前言:

 在Activity 和 Fragment 只要操作 xml 的元件，在 onCreate 时就要先加一堆
 元件宣告( findViewById )再来操作,  使用 View Binding，大部份就可以
 不用 findViewById，直接用元件的 id 名称就可以了，是不是很方便呀。
  	

大纲 :

在build.gradle(app) 中宣告

android {
 ...
  buildFeatures {
     viewBinding true
  }
}

在activity 中使用 ViewBinding

class Day18Activity : AppCompatActivity() {
  private lateinit var viewModel: Day18ViewModel
  // ViewBinding 
  private lateinit var binding: ActivityDay18Binding

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    
    //setContentView(R.layout.activity_day18)

    // ViewBinding 
    binding = ActivityDay18Binding.inflate(layoutInflater)
    val view = binding.root
    setContentView(view)

    //var dayTitle: TextView = findViewById(R.id.tvtitle)

    viewModel = ViewModelProvider(this).get(Day18ViewModel::class.java)

    viewModel.mTitle.observe(this){
        //dayTitle.text = it
       
        // ViewBinding use id name 
        binding.tvtitle.text = it
    }

    viewModel.load()

  }
}

在fragment 中使用 ViewBinding

//class Day16Fragment : Fragment(R.layout.day16_fragment) {
class Day16Fragment : Fragment() {

//ViewBinding
private var _binding: Day16FragmentBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!


override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    _binding = Day16FragmentBinding.inflate(inflater,container,false)
    val view = binding.root
    return view //super.onCreateView(inflater, container, savedInstanceState)
}

override fun onDestroyView() {
    super.onDestroyView()
    _binding = null
}


override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    // Use the 'by activityViewModels()' Kotlin property delegate
    // from the fragment-ktx artifact
    val model: Day16ViewModel by activityViewModels()

    //var textView:TextView = this.requireView().findViewById(R.id.fragmenttext)
    //textView.text = model.getArticleTitle()
    //ViewBinding use id
    binding.fragmenttext.text = model.getArticleTitle()

    println("Fragment call ViewModel get Acticle Title = "+model.getArticleTitle())
  }

}

与 findViewById 的区别

Null safety 空安全：由於视图绑定创建了对视图的直接引用，因此不存在因视图 ID 
                   无效而导致空指针异常的风险。
Type safety 类型安全：每个绑定类中的字段的类型与其在 XML 文件中引用的视图相匹配。

参考:

https://developer.android.com/topic/libraries/view-binding