一、ViewModel介绍

ViewModel属于lifecycle（生命周期感知型组件）中的一员，通常与LiveData、DataBinding一起使用，它们是MVVM架构的重要成员。ViewModel类旨在以注重生命周期的方式存储和管理界面相关的数据。ViewModel的生命周期比Activty的生命周期长，只有Activity真的销毁时ViewModel的生命周期才会结束；比如Activity旋转和按home键的销毁重建并不会导致ViewModel销毁。另外ViewModel是将数据存储在内存中所以能够比onSaveInstanceState反序列化存储的数据多，因为Bundle本身就不能存储大量的数据。

二、ViewModel使用

ViewModel 是一个抽象类必须继承ViewModel实现自己的子类

public class TestViewModel extends ViewModel {
    MutableLiveData<String> data = new MutableLiveData<>();

    
    public MutableLiveData<String> getData() {
        return data;
    }

}

Activity中创建ViewModel

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        //创建ViewModel提供者，this必须是实现了ViewModelStoreOwner
        ViewModelProvider viewModelProvider = new ViewModelProvider(this);
        //通过get方法获取自己定义的ViewModel
        TestViewModel testViewModel = viewModelProvider.get(TestViewModel.class);
        //观察数据变化
        testViewModel.getData().observe(this, new Observer<String>() {
            @Override
            public void onChanged(String s) {
                // update ui.
                Log.d(TAG, "onChanged: "+s);
            }
        });
    }

三、ViewModel源码分析

从上面ViewModel的使用中可以看到需要先创建ViewModelProvider然后使用其get方法获取想要获取的ViewModel。那么先看下ViewModelProvider源码。

    /**
     * Creates `ViewModelProvider`. This will create `ViewModels`
     * and retain them in a store of the given `ViewModelStoreOwner`.
     *
     *
     * This method will use the
     * [default factory][HasDefaultViewModelProviderFactory.getDefaultViewModelProviderFactory]
     * if the owner implements [HasDefaultViewModelProviderFactory]. Otherwise, a
     * [NewInstanceFactory] will be used.
     */
    public constructor(
        owner: ViewModelStoreOwner
    ) : this(owner.viewModelStore, defaultFactory(owner), defaultCreationExtras(owner))

    /**
     * Creates `ViewModelProvider`, which will create `ViewModels` via the given
     * `Factory` and retain them in a store of the given `ViewModelStoreOwner`.
     *
     * @param owner   a `ViewModelStoreOwner` whose [ViewModelStore] will be used to
     * retain `ViewModels`
     * @param factory a `Factory` which will be used to instantiate
     * new `ViewModels`
     */
    public constructor(owner: ViewModelStoreOwner, factory: Factory) : this(
        owner.viewModelStore,
        factory,
        defaultCreationExtras(owner)
    )

    /**
     * Returns an existing ViewModel or creates a new one in the scope (usually, a fragment or
     * an activity), associated with this `ViewModelProvider`.
     *
     *
     * The created ViewModel is associated with the given scope and will be retained
     * as long as the scope is alive (e.g. if it is an activity, until it is
     * finished or process is killed).
     *
     * @param modelClass The class of the ViewModel to create an instance of it if it is not
     * present.
     * @return A ViewModel that is an instance of the given type `T`.
     * @throws IllegalArgumentException if the given [modelClass] is local or anonymous class.
     */
    @MainThread
    public open operator fun <T : ViewModel> get(modelClass: Class<T>): T {
        val canonicalName = modelClass.canonicalName
            ?: throw IllegalArgumentException("Local and anonymous classes can not be ViewModels")
        return get("$DEFAULT_KEY:$canonicalName", modelClass)
    }



  @Suppress("UNCHECKED_CAST")
    @MainThread
    public open operator fun <T : ViewModel> get(key: String, modelClass: Class<T>): T {
        //store是ViewModelStore对象
        val viewModel = store[key]
        //如果从ViewModelStore获取到，直接返回
        if (modelClass.isInstance(viewModel)) {
            (factory as? OnRequeryFactory)?.onRequery(viewModel)
            return viewModel as T
        } else {
            @Suppress("ControlFlowWithEmptyBody")
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        val extras = MutableCreationExtras(defaultCreationExtras)
        extras[VIEW_MODEL_KEY] = key
        // AGP has some desugaring issues associated with compileOnly dependencies so we need to
        // fall back to the other create method to keep from crashing.
        return try {
             //不存在则创建
            factory.create(modelClass, extras)
        } catch (e: AbstractMethodError) {
            factory.create(modelClass)
        }.also { store.put(key, it) }//保存ViewModel
    }

ViewModelStore

ViewModelStore 就是一个包含Map的存储VIewModel的类

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();
    }
}

可以看到ViewModelProvider创建时传入了一个ViewModelStoreOwner对象，这个对象Activity和Fragment已经帮我们实现了。ViewModelStoreOwner是一个帮我们保存ViewModel的类，那么我们通过源码分析下ViewModelStoreOwner怎么帮助我们恢复ViewModel的。

public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}

ComponentActivity

当Activity配置变化时调用，此时会调用onRetainNonConfigurationInstance方法保存ViewModelStoreOwner，获取时再从保存地方返回。

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        ContextAware,
        LifecycleOwner,
        ViewModelStoreOwner,
        HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner,
        ActivityResultRegistryOwner,
        ActivityResultCaller,
        OnConfigurationChangedProvider,
        OnTrimMemoryProvider,
        OnNewIntentProvider,
        OnMultiWindowModeChangedProvider,
        OnPictureInPictureModeChangedProvider,
        MenuHost {

   static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

    //Activity销毁时清除ViewModelStore
    getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });

}

//当Activity配置变化时调用，此时会调用onRetainNonConfigurationInstance方法
    @Override
    @Nullable
    @SuppressWarnings("deprecation")
    public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        //getViewModel方法没有被调用时viewModelStore == null
        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;
        }
        //保存viewModelStore
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }




    @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.");
        }
        ensureViewModelStore();
        return mViewModelStore;
    }


    //获取保存的mViewModelStore ，如果保存的mViewModelStore  == null则创建一个实例。
    void ensureViewModelStore() {
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }