ViewModel的使用和原理 - alonealice

ViewModel 是一种用来存储和管理UI相关数据的类。同时在配合LiveData 情况下还支持在系统配置发生改变的时候自动对数据进行保存。它可以用来管理Activity和Fragment中的数据，也可以拿来处理Fragment与Fragment之间的通信等。

ViewModel的使用

添加依赖

1 2	implementation 'androidx.appcompat:appcompat:1.0.2' implementation "androidx.lifecycle:lifecycle-extensions:2.2.0"

使用方法

public class PersonModel extends ViewModel {
    public MutableLiveData<Person> liveData = new MutableLiveData<>();
    public PersonModel() {
        liveData.postValue(new Person());
    }
    public void updateData() {
        Person value = liveData.getValue();
        value.name = "test";
        liveData.setValue(value);
    }
}

PersonModel personModel = new ViewModelProvider(this,ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication())).get(PersonModel.class);
personModel.liveData.observe(this, new Observer<Person>() {
    @Override
    public void onChanged(Person person) {
        //更新界面
    }
});

//更新数据
personModel.updateData();

当页面旋转等情况下重新调用onCreate时，ViewModel其实是没有重新创建的,还是之前那个ViewModel。ViewModel只有在activity被销毁之后才会销毁，因此ViewModel也不能持有activity引用。

源码分析

创建ViewModel的过程：

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

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

    if (modelClass.isInstance(viewModel)) {
        //noinspection unchecked
        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);
    //noinspection unchecked
    return (T) viewModel;
}

@NonNull
public static AndroidViewModelFactory getInstance(@NonNull Application application) {
    if (sInstance == null) {
        sInstance = new AndroidViewModelFactory(application);
    }
    return sInstance;
}

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);
}

ViewModelProvider 的get方法中，首先会在mViewModelStore中根据class，获取对应的实例，如果没有，则调用AndroidViewModelFactory创建，并保存到mViewModelStore。

AndroidViewModelFactory专门用来实例化那种构造方法里面有参数的class,并且ViewModel里面可能是带Context的。它是通过newInstance(application)去实例化的.如果有带application参数则是这样实例化，如果没有带application参数的话,则还是会走newInstance()方法去构建实例。

然后看一下activity中的getViewModelStore方法：

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;
}

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

Activity在横竖屏切换时悄悄保存了viewModelStore，放到了NonConfigurationInstances实例里面，横竖屏切换时保存了又恢复了回来，相当于ViewModel实例依然是原先的，也就因此避免了横竖屏切换时的数据丢失。

而ViewModelStore本身是就是一个简单存储数据的类。里面使用了hashmap用来存储数据：

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();
    }
}

再和在activity的destory方法中会将mViewModelStore中的数据清除。