dependencies {
  def lifecycle_version = "2.2.0"
  def arch_version = "2.1.0"

  // ViewModel
  implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
  // Saved state module for ViewModel
  implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"
}

class MyViewModel : ViewModel() {
  //延迟
  private val users: MutableLiveData<List<User>> by lazy {
    MutableLiveData().also {
      loadUsers()
    }
  }
	
  fun getUsers(): LiveData<List<User>> {
    return users
  }
	
  // 异步
  private fun loadUsers() {
    // Do an asynchronous operation to fetch users.
  }
}
    

class MyActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
		
    val model: MyViewModel by viewModels()
    model.getUsers().observe(this, Observer<List<User>>{ users ->
        // update UI
   	})
  }
}
    

//Activity 不需要执行任何操作，也不需要对此通信有任何了解。
//除了 SharedViewModel 约定之外，Fragment 不需要相互了解。
//每个 Fragment 都有自己的生命周期，而不受另一个 Fragment 的生命周期的影响。
class SharedViewModel : ViewModel() {
  val selected = MutableLiveData<Item>()

  fun select(item: Item) {
    selected.value = item
  }
}

class MasterFragment : Fragment() {

  private lateinit var itemSelector: Selector

  private val model: SharedViewModel by activityViewModels()

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    
    itemSelector.setOnClickListener { item ->
       // Update the UI
    }
    
  }
}

class DetailFragment : Fragment() {

  private val model: SharedViewModel by activityViewModels()

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    model.selected.observe(viewLifecycleOwner, Observer<Item> { item ->
          // Update the UI
    })
  }
}
    

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.528"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.313" />

    <Button
        android:id="@+id/btn1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:text="@string/plusbtn"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.547"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tv" />

</androidx.constraintlayout.widget.ConstraintLayout>

class NumberPlusViewModel: ViewModel() {
    var number = 0
}

class MainActivity : AppCompatActivity() {

    private val numberPlusViewModel by viewModels<NumberPlusViewModel>();

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        tv.text = numberPlusViewModel.number.toString()
        btn1.setOnClickListener {
            numberPlusViewModel.number++
            tv.text = numberPlusViewModel.number.toString()
        }

    }
}

dependencies {
   implementation "androidx.activity:activity-ktx:1.1.0"
}

// 即：
//Lazy<VM>	ComponentActivity.viewModels(noinline factoryProducer: () -> ViewModelProvider.Factory = null)

//返回一个延迟委托以访问 ComponentActivity 的 ViewModel。如果已指定 factoryProducer，则它返回的 ViewModelProvider.Factory 将用于首次创建 ViewModel。

compileOptions {
  sourceCompatibility JavaVersion.VERSION_1_8
  targetCompatibility JavaVersion.VERSION_1_8
}
kotlinOptions {
  jvmTarget = '1.8'
}