Android’s Data Binding with Kotlin

Encora | September 26, 2017

I assume you are (or are trying to be) an Android developer. I ask then, have you tried using Data Binding?

It’s a library which allows you to flexibly bind the data of your models directly to xml views.

My first experience with this was a year ago trying to set up more than 10 different fonts for the Android App I was developing at that moment. For me, it was the discovery of a powerful tool. By then, another powerful tool I was flirting with during that period of time was the programming language called Kotlin.

Now, in this post I want to show you the approach I used to mix the power of Data Binding with Kotlin by using a simple example.

First, after having a created Android project in Android Studio, we need to add the Data Binding and Kotlin dependencies to the build.gradle file of our app. 

 


//build.gradle 
apply plugin: 'kotlin-android' 
apply plugin: 'kotlin-kapt' 
android { 
	.... 
    dataBinding { 
    	enabled = true 
    } 
 } 
 dependencies { 
 	... 
    // notice that the compiler version must be the same as our gradle version 
    kapt 'com.android.databinding:compiler:2.3.1' 
}

That’s all the configuration we need to start using Data Binding with Kotlin. Thank you very much for reading this … Now let’s continue the fun by viewing the code.

First we need to create a model. In this case, we’ll create a basic one like User.kt


data class User(val name: String, val age: Int)

 

In our activity_main.xml we can do something like this:

 
< ?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" >
<!-- Inside the layout tag it is possible to set the data tag in order to set one or many variables. For this example we are having an User property-->
  <data> 
  	<variable name="user" type="com.kuma.sample.User" /> 
  </data>
  <LinearLayout android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" tools:context="com.kuma.sample.MainActivity" > 
    <TextView android:id="@+id/user_name_text_view" android:layout_width="match_parent" android:layout_height="wrap_content" android:padding="16dp" android:text="@{user.name}" tools:text="Name" />
    <TextView android:id="@+id/user_age_text_view" android:layout_width="match_parent" android:layout_height="wrap_content" android:padding="16dp" android:text="@{Integer.toString(user.age)}" tools:text="XX" />
  </LinearLayout>
</layout> 

 Remember to always set your usual xml view inside the <layout> tag and attach all the “xmlns:” properties to it. Otherwise, it will display a compilation error, since the generated files will have duplicated properties.

And here comes the Binding:

package com.kuma.sample


import android.databinding.DataBindingUtil
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import com.kuma.kotlinsteps.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         val binding: ActivityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
         val user = User("Kuma", 23)
         binding.setVariable(BR.user, user)
         binding.executePendingBindings()
   }
}

In that code snippet there are some things to note:

 

  • There now exists a class called ActivityMainBinding, which is autogenerated from the activity_main.xml and this contains all the references to use the views that the xml contains.
  • The way of making an instance of ActivityMainBinding is a little different than how we set the xml layout to an activity.
  • There is also a new BR class which is some kind of secondary R class used to store the variables declared on the data tag of the xml.
  • After setting the variable to the binding object, it is necessary to call the executePendingBindings() in order to set the user variable attributes to the marked views.
  • After compiling this you’ll be able to see that the Data has been set to your view without having to write any textView.text = user.name
  • This was my simple approach to use DataBinding inside a Kotlin project. I hope this is of use to you.

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