ViewModel已经用了好久了，但是其内部原理还不是很熟悉，今天来简单了解下。
首先要确定三个问题，
第一，ViewModel是怎样保持唯一的，换句话说是怎样保持Fragment和Activity重建后还是原来的ViewModel？
第二，ViewModel的生命周期是怎样的？
第三，如何避免内存泄漏？
1.ViewModel是怎样保持唯一的？
我们看下ViewModel创建时的源码：
loginViewModel = ViewModelProvider(this
,ViewModelProvider.NewInstanceFactory()).get(LoginViewModel::class.java)
看下其内部方法：
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
@NonNull
@MainThread
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);
}
我们先看下ViewModelProvider()构造方法：
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}
把mFactory 和mViewModelStore 赋值。
接着我们看下get()方法：
@SuppressWarnings("unchecked")
@NonNull
@MainThread
public T get(@NonNull String key, @NonNull Class modelClass) {
ViewModel viewModel = mViewModelStore.get(key); // ---->①
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.
}
}
if (mFactory instanceof KeyedFactory) {
viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
} else {
viewModel = (mFactory).create(modelClass);
}
mViewModelStore.put(key, viewModel);
return (T) viewModel;
}
看代码①处，我们要从mViewModelStore获取Viewmodel，如果获取为空，那么再新建一个Viewmodel并加入到mViewModelStore中，我们看下这个mViewModelStore是什么？
public class ViewModelStore {
private final HashMap 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 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();
}
}
从ViewModelStore源码可以看出来，它内部有个HashMap保存了ViewModel信息。这儿我们应该意识到，Activity和Fragment销毁后还能保持不变，肯定是这个mViewModelStore没有被销毁。那我们看下这个mViewModelStore是什么时候被赋值和保存的？
从前面可知
this(owner.getViewModelStore(), factory);
mViewModelStore是通过getViewModelStore()方法获取的，我么继续追踪源码，发现在：
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
@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;
}
在ComponentActivity 中实现了这个方法，我们看到mViewModelStore 是从NonConfigurationInstances 获取的，而NonConfigurationInstances 是通过getLastNonConfigurationInstance创建的，继续追踪:
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback, WindowControllerCallback,
AutofillManager.AutofillClient {
/* package */ NonConfigurationInstances mLastNonConfigurationInstances;
@Nullable
public Object getLastNonConfigurationInstance() {// --- >①
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window, ActivityConfigCallback activityConfigCallback) {
attachBaseContext(context);
mFragments.attachHost(null /*parent*/);
mWindow = new PhoneWindow(this, window, activityConfigCallback);
mWindow.setWindowControllerCallback(this);
mWindow.setCallback(this);
mWindow.setOnWindowDismissedCallback(this);
mWindow.getLayoutInflater().setPrivateFactory(this);
if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
mWindow.setSoftInputMode(info.softInputMode);
}
if (info.uiOptions != 0) {
mWindow.setUiOptions(info.uiOptions);
}
mUiThread = Thread.currentThread();
mMainThread = aThread;
mInstrumentation = instr;
mToken = token;
mIdent = ident;
mApplication = application;
mIntent = intent;
mReferrer = referrer;
mComponent = intent.getComponent();
mActivityInfo = info;
mTitle = title;
mParent = parent;
mEmbeddedID = id;
mLastNonConfigurationInstances = lastNonConfigurationInstances; //-----> ②
if (voiceInteractor != null) {
if (lastNonConfigurationInstances != null) {
mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
} else {
mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
Looper.myLooper());
}
}
mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
if (mParent != null) {
mWindow.setContainer(mParent.getWindow());
}
mWindowManager = mWindow.getWindowManager();
mCurrentConfig = config;
mWindow.setColorMode(info.colorMode);
setAutofillCompatibilityEnabled(application.isAutofillCompatibilityEnabled());
enableAutofillCompatibilityIfNeeded();
}
}
从上面的源码知道①②可知，mLastNonConfigurationInstances 实在attach()方法中调用赋值的。那么attach()方法又是在哪儿调用呢，继续追踪
public final class ActivityThread extends ClientTransactionHandler {
/** Core implementation of activity launch. */
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
ComponentName component = r.intent.getComponent();
if (component == null) {
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}
if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (localLOGV) Slog.v(
TAG, r + ": app=" + app
+ ", appName=" + app.getPackageName()
+ ", pkg=" + r.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.getAppDir());
if (activity != null) {
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (r.overrideConfig != null) {
config.updateFrom(r.overrideConfig);
}
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
appContext.setOuterContext(activity);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
if (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
checkAndBlockForNetworkAccess();
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onCreate()");
}
r.activity = activity;
}
r.setState(ON_CREATE);
// updatePendingActivityConfiguration() reads from mActivities to update
// ActivityClientRecord which runs in a different thread. Protect modifications to
// mActivities to avoid race.
synchronized (mResourcesManager) {
mActivities.put(r.token, r);
}
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}
}
Activity的attach()方法实在ActivityThread.performLaunchActivity方法调用，并且lastNonConfigurationInstances在ActivityClientRecord中取出的。我们这里简单梳理下Activity启动流程：
20190610223432302.png
从上图可知，startActivity必须经过performLaunchActivity()这个方法，而且performLaunchActivity()这个方法是由ActivityThread控制的，Activity的销毁不会影响到它。换句话说，其实ViewModelStore其实是保存在ActivityClientRecord，而ActivityClientRecord是唯一的且不受Activity的影响，故缓存的ViewModel也不受影响。
Fragment其实也差不多，这里就不再分析了。
但是又两个点注意下：
第一个，我们看下ComponentActivity的构造方法
public ComponentActivity() {
Lifecycle lifecycle = getLifecycle();
//noinspection ConstantConditions
if (lifecycle == null) {
throw new IllegalStateException("getLifecycle() returned null in ComponentActivity's "
+ "constructor. Please make sure you are lazily constructing your Lifecycle "
+ "in the first call to getLifecycle() rather than relying on field "
+ "initialization.");
}
if (Build.VERSION.SDK_INT >= 19) {
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_STOP) {
Window window = getWindow();
final View decor = window != null ? window.peekDecorView() : null;
if (decor != null) {
decor.cancelPendingInputEvents();
}
}
}
});
}
getLifecycle().addObserver(new LifecycleEventObserver() {//----->①
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
if (19 <= SDK_INT && SDK_INT <= 23) {
getLifecycle().addObserver(new ImmLeaksCleaner(this));
}
}
在①处，当Activity正常通过OnDestroy时，会清除ViewModelStore内部的ViewModel的缓存，ViewModel内部也会调用clear()方法销毁。
第二点，ViewModelStore是如何保证正常情况下不被销毁的？
如果是横竖屏切换，则通过onRetainNonConfigurationInstance()方法实现的，代码如下：
/**
* 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;
}
这里有个说明，翻译下：保留所有适当的非配置状态。你不能自己覆盖它！如果您想保留自己的非配置状态，请使用 {@link androidx.lifecycle.ViewModel}。
屏幕发生旋转，会调用这个方法，保存mViewModelStore。(ps：也可以参考https://blog.51cto.com/u_15200109/2786154)
如果发生意外销毁，这就跟SavedStateHandleController、SavedStateRegistryOwner、SavedStateRegistry、SavedStateRegistryController、SavedStateProvider等几个类有关了。具体参考https://www.jianshu.com/p/82d49ea06029
2.ViewModel的生命周期是怎样的？
我们可以看下官方给出的一张图，如下：
167af49298be5d2c52a98226d2809e79.png
其实前面也已经说过了，在ComponentActivity构造中对Activity生命周期进行监听，当Activity调用onDestroy()后，将会调用ViewModelStore的clear()方法，进而调用ViewModel的clear()方法。假如Activity一直没有onDestroy，那么ViewModel的生命周期就和App的进程一样一直存活。
3.如何避免内存泄漏？
ViewModel没有持有Activity和Fragment的引用，也没有View的引用，如果需要的话，可以引用Application的Context，也强烈建议不要在ViewMode内部持有它们的引用，ViewModel就不会发生内存泄漏问题。