###1.用法 我们先看一下VIewModel的简单用法
MyViewModel model = ViewModelProviders.of(this).get(MyViewModel.class);
简单的一句话就可以在activity或者fragment里获取VIewModel对象，然后可以对他执行操作。然后通过注册观察者通知ui组件更新内容
model.getUsers().observe(this, users -> {
// update UI
});
###2.大致流程以及源码解析
既然大家知道VIewModel的用法，那么它的内部实现是什么样的，这里我先说一下大致的流程，就是ViewModelProviders通过of方法获取ViewModelProvider对象，然后通过ViewModelProvider的get方法获取到的ViewModel对象。既然大家知道大致的流程，那边下边我们来看一下具体的代码实现吧。 首先看一下ViewModelProvider的of方法，看看他是如何创建ViewModelProvider的。
/**
* Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity
* is alive. More detailed explanation is in {@link ViewModel}.
* <p>
* It uses the given {@link Factory} to instantiate new ViewModels.
*
* @param activity an activity, in whose scope ViewModels should be retained
* @param factory a {@code Factory} to instantiate new ViewModels
* @return a ViewModelProvider instance
*/
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
中间的方法是为了适配非v4包下的activity或者fragment，所以先不看那个方法，先看一下最后一句。new了一个ViewModelProvider对象，需要两个参数，一个ViewModelStore和一个factory。第二个参数很简单就是一个工厂创建出来的。那么接下来看第一个参数ViewModelStore是如何创建的。
/**
* Returns the {@link ViewModelStore} of the given activity.
*
* @param activity an activity whose {@code ViewModelStore} is requested
* @return a {@code ViewModelStore}
*/
@NonNull
@MainThread
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
return holderFragmentFor(activity).getViewModelStore();
}
这里我们还是先跳过中间的部分，还是适配非v4包下的activity和fragment，直接看最后的return，这里调用了一个静态方法holderFragmentFor，我们进入到这个方法去看看他执行了什么操作。
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
return sHolderFragmentManager.holderFragmentFor(activity);
}
这里返回了HolderFragment，这个是我们的重点，待会我们说他。这个HolderFragment内部有一个成员变量，直接返回了ViewModelStore，下边是源码。
public class HolderFragment extends Fragment implements ViewModelStoreOwner {
private ViewModelStore mViewModelStore = new ViewModelStore();
@NonNull
@Override
public ViewModelStore getViewModelStore() {
return mViewModelStore;
}
}
这里省去了部分代码，这里我们找到了ViewModelStore是如何创建的。知道了如何创建ViewModeStore，我们来看看它做了什么
/**
* Class to store {@code ViewModels}.
* <p>
* An instance of {@code ViewModelStore} must be retained through configuration changes:
* if an owner of this {@code ViewModelStore} is destroyed and recreated due to configuration
* changes, new instance of an owner should still have the same old instance of
* {@code ViewModelStore}.
* <p>
* If an owner of this {@code ViewModelStore} is destroyed and is not going to be recreated,
* then it should call {@link #clear()} on this {@code ViewModelStore}, so {@code ViewModels} would
* be notified that they are no longer used.
* <p>
* {@link android.arch.lifecycle.ViewModelStores} provides a {@code ViewModelStore} for
* activities and fragments.
*/
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);
}
/**
* 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();
}
}
ViewModelProvider整个创建过程已经介绍完了，下边我们来看看他的get方法是如何获取ViewModel对象的，在看get方法之前先看一下ViewModelStore
/**
* Class to store {@code ViewModels}.
* <p>
* An instance of {@code ViewModelStore} must be retained through configuration changes:
* if an owner of this {@code ViewModelStore} is destroyed and recreated due to configuration
* changes, new instance of an owner should still have the same old instance of
* {@code ViewModelStore}.
* <p>
* If an owner of this {@code ViewModelStore} is destroyed and is not going to be recreated,
* then it should call {@link #clear()} on this {@code ViewModelStore}, so {@code ViewModels} would
* be notified that they are no longer used.
* <p>
* {@link android.arch.lifecycle.ViewModelStores} provides a {@code ViewModelStore} for
* activities and fragments.
*/
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);
}
/**
* 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();
}
}
ViewModelStore很简单内部维护一个HashMap，提供三个方法，存、取和清空。
public class ViewModelProvider {
/**
* Returns an existing ViewModel or creates a new one in the scope (usually, a fragment or
* an activity), associated with this {@code ViewModelProvider}.
* <p>
* 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.
* @param <T> The type parameter for the ViewModel.
* @return A ViewModel that is an instance of the given type {@code T}.
*/
@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);
}
/**
* Returns an existing ViewModel or creates a new one in the scope (usually, a fragment or
* an activity), associated with this {@code ViewModelProvider}.
* <p>
* 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 key The key to use to identify the ViewModel.
* @param modelClass The class of the ViewModel to create an instance of it if it is not
* present.
* @param <T> The type parameter for the ViewModel.
* @return A ViewModel that is an instance of the given type {@code T}.
*/
@NonNull
@MainThread
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.
}
}
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
}
}
ViewModelProvider先创建一个有规律的key，然后通过这个这个可以去ViewModelStore中取，如果匹配我们的viewModel那么就返回，如果不匹配，就通过工厂创建一个新的Viewmodel，然后存到ViewModelStore中，然后返回ViewModel对象。这样整个获取ViewModel对象的过程就分析完了，下边我们来看看那个HolderFragment，在这里我们看到ViewModel是如何销毁的。 ###3.ViewModel的死亡
public abstract class ViewModel {
/**
* This method will be called when this ViewModel is no longer used and will be destroyed.
* <p>
* It is useful when ViewModel observes some data and you need to clear this subscription to
* prevent a leak of this ViewModel.
*/
@SuppressWarnings("WeakerAccess")
protected void onCleared() {
}
}
Vm 只有一个onCleared方法，那么他是在何时调用呢？这里就要看之前提到的HolderFragment
public class HolderFragment extends Fragment {
private static final String LOG_TAG = "ViewModelStores";
private static final HolderFragmentManager sHolderFragmentManager = new HolderFragmentManager();
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static final String HOLDER_TAG =
"android.arch.lifecycle.state.StateProviderHolderFragment";
private ViewModelStore mViewModelStore = new ViewModelStore();
public HolderFragment() {
setRetainInstance(true);
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
sHolderFragmentManager.holderFragmentCreated(this);
}
@Override
public void onDestroy() {
super.onDestroy();
mViewModelStore.clear();
}
public ViewModelStore getViewModelStore() {
return mViewModelStore;
}
static class HolderFragmentManager {
private Map<Activity, HolderFragment> mNotCommittedActivityHolders = new HashMap<>();
private Map<Fragment, HolderFragment> mNotCommittedFragmentHolders = new HashMap<>();
……
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;
}
……
}
Vm 创建的时候提到过 实例化了一个 HolderFragment 。并且实例化的时候通过上面createHolderFragment 方法将其fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss(); 我们知道commit 之后 fragment 将会拥有灵魂，获得生命周期。再看其onDestroy方法里 调用了 mViewModelStore.clear();
###4.那么为什么旋转屏幕为什么不会onClear 因为在HolderFragment的构造方法里调用了setRetainInstance(true); 这一节先介绍到这，下边会跟进为什么VIewModel会和activity声明周期结合，并且如何把数据更新的。
