一、ViewModel的简介
2018年谷歌IO大会上正式发布了JetPack组件，其中包括Databing、Lifecycles、LiveData、Navigation、Paging、Room、ViewModel、WorkManager等一系列框架。并且发布androidx包，这些框架的最新版本组件将引入到androidx包下。
JetPack架构组件的发布，意味着google终于拿出了官方推荐的Android开发架构，一直以来在Android开发上老生常谈三种开发模式，MVC、MVP、MVVM。整体的思想就是做一些代码封装和操作上的分离，具体三种架构的区别不在这详细描述。说一点儿个人对这三中开发模式的理解。
MVC：Activity承担C的任务，强行carry大量业务和数据代码。
MVP：Presenter承担主要代码任务，连接数据层和视图层的交互。
MVVM：Model负责数据、View负责展示、ViewModel负责Model和View的交互，同时，最好的一点是View层自动监听ViewModel的数据变化。
在JetPack组件出现之前，一直以来构建MVVM架构的方式都是代用Databing来充当ViewModel，而Databing是采用标签方式写入布局文件中的，这样一来当出现问题的时候不是太好进行调试。所以ViewMode的出现对于MVVM模式开发具有重大的意义。
二、ViewModel的用法和特点
ViewModel的使用是要结合LiveData框架进行的，LiveData框架也是JetPack组件的一部分，这里先不详细进行介绍。ViewModel有两个特点，一是更加方便的保存数据，第二个特点，也是最重要的特点，就是保证数据不受Activity的销毁重建所影响，当Activity销毁重建后仍然能收到之前的数据。
1）继承ViewModel
class MainViewModel : ViewModel() {
private val repertory: MainRepository by lazy { MainRepository() }
var data: MutableLiveData = MutableLiveData()
fun getDataFromServer(){
repertory.getDataFromServer(data)
}
}
ViewModel中持有LiveData，LiveData是ViewModel持有数据的载体。
2）Activity通过ViewModelProviders获取ViewModel
class MainActivity : AppCompatActivity() {
private lateinit var mModel: MainViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
initData()
}
private fun initData() {
mModel = ViewModelProviders.of(this)[MainViewModel::class.java]
mModel.data?.observe(this, Observer {
val mainAdapter = MainAdapter(this, it)
val linearLayoutManager = LinearLayoutManager(this)
rv.layoutManager = linearLayoutManager
rv.adapter = mainAdapter
})
mModel.getDataFromServer()
}
}
3）通过ViewModel中的方法获取数据然后改变LiveData状态，通过响应式的方式通知到Activity进行视图更新。用法上比较简单。
三、源码分析
个人认为对于一个框架的学习仅仅掌握框架的用法是不够的，因为这太简单了，曾经看到过一篇博客，里面有一句话说的很好，”如果你只会用这个框架，而不清楚其中的原理和设计，那么即便这个框架性能再好，再牛逼，那也是写框架的那个人牛逼，而不是你牛逼“。我认为这句话说的很好，所以当我们学习一中新框架的时候，不能单单只会用，一定要清楚其中的原理，尽量去理解作者的设计思想，就像品茶一样，这样才能品出里面的滋味。不扯闲篇了，由于作者能力也有限，所以以上和以下分析，如有不同观点或作者描述有误请给予批评指正，欢迎来喷。
1）先从获取ViewModel的类入手，”ViewModelProviders“从类名中也能分析出这个类是用于提供ViewModel的。源码如下：
public class ViewModelProviders {
public ViewModelProviders() {
}
private static Application checkApplication(Activity activity) {
Application application = activity.getApplication();
if (application == null) {
throw new IllegalStateException("Your activity/fragment is not yet attached to "
+ "Application. You can't request ViewModel before onCreate call.");
}
return application;
}
private static Activity checkActivity(Fragment fragment) {
Activity activity = fragment.getActivity();
if (activity == null) {
throw new IllegalStateException("Can't create ViewModelProvider for detached fragment");
}
return activity;
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment) {
ViewModelProvider.AndroidViewModelFactory factory =
ViewModelProvider.AndroidViewModelFactory.getInstance(
checkApplication(checkActivity(fragment)));
return new ViewModelProvider(ViewModelStores.of(fragment), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
ViewModelProvider.AndroidViewModelFactory factory =
ViewModelProvider.AndroidViewModelFactory.getInstance(
checkApplication(activity));
return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment, @NonNull Factory factory) {
checkApplication(checkActivity(fragment));
return new ViewModelProvider(ViewModelStores.of(fragment), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@NonNull Factory factory) {
checkApplication(activity);
return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
@SuppressWarnings("WeakerAccess")
@Deprecated
public static class DefaultFactory extends ViewModelProvider.AndroidViewModelFactory {
/**
* Creates a {@code AndroidViewModelFactory}
*
* @param application an application to pass in {@link AndroidViewModel}
* @deprecated Use {@link ViewModelProvider.AndroidViewModelFactory} or
* {@link ViewModelProvider.AndroidViewModelFactory#getInstance(Application)}.
*/
@Deprecated
public DefaultFactory(@NonNull Application application) {
super(application);
}
}
}
我们从中可以看到，这个类中的方法全是静态的，说明这个类相当于一个工具类，从of方法中我们可以看到真正实现ViewModel创建的类是ViewModelProvider，而这个类的构建又需要通过ViewModelStores的of方法获取，ViewModelStores的代码如下：
public class ViewModelStores {
private ViewModelStores() {
}
@NonNull
@MainThread
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
return holderFragmentFor(activity).getViewModelStore();
}
@NonNull
@MainThread
public static ViewModelStore of(@NonNull Fragment fragment) {
if (fragment instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) fragment).getViewModelStore();
}
return holderFragmentFor(fragment).getViewModelStore();
}
}
从这个类中可以发现获取ViewModelStore是通过构建一个HoldFragment，而这个HoldFragment的作用就是持有ViewModelStore的引用，然后将这个Fragment添加到Activity中，而且这个Fragmet有一个重要的特点，在实例化的时候会调用setRetainInstance(true)方法，这个方法的作用是让Fragment不受Activity销毁重建影响，这样一来就能保证ViewModel不会由于Activity的销毁重建导致数据丢失，这是ViewModel的一个重要特性。这种设计确实很巧妙，HolderFragment的代码如下：
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class HolderFragment extends Fragment implements ViewModelStoreOwner {
.......省略部分代码
public HolderFragment() {
setRetainInstance(true);
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
sHolderFragmentManager.holderFragmentCreated(this);
}
@Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
}
@Override
public void onDestroy() {
super.onDestroy();
mViewModelStore.clear();
}
@NonNull
@Override
public ViewModelStore getViewModelStore() {
return mViewModelStore;
}
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
return sHolderFragmentManager.holderFragmentFor(activity);
}
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(Fragment fragment) {
return sHolderFragmentManager.holderFragmentFor(fragment);
}
.....省略部分代码
private static HolderFragment createHolderFragment(FragmentManager fragmentManager) {
HolderFragment holder = new HolderFragment();
fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
return holder;
}
HolderFragment holderFragmentFor(FragmentActivity activity) {
FragmentManager fm = activity.getSupportFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedActivityHolders.get(activity);
if (holder != null) {
return holder;
}
if (!mActivityCallbacksIsAdded) {
mActivityCallbacksIsAdded = true;
activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
}
holder = createHolderFragment(fm);
mNotCommittedActivityHolders.put(activity, holder);
return holder;
}
HolderFragment holderFragmentFor(Fragment parentFragment) {
FragmentManager fm = parentFragment.getChildFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedFragmentHolders.get(parentFragment);
if (holder != null) {
return holder;
}
parentFragment.getFragmentManager()
.registerFragmentLifecycleCallbacks(mParentDestroyedCallback, false);
holder = createHolderFragment(fm);
mNotCommittedFragmentHolders.put(parentFragment, holder);
return holder;
}
}
之后来到ViewModelProvider，代码量不是很多，但是里面的设计确实很有意思。首先来分析它的成员变量，成员变量只有两个，Factory从字面的意思就能理解，它是生产ViewModel的工厂，所以这里用到了工厂模式。ViewModelStore它用来存储ViewModel，相当于ViewModel的缓存。内部是一个HashMap，key是ViewModel的类名称。ViewModelProvider代码如下：
public class ViewModelProvider {
....省略部分代码
private final Factory mFactory;
private final ViewModelStore mViewModelStore;
......
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
this.mViewModelStore = store;
}
public T get(@NonNull Class 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 get(@NonNull String key, @NonNull Class 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.
}
}
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
.......省略部分代码
}
2）Factory是ViewModelProvider中的一个接口，定义如下：
public interface Factory {
@NonNull
T create(@NonNull Class modelClass);
}
通过create方法创建ViewModel。Factory有两个实现类AndroidViewModelFactory、NewInstanceFactory都是ViewModelProvider中的静态内部类。同时AndroidViewModelFactory又继承于NewInstanceFactory。有一种依赖注入的思想在里面。接下来我们来分析两个Factory的实现类。NewInstanceFactory最终通过newinstance方法创建ViewModel实例。
public static class NewInstanceFactory implements Factory {
public T create(@NonNull Class modelClass) {
//noinspection TryWithIdenticalCatches
try {
return modelClass.newInstance();
} catch (InstantiationException e) {
throw new RuntimeException("Cannot create an instance of " + modelClass, e);
} catch (IllegalAccessException e) {
throw new RuntimeException("Cannot create an instance of " + modelClass, e);
}
}
}
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
private static AndroidViewModelFactory sInstance;
@NonNull
public static AndroidViewModelFactory getInstance(@NonNull Application application) {
if (sInstance == null) {
sInstance = new AndroidViewModelFactory(application);
}
return sInstance;
}
private Application mApplication;
public AndroidViewModelFactory(@NonNull Application application) {
mApplication = application;
}
@NonNull
@Override
public T create(@NonNull Class 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);
}
}
最后，真正存储ViewModel的容器是ViewModelStore，ViewModelStore中维护了一个HashMap，代码如下：
public class ViewModelStore {
private final HashMap mMap = new HashMap<>();
final void put(String key, ViewModel viewModel) {
ViewModel oldViewModel = mMap.get(key);
if (oldViewModel != null) {
oldViewModel.onCleared();
}
mMap.put(key, viewModel);
}
final ViewModel get(String key) {
return mMap.get(key);
}
/**
* Clears internal storage and notifies ViewModels that they are no longer used.
*/
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.onCleared();
}
mMap.clear();
}
}
文章代码库：https://github.com/24KWYL/MVVM.git