我们认识中的ViewModel是什么
Viewmodel其实就是activity中一个普通的实体。Activity持有viewmodel的引用,业务逻辑在viewmodel进行,关于界面和ui的操作在activity中进行,而添加和释放资源和viewmodel有关的那么就在viewmodel中设置函数,然后在activity中调用。ViewModel作为Activity/Fragment与其他组件的连接器,负责转换和聚合Model中返回的数据,使这些数据易于显示,并把这些数据改变及时的通知给Activity/Fragment。
ViewModel的生命周期
从图中看出,Activity的生命周期不断变化,经历了被销毁重新创建,而ViewModel的生命周期没有发生变化。
从创建一个viewModel入手分析源码:
MyViewModel myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
进入ViewModelProviders.of(this)方法:
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
return of(activity, null);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) {
Application application = checkApplication(checkActivity(fragment));
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(fragment.getViewModelStore(), factory);
}
@NonNull
@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);
}
checkApplication(activity)会获取该activity所属的application。
ViewModelProvider.AndroidViewModelFactory.getInstance(application)会创建一个单例的AndroidViewModelFactory,并从activity或fragment中获取viewmodelstore,作为两个个参数传入ViewModelProvider构造方法,最后会新建一个ViewModelProvider作为返回。
ViewModelStore代码实现如下:
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();
}
}
ViewModelStore类通过hashmap来存储和移除Viewmodel。
AndroidViewModelFactory类代码如下:
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);
}
}
ViewModel是抽象类,AndroidViewModel继承于ViewModel,AndroidViewModelFactory从它的create方法可以看出,通过反射生成ViewModel的实现类。
get(MyViewModel.class)部分
进入到了ViewModelProvider进行执行,ViewModelProvider代码如下:
public class ViewModelProvider {
private static final String DEFAULT_KEY =
"androidx.lifecycle.ViewModelProvider.DefaultKey";
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}
@NonNull
@MainThread
public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
String canonicalName = modelClass.getCanonicalName(); //注释1
if (canonicalName == null) {
throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
}
return get(DEFAULT_KEY + ":" + canonicalName, modelClass); //注释2
}
@NonNull
@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;
}
}
过程分析:注释1处得到类的名称,对这个名称进行字符串拼接,拼接上前缀androidx.lifecycle.ViewModelProvider.DefaultKey,作为注释2处方法的参数,作为key值去ViewModelStore中获取viewmodel实例。如果ViewModel能转换为modelClass类的对象,直接返回该ViewModel。否则就会通过AndroidViewModelFactory的create方法通过反射创建一个ViewModel,并将其存储到ViewModelStore中。
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