Android Studio：未解决的参考：box_three_text

Question

我尝试制作一个应用程序在Android中练习布局，但我收到错误：

未解决的参考：box_three_text

看截图： [1]：https://i.stack.imgur.com/W3uGt.jpg

为什么讲师代码有效而我的无效？

这是我的代码：

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>

我的系统：Ubuntu 20.04 Android Studio 2021.1.1 Canary 11

Answer 1

您必须在使用变量之前声明它们。您可以同时使用 databinding / viewbinding / findViewById ：

执行以下操作：

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)
        }
    }
}

Answer 2

该项目正在使用 Kotlin 合成导入。

你需要这个导入 MainActivity.kt

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

并检查您的build.gradle 文件（模块）是否具有kotlin-android-extensions 插件

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

但是，你下面的例子可能已经过时了，目前推荐Migrate from Kotlin synthetics to Jetpack view binding