1.1.0-beta5' recycler.xml <?xml version= "1.0" encoding= "utf-8" ?> <android.support.constraint.ConstraintLayout xmlns:android= "http://schemas.android.com/apk/res/android" android:layout_width= "match_parent" android:layout_height= "match_parent" xmlns:ap
Read more