ViewModel 源码解析

1.ViewModel 的创建

ViewModelProvider(this)[ViewModelTest::class.java]

默认构造
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
      
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

owner.getViewModelStore()
ViewModelStoreOwner

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        ContextAware,
        LifecycleOwner,
        ViewModelStoreOwner,
        HasDefaultViewModelProviderFactory,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner,
        ActivityResultRegistryOwner,
        ActivityResultCaller {
      
    @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;
    }
 }
  //
 void ensureViewModelStore() {
        if (mViewModelStore == null) {
            //先从最后一次保存的状态里获取
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            //没有获取到直接创建新的
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }


@SuppressWarnings("WeakerAccess")
public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}

ViewModelProvider(this)默认执行的Factory 为AndroidViewModelFactory

@SuppressLint("LambdaLast")
    public SavedStateViewModelFactory(@Nullable Application application,
            @NonNull SavedStateRegistryOwner owner,
            @Nullable Bundle defaultArgs) {
        mSavedStateRegistry = owner.getSavedStateRegistry();
        mLifecycle = owner.getLifecycle();
        mDefaultArgs = defaultArgs;
        mApplication = application;
        mFactory = application != null
                ? ViewModelProvider.AndroidViewModelFactory.getInstance(application)
                : ViewModelProvider.NewInstanceFactory.getInstance();
    }

ViewModelProvider(this)[ViewModelTest::class.java]的创建

@NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
      //首先从缓存中获取
        ViewModel viewModel = mViewModelStore.get(key);

        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 {
            //从工厂中创建
            viewModel = mFactory.create(modelClass);
        }
        //保存在ViewModelStore的HashMap里
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

AndroidViewModelFactory.create()
反射创建ViewModel对象

        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
                //noinspection TryWithIdenticalCatches
                try {
                //反射创建ViewModel对象
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
                } catch (NoSuchMethodException e) {
                 
            }
            return super.create(modelClass);
        }

ViewModelStore

public class ViewModelStore {
  //key:   androidx.lifecycle.ViewModelProvider.DefaultKey:com.demo.arouter.ViewModelTest 
 //value:viewModel对象
    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);
    }

    Set<String> 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();
    }
}

当手机配置发生改变（旋转屏幕）Activity 会销毁，重走onCreate
在onDestory之前执行

public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        Object custom = onRetainCustomNonConfigurationInstance();
        
        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            //如果为空，从最后一个保存配置中读取
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                viewModelStore = nc.viewModelStore;
            }
        }

        if (viewModelStore == null && custom == null) {
            return null;
        }
        //创建 instances实例保存VewModelStore
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

NonConfigurationInstances

static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

//状态恢复

 public ComponentActivity() {
        Lifecycle lifecycle = getLifecycle();
        //清除viewmodelStore里的viewmodel
        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });
        //当执行onCreate 时候恢复数据
        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                ensureViewModelStore();
                getLifecycle().removeObserver(this);
            }
        });

       
    }

2.onRetainNonConfigurationInstace,getLastNonConfigurationInstance()怎么保存和恢复的呢

1.handleDestroyActivity

@Override
    public void handleDestroyActivity(ActivityClientRecord r, boolean finishing, int configChanges,
            boolean getNonConfigInstance, String reason) {
        performDestroyActivity(r, finishing, configChanges, getNonConfigInstance, reason);
  
void performDestroyActivity(ActivityClientRecord r, boolean finishing,
            int configChanges, boolean getNonConfigInstance, String reason) {
       ...
        //如果是配置发生改变
        if (getNonConfigInstance) {
           
            //把 retainNonConfigurationInstances的值复制给ActivityClientRecord的lastNonConfigurationInstances
          r.lastNonConfigurationInstances = r.activity.retainNonConfigurationInstances();
        }
      ...
      mInstrumentation.callActivityOnDestroy(r.activity);

ActivityClientRecord 是保存在ActivityThread.java下的mActivities中

// final ArrayMap<IBinder, ActivityClientRecord> mActivities = new ArrayMap<>();

r.lastNonConfigurationInstances = r.activity.retainNonConfigurationInstances();

NonConfigurationInstances retainNonConfigurationInstances() {
        //调用onRetainNonConfiguration（）
        Object activity = onRetainNonConfigurationInstance();
        HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
        FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();

       
        mFragments.doLoaderStart();
        mFragments.doLoaderStop(true);
        ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();

        if (activity == null && children == null && fragments == null && loaders == null
                && mVoiceInteractor == null) {
            return null;
        }
      //activity保存在 NonConfigurationInstances
        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;
    }
  

NonConfigurationInstances

static final class NonConfigurationInstances {
        Object activity;
        HashMap<String, Object> children;
        FragmentManagerNonConfig fragments;
        ArrayMap<String, LoaderManager> loaders;
        VoiceInteractor voiceInteractor;
    }

执行完onDestory 对象已经保存在 r.lastNonConfigurationInstances

handleLaunchActivity

 final Activity a = performLaunchActivity(r, customIntent);

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ....
    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, r.shareableActivityToken);
  ....

}

将保存的对象赋值给Activity的 成员变量 mLastNonConfigurationInstances

@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    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,
            IBinder shareableActivityToken) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        mWindow.setWindowControllerCallback(mWindowControllerCallback);
        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;
        mAssistToken = assistToken;
        mShareableActivityToken = shareableActivityToken;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mReferrer = referrer;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        ///赋值给mLastNonConfigurationInstances
        mLastNonConfigurationInstances = lastNonConfigurationInstances;

getLastNonConfigurationInstance 获取的就是mLastNonConfigurationInstance的值

  @Nullable
    public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;
    }

3. 屏幕旋转事件

ActivityManagerService.updateConfiguration

@Override
    public boolean updateConfiguration(Configuration values) {
        return mActivityTaskManager.updateConfiguration(values);
    }

ActivityTaskManagerService

    @Override
    public boolean updateConfiguration(Configuration values) {
      ...
    updateConfigurationLocked(values, null, false, false /* persistent */,
                        UserHandle.USER_NULL, false /* deferResume */,
                        mTmpUpdateConfigurationResult);
      ...
    }

boolean updateConfigurationLocked(Configuration values, ActivityRecord starting,
            boolean initLocale, boolean persistent, int userId, boolean deferResume,
            ActivityTaskManagerService.UpdateConfigurationResult result) {
            ...
            if (!deferResume) {
                kept = ensureConfigAndVisibilityAfterUpdate(starting, changes);
            }
            ...
        
        return kept;
    }


ensureConfigAndVisibilityAfterUpdate() -> relaunchActivityLocked(preserveWindow);

relaunchActivityLocked

try {
        
            final ClientTransactionItem callbackItem = ActivityRelaunchItem.obtain(pendingResults,
                    pendingNewIntents, configChangeFlags,
                    new MergedConfiguration(getProcessGlobalConfiguration(),
                            getMergedOverrideConfiguration()),
                    preserveWindow);
            final ActivityLifecycleItem lifecycleItem;
            if (andResume) {
                lifecycleItem = ResumeActivityItem.obtain(isTransitionForward());
            } else {
                lifecycleItem = PauseActivityItem.obtain();
            }
            final ClientTransaction transaction = ClientTransaction.obtain(app.getThread(), appToken);
            transaction.addCallback(callbackItem);
            transaction.setLifecycleStateRequest(lifecycleItem);
            mAtmService.getLifecycleManager().scheduleTransaction(transaction);

执行ActivityRelaunchItem

@Override
    public void execute(ClientTransactionHandler client, ActivityClientRecord r,
            PendingTransactionActions pendingActions) {
        if (mActivityClientRecord == null) {
            if (DEBUG_ORDER) Slog.d(TAG, "Activity relaunch cancelled");
            return;
        }
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
        client.handleRelaunchActivity(mActivityClientRecord, pendingActions);
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    }

ActivityThread.handleRelaunchActivity()

    @Override
    public void handleRelaunchActivity(ActivityClientRecord tmp,
            PendingTransactionActions pendingActions) {
          
        handleRelaunchActivityInner(r, configChanges, tmp.pendingResults, tmp.pendingIntents,
                pendingActions, tmp.startsNotResumed, tmp.overrideConfig, "handleRelaunchActivity");
  }

handleRelaunchActivityInner

private void handleRelaunchActivityInner(ActivityClientRecord r, int configChanges,
            List<ResultInfo> pendingResults, List<ReferrerIntent> pendingIntents,
            PendingTransactionActions pendingActions, boolean startsNotResumed,
            Configuration overrideConfig, String reason) {
        // Preserve last used intent, it may be set from Activity#setIntent().
        final Intent customIntent = r.activity.mIntent;
        // Need to ensure state is saved.
        if (!r.paused) {
            //执行onPause
            performPauseActivity(r, false, reason, null /* pendingActions */);
        }
        if (!r.stopped) {
          // onStop
            callActivityOnStop(r, true /* saveState */, reason);
        }
        //onDestory 也是在这里保存调用onRetainNonConfigurationInstance 来保存状态的
        handleDestroyActivity(r, false, configChanges, true, reason);

        r.activity = null;
        r.window = null;
        r.hideForNow = false;
        r.nextIdle = null;
        // Merge any pending results and pending intents; don't just replace them
        if (pendingResults != null) {
            if (r.pendingResults == null) {
                r.pendingResults = pendingResults;
            } else {
                r.pendingResults.addAll(pendingResults);
            }
        }
        if (pendingIntents != null) {
            if (r.pendingIntents == null) {
                r.pendingIntents = pendingIntents;
            } else {
                r.pendingIntents.addAll(pendingIntents);
            }
        }
        r.startsNotResumed = startsNotResumed;
        r.overrideConfig = overrideConfig;
        //重新执行 ， getLastNonConfigurationInstance 的 attach 完成赋值
        handleLaunchActivity(r, pendingActions, customIntent);
    }