Android ——MVVM基本框架（ViewModel）

自己對MVVM的理論知識了解了許多，但是對於Android中究竟要如何體現，一直都不是很明了，今天在在官方API里，看到了一個經典的MVVM架構。

ViewModel is a class that is responsible for preparing and managing the data for an Activity or a Fragment. It also handles the communication of the Activity / Fragment with the rest of the application (e.g. calling the business logic classes).
A ViewModel is always created in association with a scope (an fragment or an activity) and will be retained as long as the scope is alive. E.g. if it is an Activity, until it is finished.
In other words, this means that a ViewModel will not be destroyed if its owner is destroyed for a configuration change (e.g. rotation). The new instance of the owner will just re-connected to the existing ViewModel.
The purpose of the ViewModel is to acquire and keep the information that is necessary for an Activity or a Fragment. The Activity or the Fragment should be able to observe changes in the ViewModel. ViewModels usually expose this information via LiveData or Android Data Binding. You can also use any observability construct from you favorite framework.
ViewModel's only responsibility is to manage the data for the UI. It should never access your view hierarchy or hold a reference back to the Activity or the Fragment.

 

ViewModel 是一個類，負責為 Activity 或 Fragment 准備和管理數據。它還處理 Activity/Fragment 與應用程序其余部分的通信（例如調用業務邏輯類）。 ViewModel 始終與作用域（片段或活動）相關聯地創建，並且只要作用域處於活動狀態就會保留。例如。如果它是一個Activity，直到它完成。 換句話說，這意味着如果 ViewModel 的所有者因配置更改（例如旋轉）而被銷毀，則它不會被銷毀。所有者的新實例將重新連接到現有的 ViewModel。 ViewModel 的目的是獲取並保存 Activity 或 Fragment 所需的信息。 Activity 或 Fragment 應該能夠觀察到 ViewModel 中的變化。 ViewModel 通常通過 LiveData 或 Android 數據綁定公開這些信息。您還可以使用您喜歡的框架中的任何可觀察性構造。 ViewModel 的唯一職責是管理 UI 的數據。它永遠不應該訪問您的視圖層次結構或保留對 Activity 或 Fragment 的引用。

 

其中activity如下：

   public class UserActivity extends Activity {
 
        @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.user_activity_layout);
           final UserModel viewModel = ViewModelProviders.of(this).get(UserModel.class);
           viewModel.userLiveData.observer(this, new Observer () {
               @Override
               public void onChanged(@Nullable User data) {
                   // update ui.
               }
           });
           findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
                @Override
               public void onClick(View v) {
                    viewModel.doAction();
               }
           });
       }
   }

然后ViewModel如下：
 

  public class UserModel extends ViewModel {
       private final MutableLiveData<User> userLiveData = new MutableLiveData<>();
 
       public LiveData<User> getUser() {
           return userLiveData;
       }
 
       public UserModel() {
           // trigger user load.
       }
 
       void doAction() {
           // depending on the action, do necessary business logic calls and update the
           // userLiveData.
       }
   }