I try to make an application to practice Layout in Android, but I get an error:

Unresolved reference: box_three_text

See screenshot: [1]: https://i.stack.imgur.com/W3uGt.jpg

Why does the instructor code work and mine doesn't?

Here's my code:

MainActivity.kt

import android.graphics.Color
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.View

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setListeners()
    }
    private  fun setListeners(){
        val clickableViews: List<View> =
            listOf(box_one_text, box_two_text, box_three_text, box_four_text, box_five_text)
        for (item in clickableViews){
            item.setOnClickListener{makeColored(it)}
        }
    }
    fun makeColored(view: View) {
        when (view.id) {

            // Boxes using Color class colors for background
            R.id.box_one_text -> view.setBackgroundColor(Color.DKGRAY)
            R.id.box_two_text -> view.setBackgroundColor(Color.GRAY)

            // Boxes using Android color resources for background
            R.id.box_three_text -> view.setBackgroundResource(android.R.color.holo_green_light)
            R.id.box_four_text -> view.setBackgroundResource(android.R.color.holo_green_dark)
            R.id.box_five_text -> view.setBackgroundResource(android.R.color.holo_green_light)

            else -> view.setBackgroundColor(Color.LTGRAY)
        }
    }
}

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:id="@ id/constraint_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@ id/box_one_text"
        style="@style/WhiteBox"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="@dimen/margin_wide"
        android:layout_marginTop="@dimen/margin_wide"
        android:layout_marginEnd="@dimen/margin_wide"
        android:fontFamily="@font/roboto"
        android:text="@string/box_one"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@ id/box_two_text"
        style="@style/WhiteBox"
        android:layout_width="130dp"
        android:layout_height="130dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:text="@string/box_two"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@ id/box_one_text" />

    <TextView
        android:id="@ id/box_three_text"
        style="@style/WhiteBox"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="16dp"
        android:text="@string/box_three"
        app:layout_constraintBottom_toTopOf="@ id/box_four_text"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toEndOf="@ id/box_two_text"
        app:layout_constraintTop_toTopOf="@ id/box_two_text" />

    <TextView
        android:id="@ id/box_four_text"
        style="@style/WhiteBox"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginEnd="16dp"
        android:text="@string/box_four"
        app:layout_constraintBottom_toTopOf="@ id/box_five_text"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@ id/box_two_text"
        app:layout_constraintTop_toBottomOf="@ id/box_three_text" />

    <TextView
        android:id="@ id/box_five_text"
        style="@style/WhiteBox"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginEnd="16dp"
        android:text="@string/box_five"
        app:layout_constraintBottom_toBottomOf="@ id/box_two_text"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@ id/box_two_text"
        app:layout_constraintTop_toBottomOf="@ id/box_four_text" />

</androidx.constraintlayout.widget.ConstraintLayout>

My system: Ubuntu 20.04 Android Studio 2021.1.1 Canary 11

CodePudding user response：

You have to declare the variables before using them . You can either use databinding / viewbinding / findViewById for the same :

Do the following :

class MainActivity : AppCompatActivity() {
   
   private lateinit var box_one_text : TextView
   private lateinit var box_two_text : TextView
   private lateinit var box_three_text : TextView
   private lateinit var box_four_text : TextView
   private lateinit var box_five_text : TextView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        box_one_text = findViewById(R.id.box_one_text)
        box_two_text = findViewById(R.id.box_two_text)
        box_three_text = findViewById(R.id.box_three_text)
        box_four_text = findViewById(R.id.box_four_text)
        box_five_text = findViewById(R.id.box_five_text)
        setListeners()
    }
    private  fun setListeners(){
        val clickableViews: List<View> =
            listOf(box_one_text, box_two_text, box_three_text, box_four_text, box_five_text)
        for (item in clickableViews){
            item.setOnClickListener{makeColored(it)}
        }
    }
    fun makeColored(view: View) {
        when (view.id) {

            // Boxes using Color class colors for background
            R.id.box_one_text -> view.setBackgroundColor(Color.DKGRAY)
            R.id.box_two_text -> view.setBackgroundColor(Color.GRAY)

            // Boxes using Android color resources for background
            R.id.box_three_text -> view.setBackgroundResource(android.R.color.holo_green_light)
            R.id.box_four_text -> view.setBackgroundResource(android.R.color.holo_green_dark)
            R.id.box_five_text -> view.setBackgroundResource(android.R.color.holo_green_light)

            else -> view.setBackgroundColor(Color.LTGRAY)
        }
    }
}

CodePudding user response：

The project is using Kotlin synthetics imports.

You need this import in you MainActivity.kt

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

and check that your build.gradle file (Module) has the kotlin-android-extensions plugin

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-android-extensions'
}

However, the example you are following is probably outdated, it is currently recommended Migrate from Kotlin synthetics to Jetpack view binding