Learn Android with Java and Kotlin
 

Kotlin-android-extensions: accessing widgets from a Kotlin activity

We no more need findViewById() again , as Kotlin Android Extensions is here . Probably you have used the Butterknife library to avoid writing findViewById() again and again . But with Kotlin Android Extensions plugin its even easier . The process can be summerized in three steps as below :

1. Apply the ‘kotlin-android-extensions’ plugin in app level build.gradle file
apply plugin: ‘kotlin-android-extensions’

2. Add an import statement to make the widgets of the layout file accessible from Kotlin code. The import statement should look like the one below (here activity_main is the layout of our activity)
import kotlinx.android.synthetic.main.activity_main.*

3. All done , the widgets of the layout can now be accessed by their ids .
If there is a TextView in our layout whose text we want to change and tv_hello is its id , then we can use tv_hello to access all members of the textview .
e.g. tv_hello.text = “We don’t need findViewById in Kotlin”

Now lets have a look at the code , the layout contains a button( btn_change_text ) and a TextView (tv_hello). The button will be used to change the text of the TextView .



    

    

And in the activity lets add the import statement .
import kotlinx.android.synthetic.main.activity_main.*

Now our activity have some extention properties corresponding to the views of our layout i.e tv_hello of type TextView and btn_change_text of type Button. Lets add an onClickListener to the btn_change_text to change the text of tv_hello .

package com.droidmonk.beginkotlin

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

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


        btn_change_text.setOnClickListener {
            tv_hello.text = "Yay! I am a Kotlin expert"
        }
    }
}

And to all the above code work we must enable the kotlin-android-extensions plugin in our app level build.gradle as discussed in step 1. Thus our build file look like the one below :


apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
android {
    compileSdkVersion 26
    buildToolsVersion "26.0.1"
    defaultConfig {
        applicationId "com.droidmonk.beginkotlin"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
    compile 'com.android.support:appcompat-v7:26.0.0-alpha1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'
}
repositories {
    mavenCentral()
}

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