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用來記錄ViewModel的源碼閱讀
閱讀版本
androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.0
androidx.activity:activity-ktx:1.3.0

閱讀目的

為了搞清ViewModel生命周期和Activity重置原理

源代碼閱讀

  val model: EnglishViewModel by viewModels()

初始化ViewModel時(shí)通過kotlin的委托，委托到了viewModels()方法體中

@MainThread//聲明主線程使用
//通過ComponentActivity的擴(kuò)展函數(shù)置頂viewModels的使用范圍在ComponentActivity
public inline fun <reified VM : ViewModel> ComponentActivity.viewModels(
//默認(rèn)入?yún)⒎椒w為空，方法構(gòu)造用Lambda語法定義方法體為空入?yún)⒎祷谾actory類型
    noinline factoryProducer: (() -> Factory)? = null
): Lazy<VM> {
若入?yún)⒎祷谾actory的方法體為空碗殷，使用defaultViewModelProviderFactory方法體
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }
//返回真正的委托函數(shù)
    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}

這個(gè)類只是去處理了一個(gè)委托方法默認(rèn)參數(shù)衙耕，真正的委托函數(shù)在ViewModelLazy方法，所以ViewModelLazy方法里應(yīng)該有初始化ViewModel的代碼疏遏，但是我們需要先研究factoryPromise和viewModelStore兩個(gè)方法入?yún)⒍及耸裁?br> 由于是ComponentActivity的擴(kuò)展函數(shù)，所以defaultViewModelProviderFactory方法在ComponentActivity中，代碼如下

    @NonNull
    @Override
    public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
//空異常
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
//懶加載方式創(chuàng)建SavedStateViewModelFactory
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
    }

通過以上代碼察覺脾拆，defaultViewModelProviderFactory方法執(zhí)行后得到的是一個(gè)SavedStateViewModelFactory馒索，先不往下追，再看viewModelStore干了什么名船，和defaultViewModelProviderFactory同理绰上，viewModelStore也是ComponentActivity成員方法，代碼如下

  @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
//異常處理
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
//處理viewModelStore的初始化
        ensureViewModelStore();
        return mViewModelStore;
    }

    @SuppressWarnings("WeakerAccess") /* synthetic access */
    void ensureViewModelStore() {
//判空
        if (mViewModelStore == null) {
//這里通過getLastNonConfigurationInstance獲取上次存儲(chǔ)數(shù)據(jù)渠驼？蜈块？？迷扇？橫豎平切換百揭，數(shù)據(jù)存儲(chǔ)核心代碼估計(jì)就是這個(gè)破玩意了
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
//以下代碼就是判斷緩存的東西有沒有，有就拿蜓席，沒有就自己創(chuàng)建
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }

最終發(fā)現(xiàn)viewModelStore是通過getLastNonConfigurationInstance獲取的器一，也就是說銷毀和切換操作viewModelStore是有數(shù)據(jù)存儲(chǔ)的，配合getLastNonConfigurationInstance的方法是onRetainNonConfigurationInstance瓮床，所以可以在當(dāng)前類找到以下代碼

  @Override
    @Nullable
    @SuppressWarnings("deprecation")
    public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        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;
    }

邏輯很簡單盹舞，就是如果當(dāng)前存在ViewModelStore就保存當(dāng)前的，若果緩存存在就保存緩存的隘庄，那ViewModel的生命周期應(yīng)該就是整個(gè)Activity無論切入后臺還是如何屏幕翻轉(zhuǎn)還是切換語言踢步，ViewModelStore可以保存一個(gè)當(dāng)前實(shí)例
那ViewModelStore是什么東西？代碼如下

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);
    }

    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();
    }
}

好吧丑掺，ViewModelStore里只是一個(gè)大Map获印，存了String和ViewModel，至此ViewModel的生命周期一半已經(jīng)和Activity掛鉤了街州，在這里我們看到了clear方法兼丰，這個(gè)clear應(yīng)該是清除ViewModel的，我們大膽猜測應(yīng)該是Activity執(zhí)行onDestory的時(shí)候唆缴，執(zhí)行鳍征，所以回到ComponentActivity搜索onDestory。
onDestory沒有搜索到面徽，但是發(fā)現(xiàn)ComponentActivity實(shí)現(xiàn)了一大坨LifecycleOwner艳丛，所以我們搜索ON_DESTROY,果然在構(gòu)造函數(shù)里看到如下代碼

        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();
                    }
                }
            }
        });

通過 isChangingConfigurations 來判斷是否是真實(shí)銷毀，如果是真實(shí)銷毀趟紊，就將ViewModelStore里的ViewModel全部清理了氮双。
好吧看到這里，其實(shí)viewModel的生命周期已經(jīng)幾乎完成了霎匈，但是戴差，創(chuàng)建流程還沒有走完，所以回到ViewModelLazy中查看創(chuàng)建流程

public class ViewModelLazy<VM : ViewModel> (
    private val viewModelClass: KClass<VM>,
    private val storeProducer: () -> ViewModelStore,
    private val factoryProducer: () -> ViewModelProvider.Factory
) : Lazy<VM> {
    private var cached: VM? = null

    override val value: VM
        get() {
            val viewModel = cached
            return if (viewModel == null) {
                val factory = factoryProducer()
                val store = storeProducer()
//真正的創(chuàng)建流程铛嘱，通過also做了一個(gè)緩存賦值暖释，so why?因?yàn)閮?nèi)聯(lián)嗎袭厂？
                ViewModelProvider(store, factory).get(viewModelClass.java).also {
                    cached = it
                }
            } else {
                viewModel
            }
        }
//委托處理判斷值是否已存在
    override fun isInitialized(): Boolean = cached != null
}

創(chuàng)建的核心流程在ViewModelProvider的get里，ViewModelProvider的構(gòu)造函數(shù)就是做了store和factory的賦值
所以直奔get方法

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
//拿了一個(gè)類唯一標(biāo)識
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
  //DEFAULT_KEY= "androidx.lifecycle.ViewModelProvider.DefaultKey";
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }

上段代碼可以知道饭入，get方法的key是通過DEFAULT_KEY+getCanonicalName生成的嵌器，保證了ViewModel的key唯一性。
來看get的主方法

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        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 {
            viewModel = mFactory.create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

邏輯也不復(fù)雜谐丢，就是判斷viewModel是否為空 （通過isInstance判斷這點(diǎn)很高明，判斷了空和是否是ViewModel的子類）蚓让，有值返回乾忱，沒有就通過create創(chuàng)建。
那核心流程應(yīng)該就是一開始我們看的SavedStateViewModelFactory了历极，回到SavedStateViewModelFactory仔細(xì)研讀
SavedStateViewModelFactory extends ViewModelProvider.KeyedFactory extends OnRequeryFactory
所以上面的所有判斷流程如果存在值的話應(yīng)該都會(huì)執(zhí)行窄瘟，所以我們先看create方法

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull String key, @NonNull Class<T> modelClass) {
//判斷是否是AndroidViewModel 
        boolean isAndroidViewModel = AndroidViewModel.class.isAssignableFrom(modelClass);

        Constructor<T> constructor;
//如果是AndroidViewModel 獲取 帶有mApplication和handler參數(shù)的構(gòu)造函數(shù)
        if (isAndroidViewModel && mApplication != null) {
            constructor = findMatchingConstructor(modelClass, ANDROID_VIEWMODEL_SIGNATURE);
        } else {
            constructor = findMatchingConstructor(modelClass, VIEWMODEL_SIGNATURE);
        }
        // doesn't need SavedStateHandle
//如果沒有handler的構(gòu)造函數(shù)，直接創(chuàng)建趟卸。 mFactory在構(gòu)造函數(shù)中創(chuàng)建的蹄葱，下面會(huì)看
        if (constructor == null) {
            return mFactory.create(modelClass);
        }
//這里主要負(fù)責(zé)創(chuàng)建SavedStateHandleController  這里就是SavedStateHandler相關(guān)邏輯了
        SavedStateHandleController controller = SavedStateHandleController.create(
                mSavedStateRegistry, mLifecycle, key, mDefaultArgs);
        try {
            T viewmodel;
//通過反射構(gòu)造函數(shù)創(chuàng)建ViewModel
            if (isAndroidViewModel && mApplication != null) {
                viewmodel = constructor.newInstance(mApplication, controller.getHandle());
            } else {
                viewmodel = constructor.newInstance(controller.getHandle());
            }
            viewmodel.setTagIfAbsent(TAG_SAVED_STATE_HANDLE_CONTROLLER, controller);
            return viewmodel;
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Failed to access " + modelClass, e);
        } catch (InstantiationException e) {
            throw new RuntimeException("A " + modelClass + " cannot be instantiated.", e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException("An exception happened in constructor of "
                    + modelClass, e.getCause());
        }
    }

看完以上代碼，ViewModel的創(chuàng)建流程已經(jīng)80%了锄列，剩下就是mFactory直接Create出來的ViewModel和SavedStateHandler相關(guān)的邏輯了图云。先看簡單的

    @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();
    }

在SavedStateViewModelFactory構(gòu)造函數(shù)中，mFactory通過application判斷是創(chuàng)建了AndroidViewModelFactory和NewInstanceFactory邻邮，這兩個(gè)factory的create的方法只是 return modelClass.newInstance();和return modelClass.getConstructor(Application.class).newInstance(mApplication);的區(qū)別竣况，代碼如下

//NewInstanceFactory
  @SuppressWarnings("ClassNewInstance")
        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            //noinspection TryWithIdenticalCatches
            try {
                return modelClass.newInstance();
            } catch (InstantiationException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            }
        }
//AndroidViewModelFactory
   @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
                //noinspection TryWithIdenticalCatches
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
                } catch (NoSuchMethodException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InstantiationException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InvocationTargetException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                }
            }
            return super.create(modelClass);
        }

好了，創(chuàng)建流程就這么簡單筒严，至此ViewModel的生命周期和創(chuàng)建流程就很清晰了丹泉。先總結(jié)一下流程
創(chuàng)建流程
viewModels 通過擴(kuò)展函數(shù)的方式，執(zhí)行了ComponentActivity里的getDefaultViewModelProviderFactory和getViewModelStore鸭蛙，分別創(chuàng)建了SavedStateViewModelFactory 和ViewModelStore摹恨，并將實(shí)例委托給了ViewModelLazy類
其中ViewModelStore的生命周期通過getLastNonConfigurationInstance和onRetainNonConfigurationInstance組合方式對后臺切換，屏幕翻轉(zhuǎn)等做了數(shù)據(jù)緩存娶视，并且通過ComponentActivity的構(gòu)造函數(shù)用Lifecycle監(jiān)聽了ON_DESTROY晒哄，在actvity銷毀時(shí)清除ViewModelStore中存儲(chǔ)的所有ViewModel，完成了ViewModelStore對應(yīng)的Activity生命周期歇万，也是ViewModelStore中ViewModel的生命周期
ViewModelLazy通過繼承Lazy方法揩晴，重寫get（）實(shí)現(xiàn)了viewModels的委托實(shí)例，然后用ViewModelProvider的get方法獲取ViewModel
ViewModelProvider的get方法先會(huì)判斷ViewModelStore中是否有緩存贪磺，然后通過SavedStateViewModelFactory在構(gòu)造函數(shù)中的Factory創(chuàng)建ViewModel
下面是我自己畫的簡易代碼邏輯

簡易代碼邏輯

最后我們遺留了一個(gè)問題硫兰，SavedStateHandleController究竟干了什么，下篇文章會(huì)繼續(xù)閱讀ViewModel源碼閱讀（二）SavedStateHandle的基本數(shù)據(jù)緩存原理