前言

ViewModel可以感知activity的生命周期來管理UI相關的數據，在屏幕旋轉后數據仍然存在。在傳統(tǒng)的UI控制器中，譬如activity或fragment屏幕發(fā)生轉動時，可以使用onSaveInstanceState（）方法并從onCreate（）中的包中恢復其數據，但此方法僅適用于可以序列化然后反序列化的少量數據。還有一個就是資源對象使用完成時，對資源對象的回收以避免內存泄漏。而且，在配置更改時，會造成資源對象的重建不僅浪費資源，還需要額外的管理。此外，UI控制器還要負責網絡請求，太過臃腫。

創(chuàng)建viewModel的創(chuàng)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) {
           注釋① //創(chuàng)建一個工廠
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(activity.getViewModelStore(), factory);
    }

Factory是個接口，里面就一個create()方法，有兩個實現(xiàn)類，一個是NewInstanceFactory，AndroidViewModelFactory ，區(qū)別是第二個是帶有application的，第一個是不帶參數的。這里就看看第二個。

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 extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
               //通過反射去創(chuàng)建ViewModel的實例。
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
}
return super.create(modelClass);
}

注釋①：這個create目前還沒掉，還沒生成想要的viewModel。
接著往下看new ViewModelProvider(activity.getViewModelStore(), factory);把創(chuàng)建好的工廠傳遞出去
先看看第一個形參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) {
          //獲取最近一次橫豎屏切換時保存下來的數據
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
            //這就是一個存放ViewMode的容器
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }

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

前面提到，橫豎屏切換時，使用viewModel不會影響到數據的變化，接下來揭開它神秘的面紗。

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;
        }
        //這里開始創(chuàng)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，這里將ViewModel保存到了NonConfigurationInstances 中，在橫豎屏切換時，還是用的原來的ViewMode，所以數據不會發(fā)生丟失。

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

       if (mLastNonConfigurationInstances != null) {
         //這里開始取
           mFragments.restoreLoaderNonConfig(mLastNonConfigurationInstances.loaders);
       }
}

關于資源回收

在ComponentActivity中對viewModel清空。

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