Android MVVM架构

MVVM（Model-View-ViewModel）设计架构旨在将图形用户界面与业务逻辑的开发分离开来，核心思想为数据驱动视图

 

• Model

  与MVP基本相同，Model表示应用程序的数据访问层，包括本地数据库或远程服务器等，一般情况下ViewModel会持有Model

• ViewModel

  ViewModel类似于MVP中的Presenter，负责处理程序的业务逻辑或者数据逻辑。但与之不同的是ViewModel不会持有View的引用，通过订阅-发布机制通知View更新UI

  该层的一个重要实现策略是将其与View分离，即ViewModel不应该意识到与之交互的View

• View

  View把UI事件传递到ViewModel，ViewModel做出具体业务逻辑处理并更新数据

  View通过订阅ViewModel中的数据，在数据变更时更新UI

  View层尽可能的下沉非UI相关代码到ViewModel

 

ViewModel

ViewModel是Android Architecture Components中的一员，是Android中对MVVM架构中ViewModel的实现

源码

public abstract class ViewModel {
    /**
     * This method will be called when this ViewModel is no longer used and will be destroyed.
     * <p>
     * It is useful when ViewModel observes some data and you need to clear this subscription to
     * prevent a leak of this ViewModel.
     */
    @SuppressWarnings("WeakerAccess")   
    protected void onCleared() {
    }
}

构造ViewModel

1.无参构造

public class TestFragment extends Fragment {

    TestViewModel testViewModel;
    ……

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        testViewModel = ViewModelProviders.of(this).get(TestViewModel.class);
    }

    ……

}

 

2.有参构造

 

 

public class ViewModelFactory extends ViewModelProvider.NewInstanceFactory {
    private static ViewModelFactory factory;

    public static ViewModelFactory getInstance() {
        if (factory == null) {
            factory = new ViewModelFactory();
        }
        return factory;
    }
    /**
     * Model层，包括本地数据库和远程服务器
     * DataBase、HTTPClient 为伪代码
     */
    private final DataBase db;
    private final HttpClient httpClient;

    public ViewModelFactory() {
        db = new DataBase();
        httpClient = new HttpClient();
    }
    
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {

        //通过传入的Class对象判断构造的ViewModel实例
        if (AViewModel.class.isAssignableFrom(modelClass)) {
            return (T) new AViewModel(db); ;
        } else if (BViewModel.class.isAssignableFrom(modelClass)) {
            return (T) new BViewModel(httpClient); ;
        } else if (CViewModel.class.isAssignableFrom(modelClass)) {
            return (T) new CViewModel(db, httpClient); ;
        }

        //……

        return super.create(modelClass);
    }
}

public class TestFragment extends Fragment {

    TestViewModel testViewModel;
    ……

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        testViewModel = ViewModelProviders
                        .of(this, ViewMoedlFactory.getInstance())
                        .get(TestViewModel.class);
    }

    ……

}

ViewModel生命周期

ViewMoedl会在配置变更（如屏幕旋转）时保持存活