ViewModel浅析

ViewModel用来可感知生命周期的方式存储和管理UI相关数据，当系统配置发生变更的时候，如屏幕旋转，数据不会丢失。

一、ViewModel源码分析

ViewModelProvider(this).get(NameViewModel::class.java)

我们先来看ViewModelProvider的构造方法

public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
    this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
            ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
            : NewInstanceFactory.getInstance());
}

 public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;
    }

owner.getViewModelStore()—>ComponentActivity.getViewModelStore(),获取这个Activity相关联的ViewModelStore

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) {
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            // Restore the ViewModelStore from NonConfigurationInstances
            mViewModelStore = nc.viewModelStore;
        }
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

public class ViewModelStore {

    private final HashMap<String, ViewModel> mMap = new HashMap<>();

    final void put(String key, ViewModel viewModel) {
        ViewModel oldViewModel = mMap.put(key, viewModel);
        if (oldViewModel != null) {
            oldViewModel.onCleared();
        }
    }

    final ViewModel get(String key) {
        return mMap.get(key);
    }

    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }

    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}

第一次调用ViewModelProvider(this)会new ViewModelStore()，也就是初始化一个Map，用来存放ViewModel。后面会调用getLastNonConfigurationInstance获取最近一次系统配置发生变化（如横竖屏切换）时保存下来的数据并取出ViewModelStore，也就是恢复ViewModelStore。当然系统配置发生变化时悄悄的保存了viewModelStore到NonConfigurationInstances中，见下面onRetainNonConfigurationInstance方法：

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
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            viewModelStore = nc.viewModelStore;
        }
    }

    if (viewModelStore == null && custom == null) {
        return null;
    }

    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.custom = custom;
    nci.viewModelStore = viewModelStore;
    return nci;
}

接着看 get(NameViewModel::class.java)

public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
    String canonicalName = modelClass.getCanonicalName();
    if (canonicalName == null) {
        throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
    }
    return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}

public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
    //取缓存
    ViewModel viewModel = mViewModelStore.get(key);

    if (modelClass.isInstance(viewModel)) {
        if (mFactory instanceof OnRequeryFactory) {
            ((OnRequeryFactory) mFactory).onRequery(viewModel);
        }
        return (T) viewModel;
    } else {
        //noinspection StatementWithEmptyBody
        if (viewModel != null) {
            // TODO: log a warning.
        }
    }
    if (mFactory instanceof KeyedFactory) {
        viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
    } else {
        //无缓存，通过mFactory构建
        viewModel = (mFactory).create(modelClass);
    }
    //缓存
    mViewModelStore.put(key, viewModel);
    return (T) viewModel;
}

接着看create方法

public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
  //...
  @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
                //noinspection TryWithIdenticalCatches
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
                } catch (NoSuchMethodException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InstantiationException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InvocationTargetException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                }
            }
            return super.create(modelClass);
        }
  
}

modelClass.getConstructor(Application.class).newInstance(mApplication)通过反射构建传入的对象。
也就是第一次调用ViewModelProvider(this).get(NameViewModel::class.java)时，通过反射构建ViewModel对象并存入ViewModelStore中的map，后面直接从map中取

那么ViewModelStore什么时候时候清除呢？ComponentActivity中有个监听：Activity ON_DESTROY并且不是因为系统配置变化导致的就会清除ViewModelStore

getLifecycle().addObserver(new LifecycleEventObserver() {
    @Override
    public void onStateChanged(@NonNull LifecycleOwner source,
            @NonNull Lifecycle.Event event) {
        if (event == Lifecycle.Event.ON_DESTROY) {
            if (!isChangingConfigurations()) {
                getViewModelStore().clear();
            }
        }
    }
});

二、小结

ViewModel.png