ViewModel
定義
- 視圖模型,以感覺生命周期的形式來存儲和管理視圖相關的資料,讓資料可在發生螢幕旋轉等配置更改後繼續留存;
特點
- 頁面資料丢失(轉屏、閃退等生命周期重建現象),可以使用onSaveInstanceState()儲存資料,單僅适用于資料量少(IPC對Bundle有1M的限制),且需要支援序列化,而ViewModel對資料量和序列化均沒有要求(ViewModel生命周期長于Activity);
- 有效的将邏輯代碼和視圖控制器分開,防止視圖控制器臃腫;
- 邏輯層往往持有UI層引用并進行異步調用,而UI需要管理這些請求,確定界面銷毀後不會存在記憶體洩露,而ViewModel可以避免(ViewModel不持有UI層引用);
生命周期
- ViewModelScope
- Activity created
- onCreated()
- onStart()
- onResume()
- Activity rotated
- onPause()
- onStop()
- onDestroy()
- onCreated()
- onStart()
- onResume()
- Activity finish()
- onPause()
- onStop()
- onDestroy()
- onCleared
簡單使用
//1. 自定義ViewModel
//class MyViewModel(application: Application) : AndroidViewModel(application) {
class UserViewModel : ViewModel() {
var userName: MutableLiveData<String> = MutableLiveData()
var isLoading: MutableLiveData<Boolean> = MutableLiveData()
//2. 擷取資料的異步請求
fun getUserName() {
isLoading.value = true
GlobalScope.launch {
delay(1000)
//3.使用LiveData将資料抛出
isLoading.postValue(false)
userName.postValue("歡迎關注公衆号:今陽說")
}
}
}
class ViewModelActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_view_model)
//4. 擷取ViewModel執行個體
val userViewModel: UserViewModel = ViewModelProvider(
this, ViewModelProvider.NewInstanceFactory()
// this, ViewModelProvider.AndroidViewModelFactory(application)
).get(UserViewModel::class.java)
//5. 觀察ViewModel中的LiveData資料,更新UI
userViewModel.userName.observe(this, Observer {
tv_view_model.text=it
})
userViewModel.isLoading.observe(this, Observer {
progressBar.visibility=if (it) View.VISIBLE else View.GONE
})
userViewModel.getUserName()
}
}
- 還可以用來在不同Fragment/Activity之間共享資料,類似于RxBus等事件總線,都是基于觀察者模式
class SharedViewModel :ViewModel(){
var isReadFinish: MutableLiveData<Boolean> = MutableLiveData()
}
class DetailFragment : Fragment(){
fun onReadFinish(){
val viewModel=ViewModelProvider(
this, ViewModelProvider.NewInstanceFactory()
).get(SharedViewModel::class.java)
viewModel.isReadFinish.value=true
}
}
class ListFragment : Fragment(){
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
val viewModel=ViewModelProvider(
this, ViewModelProvider.NewInstanceFactory()
).get(SharedViewModel::class.java)
viewModel.isReadFinish.observe(this, Observer {
//更新清單UI
})
}
}
ViewModel原了解析
ViewModel類
public abstract class ViewModel {
// Can't use ConcurrentHashMap, because it can lose values on old apis (see b/37042460)
@Nullable
private final Map<String, Object> mBagOfTags = new HashMap<>();
private volatile boolean mCleared = false;
/**
* This method will be called when this ViewModel is no longer used and will be destroyed.
* <p>
* It is useful when ViewModel observes some data and you need to clear this subscription to
* prevent a leak of this ViewModel.
*/
@SuppressWarnings("WeakerAccess")
protected void onCleared() {
//在ViewModel将被清除時調用
//當ViewModel觀察了一些資料,可以在這裡做解注冊 防止記憶體洩漏
}
//在ViewModel将被清除時調用
@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();
}
...
}
ViewModelProvider類
- 使用ViewModel時我們是通過ViewModelProvider擷取其執行個體對象的,那麼我們來看一下ViewModelProvider的構造方法;
- 由下面代碼可以看出就是通過反射生成ViewModel的實作類;
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}
ViewModelProvider如何通過ViewModelStore實作配置更改重建後ViewModel依然存在的
- 上面ViewModelProvider構造方法的第一個參數通過owner.getViewModelStore()擷取ViewModelStore對象,那麼我們來看一下ComponentActivity中對ViewModelStoreOwner的實作:
@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.");
}
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;
}
- 上面代碼先從NonConfigurationInstance從擷取ViewModelStore執行個體,如果不存在,就new ViewModelStore();
- 而在onRetainNonConfigurationInstance()方法中會把mViewModelStore指派給NonConfigurationInstances;
- 在Activity因配置改變 而正要銷毀時,且新Activity會立即建立,那麼系統就會調用此方法。
- 也就說配置改變時系統把viewModelStore存在了NonConfigurationInstances中。
@Override
@Nullable
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;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
- 可以看到getViewModelStore和onRetainNonConfigurationInstance中都是通過getLastNonConfigurationInstance擷取NonConfigurationInstances對象的
@Nullable
public Object getLastNonConfigurationInstance() {
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}
- 那麼mLastNonConfigurationInstances是哪來的呢?是Activity的attach方法
- attach方法是為Activity關聯上下文環境,是在Activity 啟動的核心流程——ActivityThread的performLaunchActivity方法中調用,這裡的lastNonConfigurationInstances是存在 ActivityClientRecord中的一個元件資訊;
- 那麼ActivityThread 中的 ActivityClientRecord 是不受 activity 重建的影響,那麼ActivityClientRecord中lastNonConfigurationInstances也不受影響,那麼其中的Object activity也不受影響,那麼ComponentActivity中的NonConfigurationInstances的viewModelStore不受影響,那麼viewModel也就不受影響了。
final void attach(Context context, ActivityThread aThread, ...
NonConfigurationInstances lastNonConfigurationInstances,... ) {
...
mLastNonConfigurationInstances = lastNonConfigurationInstances;
...
}
ViewModelProvider的Factory如何使用
- ViewModelProvider構造方法的第二個入參Factory,在ViewModelProvider中已經實作了兩個供我們使用:NewInstanceFactory和AndroidViewModelFactory
/**
* Simple factory, which calls empty constructor on the give class.
*/
public static class NewInstanceFactory implements Factory {
@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);
}
}
}
/**
* {@link Factory} which may create {@link AndroidViewModel} and
* {@link ViewModel}, which have an empty constructor.
*/
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
private static AndroidViewModelFactory sInstance;
/**
* Retrieve a singleton instance of AndroidViewModelFactory.
*
* @param application an application to pass in {@link AndroidViewModel}
* @return A valid {@link AndroidViewModelFactory}
*/
@NonNull
public static AndroidViewModelFactory getInstance(@NonNull Application application) {
if (sInstance == null) {
sInstance = new AndroidViewModelFactory(application);
}
return sInstance;
}
private Application mApplication;
/**
* Creates a {@code AndroidViewModelFactory}
*
* @param application an application to pass in {@link AndroidViewModel}
*/
public AndroidViewModelFactory(@NonNull Application application) {
mApplication = application;
}
@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);
}
}
- 如果需要參數,也可以自己實作Factory,例如:
public class MainViewModelFactory implements ViewModelProvider.Factory {
private String mValue;
public MainViewModelFactory(String value){
mValue = value;
}
@NonNull
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
try {
Class[] parameterTypeArray = new Class[]{String.class};
return modelClass.getConstructor(parameterTypeArray).newInstance(mValue);
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
return null;
}
}
//或者多個參數的
class ParametrizedFactory implements ViewModelProvider.Factory {
private final Object[] mConstructorParams;
ParametrizedFactory(Object... constructorParams) {
mConstructorParams = constructorParams;
}
@Override
public <T extends ViewModel> T create(Class<T> modelClass) {
if (modelClass == null) {
throw new IllegalArgumentException("Target ViewModel class can not be null")
}
Log.w("ParametrizedFactory", "Don't use callbacks or Context parameters in order to avoid leaks!!")
try {
if (mConstructorParams == null || mConstructorParams.length == 0) {
return modelClass.newInstance();
} else {
Class<?>[] classes = new Class<?>[mConstructorParams.length];
for (int i = 0; i < mConstructorParams.length; i++) {
classes[i] = mConstructorParams[i].getClass();
}
return modelClass.getConstructor(classes).newInstance(mConstructorParams);
}
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
return null;
}
}
ViewModelProvider的get()是如何擷取ViewModel的
- 下面讓我們再看一下ViewModelProvider的get()是如何實作的
@NonNull
@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);
}
//帶key建立ViewModel,會讓使用相同key的Activity或者Fragment,建立的ViewModel資料獨立
@NonNull
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
//1.通過key在ViewModelStore中取得ViewModel;
//2.如果這個ViewModel能轉換為modelClass類的對象,直接傳回該ViewModel;
//3.否則會通過Factory建立一個ViewModel,并将其存儲到ViewModelStore中;
ViewModel viewModel = mViewModelStore.get(key);
if (modelClass.isInstance(viewModel)) {
//noinspection unchecked
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);
//noinspection unchecked
return (T) viewModel;
}
ViewModelStore類
- 上面提到将ViewModel存儲到ViewModelStore中,那麼我們來看一下是如何存儲的
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();
}
}
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