Jetpack出来很久了，最近看到别人又在扯ViewModel，带着几个问题再去重新看看

问问题之前可以先简单回顾下之前写的Jetpack mvvm 三部曲(一) ViewModel

更方便看我在代码片段添加了注释

1. ViewModel怎么创建的？

从这段创建的代码开始

MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);

先进来看下这段代码ViewModelProvider的相关方法

public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {

//这个构造参数有个值得注意的细节就是这个三目运算符,开头传的是this是一个实体类可以看作是ComponentActivity本身所以owner instanceof HasDefaultViewModelProviderFactory这个是成立的
  this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
            ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
            : NewInstanceFactory.getInstance());
}

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

@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.
        }
    }
    //第一次进来会走这个因为上面mFactory是继成自KeyedFactory
    if (mFactory instanceof KeyedFactory) {
      //这里实际上就是调用SavedStateViewModelFactory#create方法
      //篇幅关系就在这里稍微扯下 这里最终就是通过反射实例化了
        viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
    } else {
        viewModel = (mFactory).create(modelClass);
    }
    //所以这个key是可以自己定义名称的
    //这个mViewModelStore实际上就是Activtiy的mViewModelStore
    mViewModelStore.put(key, viewModel);
    return (T) viewModel;
}

image.png

这个ComponentActivity是实现了ViewModelStoreOwner,HasDefaultViewModelProviderFactory,这俩接口

@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) {
        //初始很定是为空的
        //这里去NonConfigurationInstances拿一下，看到这里ams知识很好的的同学估计已经知道第二个问题的答案了
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
             //从 NonConfigurationInstances 恢复 ViewModelStore
            mViewModelStore = nc.viewModelStore;
        }
        if (mViewModelStore == null) {
        //这里不用说了
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

下面这个是HasDefaultViewModelProviderFactory的实现

@NonNull
@Override
public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
    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 (mDefaultFactory == null) {
        mDefaultFactory = new SavedStateViewModelFactory(
                getApplication(),
                this,
                getIntent() != null ? getIntent().getExtras() : null);
    }
    return mDefaultFactory;
}
//这里的SavedStateViewModelFactory是继承ViewModelProvider.KeyedFactory

调用ViewModelProvider的构造方法传入了一个接口实现类this，所以在一个参数的构造方法调用传入的参数是ComponentActivity#mViewModelStore以及mDefaultFactory，到这里大致清楚了ViewModel的创建了

2.他是在页面重新创建的时候恢复的？

这点算ViewModel的核心吧,想要知道答案那就得还得看下ViewModelStore这个东西，上面贴了完整的ComponentActivity#getViewModelStore方法，为了方便再这贴下geViewModelStore的关键代码

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

image.png

关键点nc.viewModelStore探究下getLastNonConfigurationInstance是啥

@Nullable
public Object getLastNonConfigurationInstance() {
    return mLastNonConfigurationInstances != null
            ? mLastNonConfigurationInstances.activity : null;
}

先去找找mLastNonConfigurationInstances的赋值在找下activity的赋值

image.png

这里mLastNonConfigurationInstances是从attach方法赋值了，attach这个方法的调用是ActivityThread调用的（这块涉及到了ams的启动感兴趣的同学可以去搜索相关文章）

image.png

这里找到mLastNonConfigurationInstances的赋值了找下activity的赋值

NonConfigurationInstances retainNonConfigurationInstances() {
     //这个onRetainNonConfigurationInstance是值得关注的
    Object activity = onRetainNonConfigurationInstance();
    HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
    FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();

    // We're already stopped but we've been asked to retain.
    // Our fragments are taken care of but we need to mark the loaders for retention.
    // In order to do this correctly we need to restart the loaders first before
    // handing them off to the next activity.
    mFragments.doLoaderStart();
    mFragments.doLoaderStop(true);
    ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();

    if (activity == null && children == null && fragments == null && loaders == null
            && mVoiceInteractor == null) {
        return null;
    }

    NonConfigurationInstances nci = new NonConfigurationInstances();
    //赋值在这里
    nci.activity = activity;
    nci.children = children;
    nci.fragments = fragments;
    nci.loaders = loaders;
    if (mVoiceInteractor != null) {
        mVoiceInteractor.retainInstance();
        nci.voiceInteractor = mVoiceInteractor;
    }
    return nci;
}

上面onRetainNonConfigurationInstance方法在Activity是return null肯定是有地方重写了的；所以在
ComponentActivity找到了这个方法的重写

//这里贴下NonConfigurationInstances这个是ComponentActivity的内部静态类
static final class NonConfigurationInstances {
    Object custom;
    ViewModelStore viewModelStore;
}

/**
 * Retain all appropriate non-config state.  You can NOT
 * override this yourself!  Use a {@link androidx.lifecycle.ViewModel} if you want to
 * retain your own non config state.
 */
@Override
@Nullable
public final Object onRetainNonConfigurationInstance() {
     
    Object custom = onRetainCustomNonConfigurationInstance();

    ViewModelStore viewModelStore = mViewModelStore;
    if (viewModelStore == null) {
       //没有人调用 getViewModelStore()，
       //所以看看我们上一个 NonConfigurationInstanc 中是否有一个现有的ViewModelStore
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            viewModelStore = nc.viewModelStore;
        }
    }

    if (viewModelStore == null && custom == null) {
        return null;
    }
    
    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.custom = custom;
    //到这里不就是把viewModelStore塞进去了么
    nci.viewModelStore = viewModelStore;
    return nci;
}

这里只是保存的过程那么谁调用了Activity#retainNonConfigurationInstances方法呢？

• 在Activity横竖屏的时候ams会走到这个ActvitiyThread#performDestroyActivity方法执行Activiy#retainNonConfigurationInstances，所以重走生命周期也不会丢失ViewModel的数据在这；重新恢复页面的时候只要通过getViewModelStore方法就可以拿到原来的数据了

image.png

3.又是如何销毁的？

直接看ComponentActivity构造方法就好了监测到Lifecycle发过来的销毁信号执行清空方法

image.png

在ViewModelStore#clear方法遍历了map去清空ViewModel并且把这个HashMap清空

image.png

可以在继承ViewModel的时候去重写onCleared方法做一些额外的操作比如断开上下文联系啥的

image.png

4. Fragment中ViewModel又是怎么创建的？

• 本来想把这个和第一点一起聊的，但是为了段落分明就单独写一个吧

MyFragmentViewModel viewModel = new ViewModelProvider(this).get(MyFragmentViewModel.class);

在Fragment使用还是和Activity一样的,区别就在于Fragment中ViewModelStoreOwner,HasDefaultViewModelProviderFactory的实现
和ComponentActivity不同；只需看ViewModelStoreOwner的实现好了

@NonNull
@Override
public ViewModelStore getViewModelStore() {
    if (mFragmentManager == null) {
        throw new IllegalStateException("Can't access ViewModels from detached fragment");
    }
    return mFragmentManager.getViewModelStore(this);
}

这个mNonConfig是继承ViewMolde的子类FragmentManagerViewModel看下他是如何创建的

@NonNull
ViewModelStore getViewModelStore(@NonNull Fragment f) {
    return mNonConfig.getViewModelStore(f);
}

可以看到FragmentManagerImpl#attachController方法有3个不同创建的这个地方就排查下

image.png

首先 Activity其实都有自己的FragmentController来管里Fragment；找到
FragmentActivity；

final FragmentController mFragments = FragmentController.createController(new HostCallbacks());

image.png

这里在传了一个空的parent结合上面的分析；

image.png

创建viewModelStore是中间那个判断条件直接去看getInstance方法；

ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);

FragmentManagerViewModel#getInstance这里又去掉了ViewModelStore的get方法使用剩下的就是把这个FragmentManagerViewModel添加到Activity的Viewmodel去了；然后get实例化 MyFragmentViewModel的时候又把这个ViewModel添加到FragmentManagerViewModel去了。

@NonNull
static FragmentManagerViewModel getInstance(ViewModelStore viewModelStore) {
    ViewModelProvider viewModelProvider = new ViewModelProvider(viewModelStore,
            FACTORY);
    return viewModelProvider.get(FragmentManagerViewModel.class);
}

正因为是这样Activtiy和Fragment可以互相拿到彼此的ViewModel