Kotlin Android Extensions

If you’ve been developing Android Apps for some time, you’re probably already tired of working with findViewById in your day-to-day life to recover views. Or maybe you gave up and started using the famous Butterknife library. If that’s your case, then you’ll love Kotlin Android Extensions. Kotlin has a built-in view injection for Android, allowing to skip manual binding or need for frameworks such as ButterKnife. Some of the advantages are a nicer syntax, better static typing, and thus being less error-prone. In your project-local (not top-level) build.gradle append extensions plugin declaration below your Kotlin plugin, on top-level indentation level.

buildscript {
...
}
id : "com.android.application"
...
id: "kotlin-android"
id "kotlin-android-extensions"
...

Using Views

Assuming we have an activity with an example layout called activity_main.xml:

XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:id="@+id/my_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="My button"/>
</LinearLayout>

We can use Kotlin extensions to call the button without any additional binding like so:

Kotlin

import kotlinx.android.synthetic.main.activity_main.my_button

class MainActivity: Activity() {
  override fun onCreate(savedInstanceBundle: Bundle?) {
    super.onCreate(savedInstanceBundle)
    setContentView(R.layout.activity_main)
    // my_button is already casted
    // to a proper type of "Button"
    // instead of being a "View"
    my_button.setText("Kotlin rocks!")
   }
}

You can also import all ids appearing in layout with a * notation

// my_button can be used the same way as before

import kotlinx.android.synthetic.main.activity_main.*

Synthetic views can’t be used outside of Activities/Fragments/Views with that layout inflated:

Kotlin

import kotlinx.android.synthetic.main.activity_main.my_button

class NotAView {
  init {
    // This sample won't compile!
    my_button.setText("Kotlin rocks!")
  }
}

Product flavors

Android extensions also work with multiple Android Product Flavors. For example, if we have flavors in the build.gradle like so:

android {
  productFlavors {
    paid {
    ...
    }
    free {
    ...
    }
  }
}

And for example, only the free flavor has a buy button:

XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
  <Button
    android:id="@+id/buy_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Buy full version"/>
</LinearLayout>

We can bind to the flavor specifically:

import kotlinx.android.synthetic.free.main_activity.buy_button

Painful listener for getting noticed, when the view is completely drawn now is so simple and awesome with Kotlin’s extension

mView.afterMeasured {
// inside this block the view is completely drawn
// you can get view's height/width, it.height / it.width
}

Under the Hood

Kotlin

inline fun View.afterMeasured(crossinline f: View.() -> Unit) {
  viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
    override fun onGlobalLayout() {
     if (measuredHeight > 0 && measuredWidth > 0) {
       viewTreeObserver.removeOnGlobalLayoutListener(this)
       f()
     }
   }
 })
}