View Binding vs Kotlin Synthetics: The Right Choice for Android XML & Kotlin Development

When developing Android applications with Kotlin and XML, accessing views in your layout files is a common task. Traditionally, developers have used findViewById() to locate views. However, more modern approaches such as View Binding and Kotlin Synthetics (now deprecated) provide cleaner and more efficient ways to handle this. This post will compare View Binding and Kotlin Synthetics, outline why Kotlin Synthetics are deprecated, and recommend the best approach for current Android development practices.

Introduction to View Binding

View Binding is a feature that generates a binding class for each XML layout file present in a module. An instance of a binding class contains direct references to all views that have an ID in the corresponding layout. View Binding replaces the need to manually call findViewById() and offers compile-time safety.

Introduction to Kotlin Synthetics (Deprecated)

Kotlin Synthetics was a Kotlin compiler plugin that allowed developers to access views in XML layouts directly in their Kotlin code, without using findViewById(). While convenient, it had significant drawbacks that led to its deprecation.

Why Kotlin Synthetics Were Deprecated

Kotlin Synthetics have been deprecated in favor of View Binding due to several reasons:

• No Null Safety: Kotlin Synthetics provided no null safety. If a view with a specific ID did not exist in the layout, it would result in a runtime error.
• No Compile-Time Safety: Issues were only detected at runtime, making it harder to catch errors during development.
• Scope Issues: Access to views wasn’t always guaranteed, leading to potential runtime exceptions, especially when dealing with included layouts or dynamic content.
• Maintenance Overhead: The plugin had maintenance overhead, and supporting it was becoming cumbersome.

View Binding: Detailed Implementation

View Binding generates binding classes at compile time, offering type safety and null safety. It integrates directly into your build process and is straightforward to use.

Step 1: Enable View Binding

To enable View Binding in your project, add the following to your module-level build.gradle file:

android {
    buildFeatures {
        viewBinding true
    }
}

Step 2: Create an XML Layout File

Create a layout file with views that have IDs. For example, activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello, View Binding!"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click Me"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Step 3: Use View Binding in Your Activity

In your Activity or Fragment, use the generated binding class to access the views:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.example.viewbindingexample.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.textView.text = "View Binding Example"
        binding.button.setOnClickListener {
            // Handle button click
        }
    }
}

Explanation:

• Enable View Binding: The viewBinding true line in the build.gradle file turns on the View Binding feature for the module.
• Binding Class Generation: The binding class ActivityMainBinding is automatically generated from the layout file activity_main.xml. The class name is derived from the layout file name by converting it to Pascal case (e.g., activity_main.xml becomes ActivityMainBinding).
• Inflate the Binding: The ActivityMainBinding.inflate(layoutInflater) inflates the layout using the generated binding.
• Access Views: You can access the views directly using binding.textView and binding.button, eliminating the need for findViewById().

Comparison: View Binding vs. Kotlin Synthetics

Feature	View Binding	Kotlin Synthetics (Deprecated)
Null Safety	Provides null safety. Views are guaranteed to exist if they have IDs in the layout.	No null safety. NullPointerException can occur if a view does not exist.
Compile-Time Safety	Provides compile-time safety. Errors are caught during compilation.	No compile-time safety. Errors are caught at runtime.
Integration	Requires enabling in build.gradle.	Required a plugin which is now deprecated.
Performance	Slightly faster due to direct references.	Performance could be slightly slower due to reflection overhead.
Complexity	Minimal boilerplate, very easy to use.	Seemed easier initially but came with hidden risks and complexity.
Maintenance	Easier to maintain, as errors are caught early.	Higher maintenance due to potential runtime errors and lack of compile-time checks.

Why Choose View Binding?

• Safety: View Binding offers both null and compile-time safety, reducing the risk of runtime exceptions.
• Simplicity: It is straightforward to set up and use, requiring minimal changes to your existing code.
• Performance: Direct references to views provide a performance boost over reflection-based solutions.
• Maintenance: Compile-time checks make the codebase easier to maintain and less prone to errors.
• Future-Proof: Since Kotlin Synthetics is deprecated, View Binding is the recommended and future-proof solution.

Code Examples Comparing Both

Using View Binding:

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.example.viewbindingexample.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.textView.text = "Hello ViewBinding!"
        binding.button.setOnClickListener {
            // Handle button click
        }
    }
}

Using Kotlin Synthetics (Deprecated):

// Note: Kotlin Synthetics is deprecated and should not be used.
// This example is for historical context only.

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.* // Deprecated

class MainActivity : AppCompatActivity() {

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

        textView.text = "Hello Kotlin Synthetics!" // Deprecated
        button.setOnClickListener {
            // Handle button click
        }
    }
}

Note: The Kotlin Synthetics example is included for comparison only. You should not use Kotlin Synthetics in new projects due to its deprecation and inherent risks.

Migrating from Kotlin Synthetics to View Binding

If you are migrating from Kotlin Synthetics to View Binding, here are the steps you should follow:

1. Enable View Binding: Add viewBinding true to your module-level build.gradle.
2. Replace Imports: Remove kotlinx.android.synthetic imports.
3. Update Code: Replace direct view access with binding.viewId.
4. Clean and Rebuild: Clean and rebuild your project to ensure the new binding classes are generated.

// Before (Kotlin Synthetics)
-import kotlinx.android.synthetic.main.activity_main.*
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        textView.text = "Hello Kotlin Synthetics!"
    }
}

// After (View Binding)
+import com.example.viewbindingexample.databinding.ActivityMainBinding
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {
+   private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
+       binding = ActivityMainBinding.inflate(layoutInflater)
+       setContentView(binding.root)
+       binding.textView.text = "Hello View Binding!"
    }
}

Conclusion

View Binding is a safer, more efficient, and recommended approach for accessing views in Kotlin XML development for Android. With its compile-time and null safety features, it reduces the risk of runtime errors and makes code more maintainable. Kotlin Synthetics, while offering convenience, lacks these critical safety features and has been deprecated. By migrating to View Binding, developers can ensure a more robust and reliable Android development experience.