Kotlin Android Extensions

30 May, 2022
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"
...
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!")  }}
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()     }   } })}
ubedk
Kotlin Android
Android
Kotlin
Android
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.