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本篇內(nèi)容主要講解“Android開發(fā)Jetpack組件ViewModel如何使用”,感興趣的朋友不妨來(lái)看看。本文介紹的方法操作簡(jiǎn)單快捷,實(shí)用性強(qiáng)。下面就讓小編來(lái)帶大家學(xué)習(xí)“Android開發(fā)Jetpack組件ViewModel如何使用”吧!
學(xué)習(xí)ViewModel之前首先我們得簡(jiǎn)單了解下MVP和MVVM,因?yàn)閂iewModel是MVVM中的一個(gè)元素
MVP
MVVM
在MVP中View想要調(diào)用Model數(shù)據(jù)層,需要經(jīng)過(guò)中間層Presenter, 這樣就實(shí)現(xiàn)了View和Model的解耦,這也是MVP和MVC的差別; 但是如果一個(gè)Activity中有太多交互,那么我們的View接口數(shù)量就會(huì)很龐大達(dá)到十幾個(gè)也不足為奇,并且在View層調(diào)用了Presenter之后,會(huì)反過(guò)來(lái)調(diào)用View層,這樣顯得繁瑣;而MVVM的出現(xiàn)就解決了這個(gè)問(wèn)題
說(shuō)到MVVM的話,我們放上Google的架構(gòu)圖
MVVM中的VM指的就是ViewModel; 從上圖為沒看到,因?yàn)閂iewModel中持有了LiveData,而LiveData是一個(gè)可觀察的數(shù)據(jù)類型,在LiveData原理篇中,我們做了詳細(xì)的分析;在View層中,將被觀察的數(shù)據(jù)LiveData訂閱,并提供了一個(gè)觀察者Observer,當(dāng)數(shù)據(jù)發(fā)生變化的時(shí)候,就會(huì)回調(diào)Observer中的onChanged()方法,從而更新UI, 這個(gè)過(guò)程是系統(tǒng)源碼幫我們處理的,所以就沒有上面Presenter中調(diào)用View的那一步了
應(yīng)用的某個(gè) Activity 中可能包含用戶列表,因配置更改而重新創(chuàng)建 Activity 后,新 Activity 必須重新提取用戶列表; 對(duì)于簡(jiǎn)單的數(shù)據(jù),Activity 可以使用onSaveInstanceState() 方法從 onCreate() 中的捆綁包恢復(fù)其數(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),而不適合數(shù)量可能較大的數(shù)據(jù),如用戶列表或位圖,使用ViewModel可以解決這個(gè)問(wèn)題
另外,界面控制器經(jīng)常需要進(jìn)行異步調(diào)用,這些調(diào)用可能需要一些時(shí)間才能返回結(jié)果; 界面控制器需要管理這些調(diào)用,并確保系統(tǒng)在其銷毀后清理這些調(diào)用以避免潛在的內(nèi)存泄露;此項(xiàng)管理需要大量的維護(hù)工作,并且在因配置更改而重新創(chuàng)建對(duì)象的情況下,會(huì)造成資源的浪費(fèi),因?yàn)閷?duì)象可能需要重新發(fā)出已經(jīng)發(fā)出過(guò)的調(diào)用,使用ViewModel可以解決這個(gè)問(wèn)題
諸如 Activity 和 Fragment 之類的界面控制器主要用于顯示界面數(shù)據(jù)、對(duì)用戶操作做出響應(yīng)或處理操作系統(tǒng)通信(如權(quán)限請(qǐng)求); 如果要求界面控制器也負(fù)責(zé)從數(shù)據(jù)庫(kù)或網(wǎng)絡(luò)加載數(shù)據(jù),那么會(huì)使類越發(fā)膨脹。為界面控制器分配過(guò)多的責(zé)任可能會(huì)導(dǎo)致單個(gè)類嘗試自己處理應(yīng)用的所有工作,而不是將工作委托給其他類;以這種方式為界面控制器分配過(guò)多的責(zé)任也會(huì)大大增加測(cè)試的難度
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)存在
ViewModel的使用比較簡(jiǎn)單,我們想要使用使用的話直接繼承ViewModel或者繼承AndroidViewModel即可; AndroidViewModel源碼如下,他們倆的區(qū)別,是AndroidViewModel中多了一個(gè)Application的成員變量以及以Application為參數(shù)的構(gòu)造方法,如果你需要Application的話,就直接繼承AndroidViewModel即可
public class AndroidViewModel extends ViewModel { @SuppressLint("StaticFieldLeak") private Application mApplication; public AndroidViewModel(@NonNull Application application) { mApplication = application; } /** * Return the application. */ @SuppressWarnings({"TypeParameterUnusedInFormals", "unchecked"}) @NonNull public <T extends Application> T getApplication() { return (T) mApplication; } }
這里以我寫的一個(gè)Demo為例,這個(gè)Demo可以在Github上找到,鏈接如下JetPack Demo,這個(gè)Demo用到了所有Jetpack的組件,是學(xué)習(xí)Jetpack的輔助資料,需要的小伙伴可以下載和star。我們自定義一個(gè)AppsViewModel
如下:
class AppsViewModel(appsRepository: AppsRepository) : ViewModel() { val apps: LiveData<List<AppEntity>> = appsRepository.loadApps() }
因?yàn)槲覀冞@里傳入了一個(gè)參數(shù),所以需要定義一個(gè)Factory,代碼如下:
class AppsViewModelFactory(private val repository: AppsRepository) : ViewModelProvider.Factory { override fun <T : ViewModel?> create(modelClass: Class<T>): T { return AppsViewModel(repository) as T } }
接下來(lái)就是使用了:
class AppsListFragment : Fragment() { private lateinit var viewModel: AppsViewModel //-----1----- private val viewModel: AppsViewModel by viewModels { FactoryProvider.providerAppsFactory(requireContext()) } override fun onActivityCreated(savedInstanceState: Bundle?) { //-----2----- viewModel = ViewModelProviders.of(this,FactoryProvider.providerAppsFactory(requireContext())) .get(AppsViewModel::class.java) //-----3----- viewModel.apps.observe(viewLifecycleOwner, Observer { //Update UI }) } }
我們先聲明了一個(gè)變量viewModel,我們可以通過(guò)注釋1處的 by viewModels提供一個(gè)自定義的Factory,但是需要添加一個(gè)依賴:implementation "androidx.fragment:fragment-ktx:1.2.2;或者采用注釋2的方式直接使用ViewModelProviders.of(fragment,factory).get(class)的形式獲取實(shí)例。 然后就直接使用了,在注釋3處使用viewmodel.apps.observe將其加入生命周期觀察中,如果數(shù)據(jù)發(fā)生變化就會(huì)調(diào)用Observer的回調(diào),從而更新UI
上面我們采用ViewModelProviders.of(...).get(class)
方法獲取ViewModel,我們就從這里開始源碼開始分析,我們先看下這個(gè)類的源碼:
@Deprecated public class ViewModelProviders { @Deprecated public ViewModelProviders() { } @Deprecated @NonNull @MainThread public static ViewModelProvider of(@NonNull Fragment fragment) { return new ViewModelProvider(fragment); } @Deprecated @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity) { return new ViewModelProvider(activity); } @Deprecated @NonNull @MainThread public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) { if (factory == null) { factory = fragment.getDefaultViewModelProviderFactory(); } return new ViewModelProvider(fragment.getViewModelStore(), factory); } @Deprecated @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory) { if (factory == null) { factory = activity.getDefaultViewModelProviderFactory(); } return new ViewModelProvider(activity.getViewModelStore(), factory); } @SuppressWarnings("WeakerAccess") @Deprecated public static class DefaultFactory extends ViewModelProvider.AndroidViewModelFactory { @Deprecated public DefaultFactory(@NonNull Application application) { super(application); } } }
我們看到此類中提供了四個(gè)方法,其實(shí)著四個(gè)都以一樣,我們以第四個(gè)為例分析;第一個(gè)參數(shù)是Activity,第二個(gè)參數(shù)是一個(gè)Factory,默認(rèn)情況下我們是不需要傳入Factory這個(gè)參數(shù)的。如果不傳入的話,我們跟進(jìn)構(gòu)造方法就能看到默認(rèn)是用NewInstanceFactory來(lái)作為Factory的,看到內(nèi)部的create方法通過(guò)反射生成了一個(gè)ViewModel,實(shí)例源碼如下:
public static class NewInstanceFactory implements Factory { private static NewInstanceFactory sInstance; /** * Retrieve a singleton instance of NewInstanceFactory. * * @return A valid {@link NewInstanceFactory} */ @NonNull static NewInstanceFactory getInstance() { if (sInstance == null) { sInstance = new NewInstanceFactory(); } return sInstance; } @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); } } }
如果傳入Factory參數(shù)的話,就會(huì)用我們自定義的Factory作來(lái)生成ViewModel。
of
方法調(diào)用結(jié)束返回的是ViewModelProvider
, 然后調(diào)用的是get
方法,我們看下這個(gè)類的部分源碼:
public class ViewModelProvider { private static final String DEFAULT_KEY ="androidx.lifecycle.ViewModelProvider.DefaultKey"; private final Factory mFactory; private final ViewModelStore mViewModelStore; //-----1----- public ViewModelProvider(@NonNull ViewModelStoreOwner owner) { this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory() : NewInstanceFactory.getInstance()); } public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) { this(owner.getViewModelStore(), factory); } public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { mFactory = factory; mViewModelStore = store; } 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); } 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; } }
上面截取的部分源碼其實(shí)是ViewModelProvider最重要的方法了,我們先看此類中有兩個(gè)重要的成員變量,其中一個(gè)是mFactory, 注釋1處看到如果我們沒有傳入factory的話,默認(rèn)實(shí)現(xiàn)的是NewInstanceFactory, 印證了我們之前的說(shuō)法。還有一個(gè)是ViewModelStore, 它是怎么來(lái)的呢?因?yàn)镃omponentActivity中實(shí)現(xiàn)了接口ViewModelStoreOwner,在ViewModelProvider的構(gòu)造方法中調(diào)用owner.getViewModelStore(),這個(gè)owner就是ComponentActivity自身,然后獲取到了ViewModelStore這個(gè)變量,實(shí)際調(diào)用的源碼如下:
@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."); } //-----1----- if (mViewModelStore == null) { NonConfigurationInstances nc = (NonConfigurationInstances) getLastNonConfigurationInstance(); if (nc != null) { // Restore the ViewModelStore from NonConfigurationInstances mViewModelStore = nc.viewModelStore; } if (mViewModelStore == null) { mViewModelStore = new ViewModelStore(); } } return mViewModelStore; }
在注釋1處,判斷如果mViewModelStore == null的話,就會(huì)調(diào)取getLastNonConfigurationInstance嘗試獲取,如果獲取到了就將獲取到的賦值給mViewModelStore返回。這里就涉及到一個(gè)重要的知識(shí)點(diǎn)了,為什么說(shuō)ViewModel在橫豎屏切換的時(shí)候能夠持久的保存數(shù)據(jù),不需要像之前一樣調(diào)用onSaveInstanceState? 因?yàn)樵贏ctivity被銷毀的時(shí)候,還會(huì)調(diào)用另外一個(gè)方法onRetainNonConfigurationInstance, 我們看它在ComponentActivity中的源碼實(shí)現(xiàn):
public final Object onRetainNonConfigurationInstance() { 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; } //----1----- NonConfigurationInstances nci = new NonConfigurationInstances(); nci.custom = custom; nci.viewModelStore = viewModelStore; return nci; }
我們看到在注釋1的地方將我們之前存在的viewModelStore存儲(chǔ)到NonConfigurationInstances中了,然后在調(diào)用getViewModelStore的時(shí)候調(diào)用getLastNonConfigurationInstance這樣就保證了Activity銷毀之前和之后的viewModelStore是同一個(gè),那它里面存儲(chǔ)的ViewModel值也就是同樣的了。所以ViewModel的生命周期可以用下圖來(lái)概括:
接下來(lái)我們分析get
方法:
private static final String DEFAULT_KEY = "androidx.lifecycle.ViewModelProvider.DefaultKey" 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); }
首先獲取類的全稱的字符串名字,和DEFAULT_KEY
拼湊成一個(gè)Key,然后調(diào)用get
的重載方法如下:
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) { ViewModel viewModel = mViewModelStore.get(key); //-----1----- 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 { //-----2----- viewModel = (mFactory).create(modelClass); } mViewModelStore.put(key, viewModel); return (T) viewModel; }
注釋1處判斷我們的modelClass是不是屬于ViewModel類型的,并且判斷mFactory的類型是否屬于OnRequeryFactory類型,如果是的話,就返回值; 在注釋2處使用Factory通過(guò)反射創(chuàng)建一個(gè)viewModel, 然后將其存入mViewModelStore中。我們看下ViewModelStore的源碼:
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(); } }
ViewModelStore
的源碼很簡(jiǎn)單,內(nèi)部持有了一個(gè)HashMap對(duì)象,用來(lái)存放ViewModel。ViewModel的創(chuàng)建到此就結(jié)束了。然后就是使用的問(wèn)題, 使用如下
override fun onActivityCreated(savedInstanceState: Bundle?) { viewModel = ViewModelProviders.of(this,FactoryProvider.providerAppsFactory(requireContext())) .get(AppsViewModel::class.java) viewModel.apps.observe(viewLifecycleOwner, Observer { //Update UI }) }
到此,相信大家對(duì)“Android開發(fā)Jetpack組件ViewModel如何使用”有了更深的了解,不妨來(lái)實(shí)際操作一番吧!這里是億速云網(wǎng)站,更多相關(guān)內(nèi)容可以進(jìn)入相關(guān)頻道進(jìn)行查詢,關(guān)注我們,繼續(xù)學(xué)習(xí)!
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