[移动开发]Android ViewModel 重建恢复原理

目录

如何获取ViewModel

ViewModelProvider

ViewModelStore从哪里来

???????

如何获取ViewModel

 mViewModel = new ViewModelProvider(
                this, new ViewModelProvider.AndroidViewModelFactory(getApplication())
        ).get(getViewModelClass());

ViewModelProvider

首先

 public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
        this(owner.getViewModelStore(), factory);
    }

传入两个参数， owner 和 factory

• 第一个是保存ViewmodelStore的activity
• 第二个是生成viewmodel的工厂类

接着

    @NonNull
    @MainThread
    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);
    }

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        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 {
            viewModel = (mFactory).create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

首先从ViewModelStore中获取viewmodel ,如果正好是当前页面的ViewModel，就直接返回； 如果不是，就会利用Factory重新创建ViewModel实例，添加到ViewModelStore中后，返回。

ViewModelStore从哪里来

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
        
    static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

    private ViewModelStore mViewModelStore;
    private ViewModelProvider.Factory mDefaultFactory;
}

ViewModelStore?来自于ComponentActivity

    @NonNull
    @Override
    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;
    }

利用onRetainNonConfigurationInstance机制 在回调

    @Override
    @Nullable
    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;
    }

中初始化NonConfigurationInstances 并将当前的ViewModelStore保存，下次重建时获取最新的配置，将对象重新赋值给ViewModelStore。

然后在getViewModelStore()中 NonConfigurationInstances 这个静态内部类保存了ViewModelStroe实例 如果有的话，直接getViewModelStore中取出来返回； 没有的话，会new ViewModelStrore返回