Jetpack入门（五）ViewModel介绍及原理

前言

ViewModel可以感知activity的生命周期来管理UI相关的数据，在屏幕旋转后数据仍然存在。在传统的UI控制器中，譬如activity或fragment屏幕发生转动时，可以使用onSaveInstanceState（）方法并从onCreate（）中的包中恢复其数据，但此方法仅适用于可以序列化然后反序列化的少量数据。还有一个就是资源对象使用完成时，对资源对象的回收以避免内存泄漏。而且，在配置更改时，会造成资源对象的重建不仅浪费资源，还需要额外的管理。此外，UI控制器还要负责网络请求，太过臃肿。

创建viewModel的创建

 ViewModelProviders.of(this).get(viewModelClass)

去看看of里面做了啥事

 public static ViewModelProvider of(@NonNull FragmentActivity activity) {
        return of(activity, null);
    }

  public static ViewModelProvider of(@NonNull FragmentActivity activity,
            @Nullable Factory factory) {
        Application application = checkApplication(activity);
        if (factory == null) {
           注释① //创建一个工厂
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(activity.getViewModelStore(), factory);
    }

Factory是个接口，里面就一个create()方法，有两个实现类，一个是NewInstanceFactory，AndroidViewModelFactory ，区别是第二个是带有application的，第一个是不带参数的。这里就看看第二个。

public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
  private static AndroidViewModelFactory sInstance;
        @NonNull
        public static AndroidViewModelFactory getInstance(@NonNull Application application) {
            if (sInstance == null) {
                sInstance = new AndroidViewModelFactory(application);
            }
            return sInstance;
        }

        private Application mApplication;

        public AndroidViewModelFactory(@NonNull Application application) {
            mApplication = application;
        }

        @NonNull
        @Override
        public  T create(@NonNull Class modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
               //通过反射去创建ViewModel的实例。
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
}
return super.create(modelClass);
}

注释①：这个create目前还没掉，还没生成想要的viewModel。
接着往下看new ViewModelProvider(activity.getViewModelStore(), factory);把创建好的工厂传递出去
先看看第一个形参activity.getViewModelStore()

 public ViewModelStore getViewModelStore() {
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
        if (mViewModelStore == null) {
          //获取最近一次横竖屏切换时保存下来的数据
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
            //这就是一个存放ViewMode的容器
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }

static final class NonConfigurationInstances {
        Object custom;
        //ViewModel容器
        ViewModelStore viewModelStore;
    }

前面提到，横竖屏切换时，使用viewModel不会影响到数据的变化，接下来揭开它神秘的面纱。

public final Object onRetainNonConfigurationInstance() {
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            // No one called getViewModelStore(), so see if there was an existing
            // ViewModelStore from our last NonConfigurationInstance
            //ViewModel保存到NonConfigurationInstances 中
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
              //去取viewModelStore 
                viewModelStore = nc.viewModelStore;
            }
        }

        if (viewModelStore == null && custom == null) {
            return null;
        }
        //这里开始创建，nci和nc都是NonConfigurationInstances 
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

   public Object getLastCustomNonConfigurationInstance() {
        NonConfigurationInstances nc = (NonConfigurationInstances)
                getLastNonConfigurationInstance();
        return nc != null ? nc.custom : null;
    }

可以看出，这里将ViewModel保存到了NonConfigurationInstances 中，在横竖屏切换时，还是用的原来的ViewMode，所以数据不会发生丢失。

NonConfigurationInstances mLastNonConfigurationInstances
protected void onCreate(@Nullable Bundle savedInstanceState) {
       if (DEBUG_LIFECYCLE) Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);

       if (mLastNonConfigurationInstances != null) {
         //这里开始取
           mFragments.restoreLoaderNonConfig(mLastNonConfigurationInstances.loaders);
       }
}

关于资源回收

在ComponentActivity中对viewModel清空。

if (event == Lifecycle.Event.ON_DESTROY) {
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }