ViewModel
08 October 2020
androidx.activity.ComponentActivity
设计架构：
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+-----------------------------------------------------------------------------------------------------+
|androidx.activity.ComponentActivity(or Derived Class) |
| onResume(new instance) |
| ^ |
| | |
| getViewModelStore() onStart(new instance)->getViewModelStore() |
| ^ ^ |
| | | |
| +-------------------------------------------+ onCreate(new instance)->getViewModelStore()|
| | androidx.lifecycle.ViewModelStore | ^ |
| | (Single instance & Lazy initialization) | | |
| | -------------------------------- | onDestroy |
| | |androidx.lifecycle.ViewModel - 1| | ^ |
| | |androidx.lifecycle.ViewModel - 2| | | |
| | |androidx.lifecycle.ViewModel - 3| | | |
| | |... | |------>onRetainNonConfigurationInstance() |
| | |androidx.lifecycle.ViewModel - N| | ^ |
| | -------------------------------- | | |
| | | | |
| | clear: 1. Call clear() of each ViewModel.| onStop |
| | ^ 2. Clear all viewModels | ^ |
| +---|---------------------------------------+ | |
| | | |
| | onPause(due to configuration changed) |
| | |
| onDestroy <---onStop <--- onPause(activity is done and should be closed.) |
+-----------------------------------------------------------------------------------------------------+
上面这张架构图描述了androidx.activity.ComponentActivity, androidx.lifecycle.ViewModelStore 和 androidx.lifecycle.ViewModel之间的关系：每个androidx.activity.ComponentActivity的派生Activity都会持有1个androidx.lifecycle.ViewModelStore，只不过androidx.lifecycle.ViewModelStore直到getViewModelStore()调用时才会惰性创建1次（需要注意的是，创建这个androidx.lifecycle.ViewModelStore的前提有两个：当前Activity对象持有的mViewModelStore为null；如果当前Activity对象是由于配置改变（如旋转）重新创建的，而getViewModelStore()并没有在onCreate或onStart时调用，则获取不到上次存储的androidx.lifecycle.ViewModelStore实例），这个androidx.lifecycle.ViewModelStore中会存储所有在这个派生Activity里创建的androidx.lifecycle.ViewModel。当当前Activity正常需要销毁的时候，会调用androidx.lifecycle.ViewModelStore的clear方法：先去调用所有的androidx.lifecycle.ViewModel的clear方法以通知当前ViewModelStoreOwner（androidx.activity.ComponentActivity/androidx.fragment.app.Fragment）需要销毁了，然后在清除调所有的androidx.lifecycle.ViewModel。
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//androidx.activity.ComponentActivity.java
package androidx.activity;
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
HasDefaultViewModelProviderFactory,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
// Lazily recreated from NonConfigurationInstances by getViewModelStore()
private ViewModelStore mViewModelStore;
/**
* Default constructor for ComponentActivity. All Activities must have a default constructor
* for API 27 and lower devices or when using the default
* {@link android.app.AppComponentFactory}.
*/
public ComponentActivity() {
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
}
/**
* 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) {
// 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;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
/**
* Returns the {@link ViewModelStore} associated with this activity
* <p>
* Overriding this method is no longer supported and this method will be made
* <code>final</code> in a future version of ComponentActivity.
*
* @return a {@code ViewModelStore}
* @throws IllegalStateException if called before the Activity is attached to the Application
* instance i.e., before onCreate()
*/
@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) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
}
综上所述，下面列出几个的关键点：
- 当你的Activity是
androidx.activity.ComponentActivity的派生类，如果需要存储一些将由配置改变（比如旋转）而丢失的昂贵数据，推荐使用androidx.lifecycle.ViewModel。这是因为androidx.activity.ComponentActivity重写了onRetainNonConfigurationInstance()这个方法并添加了final修饰符，所以不允许其派生类继续重写。当然你也可以使用onRetainCustomNonConfigurationInstance和getLastCustomNonConfigurationInstance这两个方法实现，但这两个方法都已经废弃，所以不推荐使用。 getViewModelStore()不能在onCreate()生命周期之前调用。- 如果当前Activity实例是由于配置变更（比如旋转）重新创建的，
getViewModelStore()的调用除了不能在onCreate()生命周期之前调用，还不能在onStart之后调用，即只能在onCreate和onStart这两个生命周期中调用。 - 如果当前Activity实例由于配置变更（比如旋转）执行销毁流程，当销毁的时候，不移除所有存储的
androidx.lifecycle.ViewModel。 - 如果当前Activity实例正常执行销毁流程，当销毁的时候，会调用所有
androidx.lifecycle.ViewModel的clear方法，然后在androidx.lifecycle.ViewModel的内部调用onCleared方法，这样我们就可以在我们自定义的androidx.lifecycle.ViewModel的onCleared方法中执行资源释放，网络请求取消等操作。
androidx.fragment.app.Fragment
设计架构：
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+-----------------------------------------------------------------------------------------------------+
|androidx.fragment.app.Fragment(or Derived Class)| |
+------------------------------------------------+ |
|
getViewModelStore()
^
|
+-------|-----------------------------------------------------------------------------------+
| |androidx.fragment.app.FragmentManager| |
| +-------------------------------------+ |
| |
getViewModelStore(androidx.fragment.app.Fragment)
^
|
+-------|--------------------------------------------------------------------------------+
| |androidx.fragment.app.FragmentManagerViewModel| |
| +----------------------------------------------+ |
| |
getViewModelStore(androidx.fragment.app.Fragment)
^
|
+---------------------------------------------------------------------------------+
| |mViewModelStores(HashMap<String, androidx.lifecycle.ViewModelStore>)| |
| +--------------------------------------------------------------------+ |
| |
| androidx.fragment.app.Fragment@1 => androidx.lifecycle.ViewModelStore@1 |
| androidx.fragment.app.Fragment@2 => androidx.lifecycle.ViewModelStore@2 |
| androidx.fragment.app.Fragment@3 => androidx.lifecycle.ViewModelStore@3 |
| ... |
| androidx.fragment.app.Fragment@N => androidx.lifecycle.ViewModelStore@N |
| |
+---------------------------------------------------------------------------------+
在梳理androidx.lifecycle.ViewModel在androidx.fragment.app.Fragment中的设计流程之前，我们需要先了解一些FragmentManager的知识：当我们创建了一个直接/间接继承自androidx.fragment.app.FragmentActivity的Activity并交给系统，然后系统开始实例化这个Activity的时候，同时也会初始化了一个FragmentManager（只不过这个FragmentManager被androidx.fragment.app.FragmentHostCallback直接持有，而androidx.fragment.app.FragmentController又直接持有androidx.fragment.app.FragmentHostCallback, androidx.fragment.app.FragmentController又被androidx.fragment.app.FragmentActivity直接持有），所以直接在这个Activity中通过事务提交的Fragments都由这个FragmentManager直接管理，并且这些Fragment中持有的mFragmentManager也是这个FragmentManager。而在这些Fragment中创建的子Fragment由它的创建者Fragment（父Fragment）的mChildFragmentManager(在父Fragment实例化的时候初始化)负责管理。
每个FragmentManager持有一个androidx.fragment.app.FragmentManagerViewModel,由它负责管理这些Fragments（由这个FragmentManager直接管理）中使用的androidx.lifecycle.ViewModelStore。
我们在Fragment中获取到的ViewModelStore来自于androidx.fragment.app.FragmentManagerViewModel，这是一个ViewModel，它内部维护了一个K-V容器用来存储每个Fragment对应的ViewModelStore。那这个FragmentManagerViewModel是怎么创建的呢？
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// androidx.fragment.app.FragmentManager.java
package androidx.fragment.app;
public abstract class FragmentManager {
private FragmentManagerViewModel mNonConfig;
void attachController(@NonNull FragmentHostCallback<?> host,
@NonNull FragmentContainer container, @Nullable final Fragment parent) {
...
// Get the FragmentManagerViewModel
if (parent != null) {
mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
} else if (host instanceof ViewModelStoreOwner) {
ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
} else {
mNonConfig = new FragmentManagerViewModel(false);
}
}
@NonNull
private FragmentManagerViewModel getChildNonConfig(@NonNull Fragment f) {
return mNonConfig.getChildNonConfig(f);
}
}
创建FragmentManagerViewModel有三种情况：
-
第一种情况：
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ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore(); mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
这种情况发生在
androidx.fragment.app.FragmentActivity执行到生命周期onCreate的时候，此时host会attach到FragmentManager，这样之后才可以用这个FragmentManager管理Fragments。由上述代码可知，ViewModelStore来自于host，这个host实现了ViewModelStoreOwner这个接口以提供ViewModelStore，然后我们通过将新创建的FragmentManagerViewModel存储到这个ViewModelStore中。1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
// androidx.fragment.app.FragmentManagerViewModel.java package androidx.fragment.app; final class FragmentManagerViewModel extends ViewModel { private static final ViewModelProvider.Factory FACTORY = new ViewModelProvider.Factory() { @NonNull @Override @SuppressWarnings("unchecked") public <T extends ViewModel> T create(@NonNull Class<T> modelClass) { FragmentManagerViewModel viewModel = new FragmentManagerViewModel(true); return (T) viewModel; } } @NonNull static FragmentManagerViewModel getInstance(ViewModelStore viewModelStore) { ViewModelProvider viewModelProvider = new ViewModelProvider(viewModelStore, FACTORY); return viewModelProvider.get(FragmentManagerViewModel.class); } }
现在我们只要知道
host在哪，我们就知道FragmentManagerViewModel存储在哪：1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
// androidx.fragment.app.FragmentActivity.java package androidx.fragment.app; public class FragmentActivity extends ComponentActivity implements ActivityCompat.OnRequestPermissionsResultCallback, ActivityCompat.RequestPermissionsRequestCodeValidator { final FragmentController mFragments = FragmentController.createController(new HostCallbacks()); class HostCallbacks extends FragmentHostCallback<FragmentActivity> implements ViewModelStoreOwner, OnBackPressedDispatcherOwner { public HostCallbacks() { super(FragmentActivity.this /*fragmentActivity*/); } @NonNull @Override public ViewModelStore getViewModelStore() { return FragmentActivity.this.getViewModelStore(); } } }
很清楚了，
FragmentActivity继承自ComponentActivity，所以FragmentManagerViewModel存储在这个Fragment的宿主Activity中。 -
第二种情况
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mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
我们知道，一个Fragment包含两个
FragmentManager：mFragmentManager和mChildFragmentManager，后者就是用来管理在这个Fragment中创建的所有子Fragment，当这个Fragment attach到host的时候，mChildFragmentManager会在父Fragment关联的FragmentManagerViewModel中的mChildNonConfigs创建一个新的FragmentManagerViewModel（key:父Fragment）供这个mChildFragmentManager所管理的所有Fragments使用。1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
// androidx.fragment.app.Fragment package androidx.fragment.app; public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner, ViewModelStoreOwner, HasDefaultViewModelProviderFactory, SavedStateRegistryOwner { void performAttach() { mChildFragmentManager.attachController(mHost, new FragmentContainer() { @Override @Nullable public View onFindViewById(int id) { if (mView == null) { throw new IllegalStateException("Fragment " + this + " does not have a view"); } return mView.findViewById(id); } @Override public boolean onHasView() { return (mView != null); } }, this); mState = ATTACHED; mCalled = false; onAttach(mHost.getContext()); if (!mCalled) { throw new SuperNotCalledException("Fragment " + this + " did not call through to super.onAttach()"); } } }
这样意味这我们可以在
androidx.fragment.app.FragmentActivity的ViewModelStore里存储的FragmentManagerViewModel通过mChildNonConfigs和mViewModelStores直接或间接获取到附加到这个Activity上所有的Fragment和子Fragment的ViewModel。
-
第三种情况
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mNonConfig = new FragmentManagerViewModel(false);
这种已经废弃，可以不用考虑。
androidx.lifecycle.ViewModel
我们平时所使用的ViewModel都需要继承这个类，这个类设计的很简单，对于我们来说有用的方法只有一个：onCleared，要注意这个方法很重要，当我们的ViewModel需要被移除的时候，会调用这个方法，所以我们可以在这个方法对正在观察的数据去取消订阅，关闭正在加载的资源等。
ViewModel禁止持有一些如View/Activity/Fragment这些引用，想象一下旋转一个Activity的时候，如果ViewModel持有这个Activity的引用，导致这个Activity在需要被销毁的时候由于ViewModel的引用而导致无法被销毁，导致Activity内存泄漏。
ViewModel的存在也简化了Fragment与Fragment之间的通信：我们只需要定义一个用于Fragment之间共享的ViewModel，然后将它存储在这些Fragment的宿主Activity的ViewModelStore中，然后这些Fragment就都可以获取到这个Fragment，然后相互通信，不需要定义交互接口，不要在宿主Activity中处理过多的逻辑。
ViewModel也是官方推荐替换onRetainNonConfigurationInstance去保留所有适当的非配置状态（non-config state）的方法。
— Lenox Xian