Kotlin搭建RecyclerView时recyclerView未实例化即可调用方法的问题求解

你看到的无需初始化直接使用recyclerView变量的能力，来自曾经官方推出的Kotlin Android Extensions（业内也常称合成视图绑定），这是Kotlin对Android开发提供的专属扩展能力，核心作用就是省去findViewById的重复代码。

原理逻辑 #

你贴出的示例代码顶部有这行导入声明：

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

这就是合成视图绑定的对应导入：插件会自动扫描你activity_main.xml布局中所有控件的android:id属性，生成和id同名的全局变量，只要导入对应布局的synthetic包，在调用setContentView加载对应布局之后，就可以直接用变量名访问控件。示例里用到的recyclerView，就是布局文件中RecyclerView控件设置的id值。

你本地代码报错的原因 #

Kotlin Android Extensions早在2020年就被谷歌官方废弃，现在新版Android Studio创建的项目默认不会开启该插件，所以你没有做对应配置的情况下，编译器找不到recyclerView变量的声明，就会报unresolved reference错误。

两种解决方式 #

• （仅适配老代码，不推荐）开启Kotlin Android Extensions插件：在模块级build.gradle文件的plugins配置块中添加id 'kotlin-android-extensions'，同步项目后即可复用示例的写法。
• （官方推荐现行方案）使用View Binding代替：
  1. 在模块级build.gradle的android配置块中开启View Binding：

  android {
      // 其他配置
      buildFeatures {
          viewBinding true
      }
  }
  

  2. 同步项目后修改Activity代码：

  // 去掉原来的synthetic导入
  import android.os.Bundle
  import android.support.v7.widget.LinearLayoutManager
  import android.view.Window
  import android.view.WindowManager
  import android.widget.LinearLayout
  import net.simplifiedcoding.recyclerviewexample.CustomAdapter
  // 导入自动生成的View Binding类，包名替换为你自己项目的包名
  import com.yourprojectpackage.databinding.ActivityMainBinding
  
  
  class MainActivity : Activity() {
      private lateinit var binding: ActivityMainBinding
  
      override fun onCreate(savedInstanceState: Bundle?) {
          super.onCreate(savedInstanceState)
          requestWindowFeature(Window.FEATURE_NO_TITLE)
          window.setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN)
          
          // 初始化View Binding
          binding = ActivityMainBinding.inflate(layoutInflater)
          setContentView(binding.root)
  
          val users = ArrayList<Model_Details>()
          users.add(Model_Details("Kaju katli", "Kaju katli, also known as kaju Katari or kaju barfi, is an Indian dessert similar to a barfi.",R.drawable.kaju))
          users.add(Model_Details("Doughnut", "The doughnut is popular in many countries and prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and franchised specialty outlets",R.drawable.donuts))
          users.add(Model_Details("Panna cotta", "Panna cotta is an Italian dessert of sweetened cream thickened with gelatin and molded. The cream may be aromatized with rum, coffee, vanilla, or other flavorings.",R.drawable.panna_cotta))
          users.add(Model_Details("Rose Cookies", "Rose cooky is a famous South Indian snack made during festivals",R.drawable.rosecookies))
          users.add(Model_Details("Belgian waffle", "In North America, Belgian waffles are a variety of waffle with a lighter batter, larger squares, and deeper pockets than ordinary American waffles",R.drawable.belgianwaffle))
  
          val obj_adapter = CustomAdapter(users)
  
          // 通过binding访问对应控件
          binding.recyclerView.layoutManager = LinearLayoutManager(this, LinearLayout.VERTICAL, false)
          binding.recyclerView.adapter = obj_adapter
      }
  }
  

内容的提问来源于stack exchange，提问作者Louis Sankey