ViewModel的使用
引入ViewModel
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test:runner:1.2.0'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
implementation 'androidx.lifecycle:lifecycle-extensions:2.1.0'
}
简单使用
public class NameViewModel extends ViewModel {
//liveData通常和viewModel一起使用
private MutableLiveData<String> mCurrentName;
public MutableLiveData<String> getmCurrentName() {
if (mCurrentName == null) {
mCurrentName = new MutableLiveData<String>();
}
return mCurrentName;
}
//如果需要可以在这里释放资源
@Override
protected void onCleared() {
super.onCleared();
}
}
使用ViewModel在两个fragment之间进行通信
public class FragmentOne extends Fragment {
private EditText edContent;
private Button btnSend;
private NameViewModel model;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_one, container, false);
edContent = view.findViewById(R.id.et_content);
btnSend = view.findViewById(R.id.btn_send);
//获取viewModel
model = ViewModelProviders.of(getActivity()).get(NameViewModel.class);
btnSend.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
//获取到liveData后设置liveData的值 model.getmCurrentName().setValue(edContent.getText().toString());
}
});
return view;
}
}
public class FragmentTwo extends Fragment {
private TextView textName;
private NameViewModel model;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_two, container, false);
textName = view.findViewById(R.id.tv_text);
//获取viewModel
model = ViewModelProviders.of(getActivity()).get(NameViewModel.class);
//监听值的变化
model.getmCurrentName().observe(getActivity(), new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
//更新UI
textName.setText(s);
}
});
return view;
}
}
我们在fragmentOne随便输入一个数据然后发送，fragmentTwo会立刻收到数据变化实时更新。并且我们旋转屏幕后发现显示的数据不会发生变化。这个就不得不提ViewModel的生命周期了，它只有在Activity销毁之后，它才会自动销毁。下面引用一下谷歌官方的图片，将ViewModel的生命周期展示的淋漓尽致。
1018039-824fdaac57bd11f1 (1)(1).png
下面我们分析一下ViewModel的源码
我们从ViewModelProviders.of(getActivity()).get(NameViewModel.class)开始分析
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
return of(activity, null);
}
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(activity.getViewModelStore(), factory);
}
of方法主要是提供了一个ViewModelProvider对象，需要一个参数为Factory对象。Factory通过ViewModelProvider.AndroidViewModelFactory.getInstance(application)获取
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);
}
}
AndroidViewModelFactory通过构造方法给ViewModel带入Application，就可以在ViewModel里面拿到Context，因为Application是APP全局的，那么不存在内存泄露的问题，完美解决了有些ViewModel里面需要Context引用，但是又担心内存泄露的问题。
下面我们继续ViewModelProviders.of(this)方法继续分析吧，注意最后一句new ViewModelProvider(activity.getViewModelStore()， factory);第一个参数会调用activity的getViewModelStore()方法(这个方法会返回ViewModelStore，这个类是拿来存储ViewModel的,下面会说到),这里的activity是androidx.fragment.app.FragmentActivity，看一下这个getViewModelStore()方法。
@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) {
//获取最近一次横竖屏切换时保存下来的数据
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
//静态变量，常驻内存
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
Android横竖屏切换时会触发onSaveInstanceState()，而还原时会调用onRestoreInstanceState()，但是Android的Activity类还有2个方法名为onRetainNonConfigurationInstance()和getLastNonConfigurationInstance()这两个方法。
/**
保留所有fragment的状态。你不能自己覆写它！如果要保留自己的状态，请使用onRetainCustomNonConfigurationInstance（）
*/
@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;
}
@Nullable
public Object getLastNonConfigurationInstance() {
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}
Activity在横竖屏切换时悄悄保存了viewModelStore,放到了NonConfigurationInstances实例里面，横竖屏切换时保存了又恢复了回来，相当于ViewModel实例就还在啊，也就避免了横竖屏切换时的数据丢失。
下面我们来到那句构建ViewModel代码的后半段，它是ViewModelProvider的get()方法,看看实现,其实很简单。
@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);
}
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
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;
}
上面代码很好理解，先从一个缓存容器mViewModelStore中找有没有相对应的ViewModel，找到了就直接使用，没有就通过Factory的create()方法创造一个viewModel并且缓存起来。create()方法在上面讲Factory的时候有讲到。
总结：
ViewModel的源码其实并不多，主要是官方ComponentActivity提供了技术实现，onRetainNonConfigurationInstance（）保存状态，getLastNonConfigurationInstance()恢复。
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