前言:

   進入Android Jetpack 架構，還是要提一下什麼是MVVM，再來使用ViewModel。

大綱 :

MVVM

   View --------  View Model ------- Model
(Activity)       (View Model)       (DATA  --- API , SQL...)

View 
    負責使用者介面，UI 層顯示，avtivity 和 fragment 都是

View Model
     負責接受處理 View 的請求和回覆資料

Model
    負責管理和更新資料，像是資料庫存取，API 

如何使用 ViewModel
1.build.gradle (app):

dependencies {
  ….
//ViewModel
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1'
}

2.建立class 宣告續承 ViewModel():

class Day16ViewModel: ViewModel() {
   init{ println("init Day16ViewModel")} 
}

3. Activity 呼叫 ViewModel

class Day16Activity : AppCompatActivity() {
    private lateinit var viewModel: Day16ViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_day16)

// Create a ViewModel the first time the system calls an activity's onCreate() method.
// Re-created activities receive the same Day16ViewModel instance created by the first activity.
// Use the 'by viewModels()' Kotlin property delegate
// from the activity-ktx artifact
    viewModel = ViewModelProvider(this).get(Day16ViewModel::class.java)
….
  }
}

4. Fragment呼叫 ViewModel:

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()
}

5. 這樣就在activity 和 fragmnet 使用viewmodel 了

範例:

在 Activity 的 viewmodel 中設定字串為 “鐵人賽主題”，給Activity 和 fragment 的viewmodel 讀取字串顯示在畫面和log

執行結果:

參考:

https://developer.android.com/topic/libraries/architecture/viewmodel