Migrating the deprecated Kotlin Android Extensions compiler plugin

credits: https://www.techyourchance.com/asynctask-deprecated/

In Kotlin 1.4.20-M2 JetBrains deprecated Kotlin Android Extensions compiler plugin.

Actually this was expected long time ago, in this commit you can see

kotlinx.android.synthetic is no longer a recommended practice. Removing in favour of explicit findViewById

But why?

There are some well-known issues with kotlinx synthetic

• They expose a global namespace of ids that’s unrelated to the layout that’s actually inflated with no checks against invalid lookups.
• They are Kotlin only
• They don’t expose nullability when views are only present in some configuration.
• All together, these issues cause the API to increase number of crashes for Android apps.
• Also Google is promoting modularisation but synthetic properties don’t work cross-module there is an open issue since January 2018.

What are the alternatives?

• View Binding is the recommendation for view lookup as well as binding, but it does add a bit of overhead when compared to Android Kotlin Extensions.
  Compared to Kotlin Extensions, it adds compile time checking of view lookups and type safety.
• findViewById the old traditional way that works for both Kotlin and Java.

JetBrains deprecated the Kotlin Android Extensions in favour of View Binding, so we will be exploring how to migrate to View Binding in this article.

View Binding

Not to be confused with Data Binding

View binding is a feature that allows you to more easily write code that interacts with views.

Once view binding is enabled in a module, it generates a binding class for each XML layout file present in that module.

An instance of a binding class contains direct references to all views that have an ID in the corresponding layout.

View Binding is Null-safe for layouts defined in multiple configurations.

View binding will detect if a view is only present in some configurations and create a @Nullable property.

View Binding works with Java and Kotlin.

How to enable View Binding?

You don’t need to include any extra libraries to enable view binding. It’s built into the Android Gradle Plugin starting with the versions shipped in Android Studio 3.6. To enable the feature in the modules that will use it add the following into your build.gradle file.

How to use View Binding?

If view binding is enabled for a module, a binding class is generated for each XML layout file that the module contains.

Each binding class contains references to the root view and all views that have an ID.

The name of the binding class is generated by converting the name of the XML file to Pascal case and adding the word Binding to the end.

Use view binding in activities

Then you can access the views using the binding object

binding.name.text = "Some Text"

Use view binding in fragments

Using view binding in fragments requires a bit more attention, as view binding doesn’t play well with fragments, as it introduces memory leaks, if you don’t nullify the view in OnDestroy, then it’s not cleared from memory.

Then you can access the views using the binding object the same way we do in activities.

binding.name.text = "Some Text"

Under the hood

View Binding will generate one binding object for every XML layout in your module.

For example for this activity_main.xml layout file

View Binding will generate ActivityMainBinding.java

View binding will generate a correctly-typed property for each view that has a specified id. It will also generate a property called rootView.

View Binding is Kotlin friendly as all properties are annotated with @Nullable or @NonNull Kotlin knows how to expose them as null-safe types

In ActivityMainBinding.java, view binding generates a public inflate method.

It calls bind where it will take the inflated layout and bind the properties, with some error checking.

The bind method is where generated binding object will call findViewById for each view to bind.

What about the Parcelize feature from Kotlin Android Extensions?

Don’t forget that Parcelize feature in Kotlin is part of the kotlin-android-extensions compiler plugin, so removing the plugin will endup making all your Parcelable classes not compiling if they depend on the Parcelize annotation.

JetBrains extracted the Parcelize from Kotlin Android Extensions to a new plugin, kotlin-parcelize

First you will need to add kotlin-parcelize plugin to your module.

Then change your old import statement from

import kotlinx.android.parcel.Parcelize

to

import kotlinx.parcelize.Parcelize

Example

You could find this IDE error

Class 'User' is not abstract and does not implement abstract member public abstract fun describeContents(): Int defined in android.os.Parcelable

Don’t worry your app will build fine, this is a false positive error and was reported on youtrack and marked as fixed so should be shipped in the next release hopefully.

Please note this plugin is only available starting from version 1.4.20-M2 which is the same version that deprecated kotlin-android-extensions compiler plugin.

If you want to give it a try you need to install the Kotlin EAP Plugin for IntelliJ IDEA or Android Studio

• Select Tools → Kotlin → Configure Kotlin Plugin Updates.
• In the Update channel list, select the Early Access Preview 1.4.x channel
• Click Check again.
• Then click Install

Tl;dr

Here is what you should do to migrate from kotlin-android-extensions Kotlin plugin to ViewBinding and kotlin-parcelize plugin

• Remove kotlin-android-extensions plugin from your build.gradle files.
• Remove all kotlin synthetic import statements from your activities and fragments.
• Enable viewBinding feature in your modules build.gradle files.
• Add binding object in your activities and fragments and use it to set the content view and access views from your xml files.
• If you are using Parcelize annotation, then add the new kotlin-parcelize plugin into your modules build.gradle files and change the import statements as mentioned above.

References

• YouTrack | Deprecate Kotlin Android Extensions compiler plugin
• YouTrack | Move the Parcelize functionality out of the Android Extensions plugin
• JetBrains/kotlin |Parcelize: Add integration test for the new kotlin-parcelize plugin
• Developer Advocate for Android at Google comment on a reddit thread
• Android Developers | Use view binding to replace findViewById

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