Android?Jetpack庫剖析之ViewModel組件篇

更新時(shí)間：2022年07月22日 11:46:53 作者：豬飛啦~

這篇文章主要介紹了Android?Jetpack架構(gòu)組件?ViewModel詳解，ViewModel類讓數(shù)據(jù)可在發(fā)生屏幕旋轉(zhuǎn)等配置更改后繼續(xù)存在，ViewModel類旨在以注重生命周期的方式存儲和管理界面相關(guān)的數(shù)據(jù)。感興趣可以來學(xué)習(xí)一下

前言

今天讓我們一起去探究一下ViewModel的實(shí)現(xiàn)原理，描述的不對或不足還請海涵，僅作為參考

ViewModel簡介

ViewModel是一個(gè)可感知Activity或Fragment生命周期的一個(gè)架構(gòu)組件，當(dāng)視圖銷毀，數(shù)據(jù)也會被清除，所以它的本質(zhì)就是用來存儲與視圖相關(guān)的數(shù)據(jù)，讓視圖顯示控制與數(shù)據(jù)分離，即使界面配置發(fā)生改變數(shù)據(jù)也不會被銷毀，通常配合LiveData使用

ViewModel用法

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(LayoutInflater.from(baseContext))
        setContentView(binding.root)
        //獲取ViewModel實(shí)例
        val viewModel: TextViewModel = ViewModelProvider(this).get(TextViewModel::class.java)
        //訂閱數(shù)據(jù)
        viewModel.liveData.observe(this, { println(it) })
        //調(diào)用函數(shù)
        viewModel.test()
    }
    class TextViewModel : ViewModel() {
        val liveData = MediatorLiveData<String>()
        fun test() {
            liveData.postValue("Hello")
        }
    }
}

1，使用ViewModelProvider獲取ViewModel實(shí)例

2，訂閱VIewModel的LiveData

3，調(diào)用ViewModel的方法

構(gòu)造ViewModelProvider過程做了什么

    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }
    public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;

1，當(dāng)我們創(chuàng)建ViewModelProvider的時(shí)候需要傳入一個(gè)ViewModelStoreOwner對象，ViewModelStoreOwner是負(fù)責(zé)提供ViewModelStore對象的，而ComponentActivity實(shí)現(xiàn)了這個(gè)接口，所以我們默認(rèn)傳當(dāng)前的Activity即可

2，首先判斷是否有默認(rèn)的ViewModel工廠，如果沒有就創(chuàng)建一個(gè)Factory

3，F(xiàn)actory是用來創(chuàng)建ViewModel的，ViewModelStore是用來存儲ViewModel的

調(diào)用get()方法是如何構(gòu)建ViewModel

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        //獲取類名
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }
    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        //通過key到ViewModelStore中取
        ViewModel viewModel = mViewModelStore.get(key);
        
        //如果取到ViewModel 代表已經(jīng)創(chuàng)建過這個(gè)ViewModel 直接返回
        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.
            }
        }
        //通過Factor利用反射創(chuàng)建一個(gè)實(shí)例
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
        } else {
            viewModel = mFactory.create(modelClass);
        }
        //把ViewModel實(shí)例存儲到ViewModelStore中
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

1，當(dāng)我們調(diào)用get()方法時(shí)會先獲取類名，然后生成一個(gè)唯一的key

2，先通過key到ViewModelStore中取ViewModel，如果不為空代表已經(jīng)創(chuàng)建過這個(gè)ViewModel的實(shí)例，直接返回這個(gè)實(shí)例

3，如果為空就通過工廠利用java反射機(jī)制創(chuàng)建一個(gè)實(shí)例，通過鍵值對形式保存到ViewModelStore中，返回實(shí)例，看到這里是不是對為什么多個(gè)fragment可以共享同一個(gè)ViewModel的疑問豁然開朗了，因?yàn)镕ragment是依附于Activity之上的，在我們構(gòu)建ViewModelProvider的時(shí)候傳入同一個(gè)Activity，那么ViewModelProvider得到的ViewModelStore是同一個(gè)，在我們調(diào)用get()方法時(shí)就能通過key到ViewModelStore中取到同一個(gè)ViewModel實(shí)例，說白了就是共用Activity的ViewModel

Activity配置發(fā)生改變?nèi)绾尉彺鎂iewModelStore

ActivityThread{
    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) {
            performPauseActivity(r, false, reason, null /* pendingActions */);
        }
        if (!r.stopped) {
            callActivityOnStop(r, true /* saveState */, reason);
        }
        //銷毀Activity
        handleDestroyActivity(r.token, false, configChanges, true, reason);
        //啟動Activity
        handleLaunchActivity(r, pendingActions, customIntent);
    }
}

1，當(dāng)我們切換橫豎屏的時(shí)候，ActivityThread會接收到RELAUNCH_ACTIVITY消息，會調(diào)用自身的handleRelaunchActivityInner()，這個(gè)方法里面有一個(gè)參數(shù)r，類型是ActivityClientRecord，我們每打開一個(gè)Activity，ActivityThread就會生成這么個(gè)對象來記錄我們打開的Activity并保存起來

2，handleRelaunchActivityInner()這個(gè)方法里調(diào)用了handleDestroyActivity()方法去銷毀我們的Activity

ActivityThread{
        @Override
    public void handleDestroyActivity(IBinder token, boolean finishing, int configChanges,
            boolean getNonConfigInstance, String reason) {
        //執(zhí)行銷毀Activity
        ActivityClientRecord r = performDestroyActivity(token, finishing,
                configChanges, getNonConfigInstance, reason);
    }
    ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
            int configChanges, boolean getNonConfigInstance, String reason) {
        //通過token獲取Activity的記錄
        ActivityClientRecord r = mActivities.get(token);
        if (r != null) {
            //是否需要獲取配置實(shí)例
            if (getNonConfigInstance) {
                try {
                    //調(diào)用Activity的retainNonConfigurationInstances方法獲取配置實(shí)例
                    r.lastNonConfigurationInstances
                            = r.activity.retainNonConfigurationInstances();
                } catch (Exception e) {
                }
            }
            r.setState(ON_DESTROY);
        }
        //通過token移除這條Activity的記錄
        synchronized (mResourcesManager) {
            mActivities.remove(token);
        }
        return r;
    }
}

3，handleDestroyActivity()則直接調(diào)用了performDestroyActivity()方法去銷毀Activity，核心部分就是調(diào)用了Activity的retainNonConfigurationInstances()方法獲取了配置實(shí)例并復(fù)制給了ActivityClientRecord，把NonConfigurationInstances實(shí)例保存起來

Activity{
    NonConfigurationInstances retainNonConfigurationInstances() {
        //獲取NonConfigurationInstances對象
        Object activity = onRetainNonConfigurationInstance();
        HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
        FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
        mFragments.doLoaderStart();
        mFragments.doLoaderStop(true);
        ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();
        //創(chuàng)建NonConfigurationInstances實(shí)例
        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;
    }
}
ComponentActivity{
    @Override
    @Nullable
    public final Object onRetainNonConfigurationInstance() {
        Object custom = onRetainCustomNonConfigurationInstance();
        //獲取ViewModelStore
        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;
        }
        //創(chuàng)建NonConfigurationInstances實(shí)例
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }
}

4，通過查閱源碼，一層一層的往下剖析，ActivityThread是通過這樣的調(diào)用鏈來獲取我們的ViewModelStore實(shí)例并保存在ActivityClientRecord中的

Activity重建后如何恢復(fù)ViewModelStore

ActivityThread{
    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) {
            performPauseActivity(r, false, reason, null /* pendingActions */);
        }
        if (!r.stopped) {
            callActivityOnStop(r, true /* saveState */, reason);
        }
        //銷毀Activity
        handleDestroyActivity(r.token, false, configChanges, true, reason);
        //啟動Activity
        handleLaunchActivity(r, pendingActions, customIntent);
    }
}

1，當(dāng)handleDestroyActivity執(zhí)行完畢就已經(jīng)把ViewModelStore的實(shí)例獲取到并存放到ActivityClientRecord中，此時(shí)就開始執(zhí)行handleLaunchActivity()方法來啟動activity

2，handleLaunchActivity()這個(gè)方法也需要ActivityClientRecord這個(gè)參數(shù)，而此時(shí)ActivityClientRecord這個(gè)對象正是經(jīng)過handleDestroyActivity()這個(gè)方法獲取并保存了ViewModelStore 實(shí)例的對象

3，handleLaunchActivity()則調(diào)用了performLaunchActivity()方法來啟動Activity

ActivityThread{
    @Override
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);
        //去啟動Activity
        final Activity a = performLaunchActivity(r, customIntent);
        return a;
    }
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);
            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);
                //調(diào)用當(dāng)前打開的Activity的attach()方法
                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.activity = activity;
            }
            r.setState(ON_CREATE);
            //保存這條Activity記錄
            synchronized (mResourcesManager) {
                mActivities.put(r.token, r);
            }
        } catch (SuperNotCalledException e) {
            throw e;
        } catch (Exception e) {
        }
        return activity;
    }
}

4，通過代碼發(fā)現(xiàn)performLaunchActivity調(diào)用了當(dāng)前正在打開的Activity的attach方法，而這個(gè)方法需要一個(gè)NonConfigurationInstances類型的參數(shù)，這個(gè)參數(shù)里面就有我們的ViewModelStore實(shí)例

Activity{
     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);
        mFragments.attachHost(null /*parent*/);
        //賦值給mLastNonConfigurationInstances
        mLastNonConfigurationInstances = lastNonConfigurationInstances;
        setAutofillOptions(application.getAutofillOptions());
        setContentCaptureOptions(application.getContentCaptureOptions());
    }
    public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;
    }
}
ComponentActivity{
    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) {
            //先去lastNonConfigurationInstance中取，沒有再創(chuàng)建
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }
}

5，在attach方法里就把這個(gè)參數(shù)賦值給mLastNonConfigurationInstances，當(dāng)我們獲取ViewModelStore實(shí)例的時(shí)候，就會先去mLastNonConfigurationInstances中取，如果沒有再自己創(chuàng)建一個(gè)ViewModelStore實(shí)例，到這里整個(gè)調(diào)用鏈就搞明白了

生命周期綁定

ComponentActivity{
    public ComponentActivity(){
        //訂閱生命周期，當(dāng)生命周期==ON_DESTROY，清除ViewModel數(shù)據(jù)
        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();
                    }
                }
            }
        });
    }
}
ViewModelStore{
    public final void clear() {
        //遍歷所有ViewModel并調(diào)用其clear()方法清空數(shù)據(jù)
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        //清空所有ViewModel
        mMap.clear();
    }
}