NonConfigurationInstances retainNonConfigurationInstances() {
// 重点
Object activity = onRetainNonConfigurationInstance();
HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
// We're already stopped but we've been asked to retain.
// Our fragments are taken care of but we need to mark the loaders for retention.
// In order to do this correctly we need to restart the loaders first before
// handing them off to the next activity.
mFragments.doLoaderStart();
mFragments.doLoaderStop(true);
ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();
if (activity == null && children == null && fragments == null && loaders == null
&& mVoiceInteractor == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.activity = activity;
nci.children = children;
nci.fragments = fragments;
nci.loaders = loaders;
if (mVoiceInteractor != null) {
mVoiceInteractor.retainInstance();
nci.voiceInteractor = mVoiceInteractor;
}
return nci;
}
retainNonConfigurationInstances 中调用了 onRetainNonConfigurationInstance() 。 到这儿，知道了 onRetainNonConfigurationInstance() 是怎么调用的了。
回头再看看 handleRelaunchActivityInner 中最后调用了 handleLaunchActivity ，熟悉Activity 启动流程的应该知道 handleLaunchActivity 是启动 Activity 的重要步骤
private void handleRelaunchActivityInner(ActivityClientRecord r, int configChanges,
List pendingResults, List pendingIntents,
PendingTransactionActions pendingActions, boolean startsNotResumed,
Configuration overrideConfig, String reason) {
...
// 注意第三个参数为 true
handleDestroyActivity(r.token, false, configChanges, true, reason);
...
handleLaunchActivity(r, pendingActions, customIntent);
}
handleLaunchActivity
@Override
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
final Activity a = performLaunchActivity(r, customIntent);
}
performLaunchActivity 启动 Activity 的核心实现
/** Core implementation of activity launch. */
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
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);
}
}
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);
创建了 Activity 实例并调用了 activity 的 attach 方法，注意 attach 方法有一个参数 传入了 r.lastNonConfigurationInstances ，有没有很熟悉，刚刚在 performDestroyActivity 中，r.lastNonConfigurationInstances = r.activity.retainNonConfigurationInstances();
调用了 Activity 对象的 retainNonConfigurationInstances() 并将返回值赋值给了 ActivityClientRecord 类型的 r 对象的 lastNonConfigurationInstances 属性。 已经串起来了。
@UnsupportedAppUsage
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, IBinder assistToken) {
attachBaseContext(context);
...
mLastNonConfigurationInstances = lastNonConfigurationInstances;
...
在 attach 中把之前保存的 lastNonConfigurationInstances 对象又赋值进新的 Activity 实例的 mLastNonConfigurationInstances 对象中了。
再回顾一下 怎么获取 ViewModel 的
val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
// ViewModelProvider 的构造方法
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
//
@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.");
}
ensureViewModelStore();
return mViewModelStore;
}
@SuppressWarnings("WeakerAccess") /* synthetic access */
void ensureViewModelStore() {
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
}
获取 ViewModelStore 时，调用了 ensureViewModelStore() 方法，ensureViewModelStore() 在它内部会调用 getLastNonConfigurationInstance() 获取是否有缓存的 ViewModelStore 对象，若有则返回，没有则创建新 ViewModelStore 实例。
getLastNonConfigurationInstance：
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