Android ViewModel的使用總結(jié)

更新時(shí)間：2021年06月23日 09:41:48 作者：那根筆

ViewModel 是 Jetpack 的一部分。 ViewModel 類旨在以注重生命周期的方式存儲(chǔ)和管理界面相關(guān)的數(shù)據(jù)。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)留存。本文簡(jiǎn)單講解ViewModel的使用

基本使用

MainRepository

class MainRepository {
    suspend fun getNameList(): List<String> {
        return withContext(Dispatchers.IO) {
            listOf("張三", "李四")
        }
    }
}

MainViewModel

class MainViewModel: ViewModel() {
    private val nameList = MutableLiveData<List<String>>()
    val nameListResult: LiveData<List<String>> = nameList
    private val mainRepository = MainRepository()
    
    fun getNames() {
        viewModelScope.launch {
            nameList.value = mainRepository.getNameList()
        }
    }
}

MainActivity

class MainActivity : AppCompatActivity() {
    // 創(chuàng)建 ViewModel 方式 1
    // 通過(guò) kotlin 委托特性創(chuàng)建 ViewModel
    // 需添加依賴 implementation 'androidx.activity:activity-ktx:1.2.3'
    // viewModels() 內(nèi)部也是通過(guò) 創(chuàng)建 ViewModel 方式 2 來(lái)創(chuàng)建的 ViewModel
    private val mainViewModel: MainViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate  (savedInstanceState)
        setContentView(R.layout.activity_main)
        // 創(chuàng)建 ViewModel 方式 2
        val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
        mainViewModel.nameListResult.observe(this, {
            Log.i("MainActivity", "mainViewModel: nameListResult: $it")
            Log.i("MainActivity", "MainActivity: ${this@MainActivity} mainViewModel: $mainViewModel  mainViewModel.nameListResult: ${mainViewModel.nameListResult}")
        })
        mainViewModel.getNames()
    }
}

測(cè)試步驟：打開(kāi)app -> 正?？吹饺罩?/p>

18:03:02.575 : mainViewModel: nameListResult: [張三, 李四]
18:03:02.575 : com.yqy.myapplication.MainActivity@7ffa77 mainViewModel: com.yqy.myapplication.MainViewModel@29c0057  mainViewModel.nameListResult: androidx.lifecycle.MutableLiveData@ed0d744

接著測(cè)試步驟：打開(kāi)設(shè)置更換系統(tǒng)語(yǔ)言 -> 切換到當(dāng)前app所在的任務(wù) 再看日志

18:03:59.622 : mainViewModel: nameListResult: [張三, 李四]
18:03:59.622 : com.yqy.myapplication.MainActivity@49a4455 mainViewModel: com.yqy.myapplication.MainViewModel@29c0057  mainViewModel.nameListResult: androidx.lifecycle.MutableLiveData@ed0d744

神奇！MainActivity 被重建了，而 ViewModel 的實(shí)例沒(méi)有變，并且 ViewModel 對(duì)象里的 LiveData 對(duì)象實(shí)例也沒(méi)變。這就是 ViewModel 的特性。

ViewModel 出現(xiàn)之前，Activity 可以使用 onSaveInstanceState() 方法保存，然后從 onCreate() 中的 Bundle 恢復(fù)數(shù)據(jù)，但此方法僅適合可以序列化再反序列化的少量數(shù)據(jù)（IPC 對(duì) Bundle 有 1M 的限制），而不適合數(shù)量可能較大的數(shù)據(jù)，如用戶信息列表或位圖。 ViewModel 的出現(xiàn)完美解決這個(gè)問(wèn)題。

我們先看看 ViewModel 怎么創(chuàng)建的：通過(guò)上面的實(shí)例代碼，最終 ViewModel 的創(chuàng)建方法是

val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)

我們先看看 ViewModel 怎么創(chuàng)建的：通過(guò)上面的實(shí)例代碼，最終 ViewModel 的創(chuàng)建方法是

val mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)

創(chuàng)建 ViewModelProvider 對(duì)象并傳入了 this 參數(shù)，然后通過(guò) ViewModelProvider#get 方法，傳入 MainViewModel 的 class 類型，然后拿到了 mainViewModel 實(shí)例。

ViewModelProvider 的構(gòu)造方法

    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
    	// 獲取 owner 對(duì)象的 ViewModelStore 對(duì)象
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

ViewModelProvider 構(gòu)造方法的參數(shù)類型是 ViewModelStoreOwner ？ViewModelStoreOwner 是什么？我們明明傳入的 MainActivity 對(duì)象呀！看看 MainActivity 的父類們發(fā)現(xiàn)

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        ...
        // 實(shí)現(xiàn)了 ViewModelStoreOwner 接口
        ViewModelStoreOwner,
        ...{
    private ViewModelStore mViewModelStore;

	// 重寫了 ViewModelStoreOwner 接口的唯一的方法 getViewModelStore()
    @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;
    }

ComponentActivity 類實(shí)現(xiàn)了 ViewModelStoreOwner 接口。奧 ~~ 剛剛的問(wèn)題解決了。

再看看剛剛的 ViewModelProvider 構(gòu)造方法里調(diào)用了 this(ViewModelStore, Factory)，將 ComponentActivity#getViewModelStore 返回的 ViewModelStore 實(shí)例傳了進(jìn)去，并緩存到 ViewModelProvider 中

    public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        // 緩存 ViewModelStore 對(duì)象
        mViewModelStore = store;
    }

接著看 ViewModelProvider#get 方法做了什么

    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> 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);
    }

獲取 ViewModel 的 CanonicalName , 調(diào)用了另一個(gè) get 方法

    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
    	// 從 mViewModelStore 緩存中嘗試獲取
        ViewModel viewModel = mViewModelStore.get(key);
		// 命中緩存
        if (modelClass.isInstance(viewModel)) {
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            // 返回緩存的 ViewModel 對(duì)象
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        // 使用工廠模式創(chuàng)建 ViewModel 實(shí)例
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
        } else {
            viewModel = mFactory.create(modelClass);
        }
        // 將創(chuàng)建的 ViewModel 實(shí)例放進(jìn) mViewModelStore 緩存中
        mViewModelStore.put(key, viewModel);
        // 返回新創(chuàng)建的 ViewModel 實(shí)例
        return (T) viewModel;
    }

mViewModelStore 是啥？通過(guò) ViewModelProvider 的構(gòu)造方法知道 mViewModelStore 其實(shí)是我們 Activity 里的 mViewModelStore 對(duì)象，它在 ComponentActivity 中被聲明。看到了 put 方法，不難猜它內(nèi)部用了 Map 結(jié)構(gòu)。

public class ViewModelStore {
	// 果不其然，內(nèi)部有一個(gè) HashMap
    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();
        }
    }

	// 通過(guò) key 獲取 ViewModel 對(duì)象
    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();
    }
}

到這兒正常情況下 ViewModel 的創(chuàng)建流程看完了，似乎沒(méi)有解決任何問(wèn)題~ 簡(jiǎn)單總結(jié)：ViewModel 對(duì)象存在了 ComponentActivity 的 mViewModelStore 對(duì)象中。第二個(gè)問(wèn)題解決了：ViewModel 的實(shí)例緩存到哪兒了

轉(zhuǎn)換思路 mViewModelStore 出現(xiàn)頻率這么高，何不看看它是什么時(shí)候被創(chuàng)建的呢？

記不記得剛才看 ViewModelProvider 的構(gòu)造方法時(shí) ，獲取 ViewModelStore 對(duì)象時(shí)，實(shí)際調(diào)用了 MainActivity#getViewModelStore() ，而 getViewModelStore() 實(shí)現(xiàn)在 MainActivity 的父類 ComponentActivity 中。

	// ComponentActivity#getViewModelStore()
    @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;
    }

在返回 mViewModelStore 對(duì)象之前調(diào)用了 ensureViewModelStore()

    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();
            }
        }
    }

當(dāng) mViewModelStore == null 調(diào)用了 getLastNonConfigurationInstance() 獲取 NonConfigurationInstances 對(duì)象 nc，當(dāng) nc != null 時(shí)將 mViewModelStore 賦值為 nc.viewModelStore，最終 viewModelStore == null 時(shí)，才會(huì)創(chuàng)建 ViewModelStore 實(shí)例。

不難發(fā)現(xiàn)，之前創(chuàng)建的 viewModelStore 對(duì)象被緩存在 NonConfigurationInstances 中

	// 它是 ComponentActivity 的靜態(tài)內(nèi)部類
    static final class NonConfigurationInstances {
        Object custom;
        // 果然在這兒
        ViewModelStore viewModelStore;
    }

不難發(fā)現(xiàn)，之前創(chuàng)建的 viewModelStore 對(duì)象被緩存在 NonConfigurationInstances 中

	// 它是 ComponentActivity 的靜態(tài)內(nèi)部類
    static final class NonConfigurationInstances {
        Object custom;
        // 果然在這兒
        ViewModelStore viewModelStore;
    }

NonConfigurationInstances 對(duì)象通過(guò) getLastNonConfigurationInstance() 來(lái)獲取的

	// Activity#getLastNonConfigurationInstance
    /**
     * Retrieve the non-configuration instance data that was previously
     * returned by {@link #onRetainNonConfigurationInstance()}.  This will
     * be available from the initial {@link #onCreate} and
     * {@link #onStart} calls to the new instance, allowing you to extract
     * any useful dynamic state from the previous instance.
     *
     * <p>Note that the data you retrieve here should <em>only</em> be used
     * as an optimization for handling configuration changes.  You should always
     * be able to handle getting a null pointer back, and an activity must
     * still be able to restore itself to its previous state (through the
     * normal {@link #onSaveInstanceState(Bundle)} mechanism) even if this
     * function returns null.
     *
     * <p><strong>Note:</strong> For most cases you should use the {@link Fragment} API
     * {@link Fragment#setRetainInstance(boolean)} instead; this is also
     * available on older platforms through the Android support libraries.
     *
     * @return the object previously returned by {@link #onRetainNonConfigurationInstance()}
     */
    @Nullable
    public Object getLastNonConfigurationInstance() {
        return mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.activity : null;
    }

好長(zhǎng)一段注釋，大概意思有幾點(diǎn)：

onRetainNonConfigurationInstance 方法和 getLastNonConfigurationInstance 是成對(duì)出現(xiàn)的，跟 onSaveInstanceState 機(jī)制類似，只不過(guò)它是僅用作處理配置更改的優(yōu)化。
返回的是 onRetainNonConfigurationInstance 返回的對(duì)象

onRetainNonConfigurationInstance 和 getLastNonConfigurationInstance 的調(diào)用時(shí)機(jī)在本篇文章不做贅述，后續(xù)文章會(huì)進(jìn)行解釋。

看看 onRetainNonConfigurationInstance 方法

	/**
	* 保留所有適當(dāng)?shù)姆桥渲脿顟B(tài)
	*/
    @Override
    @Nullable
    @SuppressWarnings("deprecation")
    public final Object onRetainNonConfigurationInstance() {
        // Maintain backward compatibility.
        Object custom = onRetainCustomNonConfigurationInstance();

        ViewModelStore viewModelStore = mViewModelStore;
        // 若 viewModelStore 為空，則嘗試從 getLastNonConfigurationInstance() 中獲取
        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;
            }
        }
		// 依然為空，說(shuō)明沒(méi)有需要緩存的，則返回 null
        if (viewModelStore == null && custom == null) {
            return null;
        }
		
		// 創(chuàng)建 NonConfigurationInstances 對(duì)象，并賦值 viewModelStore
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

到這兒我們大概明白了，Activity 在因配置更改而銷毀重建過(guò)程中會(huì)先調(diào)用 onRetainNonConfigurationInstance 保存 viewModelStore 實(shí)例。在重建后可以通過(guò) getLastNonConfigurationInstance 方法獲取之前的 viewModelStore 實(shí)例。

現(xiàn)在解決了第一個(gè)問(wèn)題：為什么Activity旋轉(zhuǎn)屏幕后ViewModel可以恢復(fù)數(shù)據(jù)

再看第三個(gè)問(wèn)題：什么時(shí)候 ViewModel#onCleared() 會(huì)被調(diào)用

public abstract class ViewModel {
    protected void onCleared() {
    }

    @MainThread
    final void clear() {
        mCleared = true;
        // Since clear() is final, this method is still called on mock objects
        // and in those cases, mBagOfTags is null. It'll always be empty though
        // because setTagIfAbsent and getTag are not final so we can skip
        // clearing it
        if (mBagOfTags != null) {
            synchronized (mBagOfTags) {
                for (Object value : mBagOfTags.values()) {
                    // see comment for the similar call in setTagIfAbsent
                    closeWithRuntimeException(value);
                }
            }
        }
        onCleared();
    }
}

onCleared() 方法被 clear() 調(diào)用了。剛才看 ViewModelStore 源碼時(shí)好像是調(diào)用了 clear() ，回顧一下：

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();
    }
}

onCleared() 方法被 clear() 調(diào)用了。剛才看 ViewModelStore 源碼時(shí)好像是調(diào)用了 clear() ，回顧一下：

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();
    }
}

在 ViewModelStore 的 clear() 中，遍歷 mMap 并調(diào)用 ViewModel 對(duì)象的 clear() ，再看 ViewModelStore 的 clear() 什么時(shí)候被調(diào)用的：

	// ComponentActivity 的構(gòu)造方法
    public ComponentActivity() {
        ... 
        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();
                    }
                }
            }
        });
        ...
    }

觀察當(dāng)前 activity 生命周期，當(dāng) Lifecycle.Event == Lifecycle.Event.ON_DESTROY，并且 isChangingConfigurations() 返回 false 時(shí)才會(huì)調(diào)用 ViewModelStore#clear 。

	// Activity#isChangingConfigurations()
    /**
     * Check to see whether this activity is in the process of being destroyed in order to be
     * recreated with a new configuration. This is often used in
     * {@link #onStop} to determine whether the state needs to be cleaned up or will be passed
     * on to the next instance of the activity via {@link #onRetainNonConfigurationInstance()}.
     *
     * @return If the activity is being torn down in order to be recreated with a new configuration,
     * returns true; else returns false.
     */
    public boolean isChangingConfigurations() {
        return mChangingConfigurations;
    }

isChangingConfigurations 用來(lái)檢測(cè)當(dāng)前的 Activity 是否因?yàn)?Configuration 的改變被銷毀了, 配置改變返回 true，非配置改變返回 false。

總結(jié)，在 activity 銷毀時(shí)，判斷如果是非配置改變導(dǎo)致的銷毀， getViewModelStore().clear() 才會(huì)被調(diào)用。

第三個(gè)問(wèn)題：什么時(shí)候 ViewModel#onCleared() 會(huì)被調(diào)用解決！

以上就是Android ViewModel的使用總結(jié)的詳細(xì)內(nèi)容，更多關(guān)于Android ViewModel的使用的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

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