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Jetpack之ViewModel
ViewModelScope是viewModel的管理者,而ViewModelProvider是ViewModel的间接管理者。
我们一般使用的时候都是ViewModel持有LiveData
使用
我们一般获取ViewModel对象都是使用ViewModelProvider的get()方法。
在Activity或者Fragment 里调用
val viewProvider:ViewModelProvider = ViewModelProvider(MainActivity@this)
val viewModel:MainViewModel = viewProvider .get(MainViewModel::class.java)
首先得到ViewModelProvider对象
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
get方法
public T get(@NonNull Class modelClass) {
//取出modelClass的全限定名
String canonicalName = modelClass.getCanonicalName();
if (canonicalName == null) {
throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
}
// DEFAULT_KEY ="androidx.lifecycle.ViewModelProvider.DefaultKey"
//生成新的Key
return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
最终调用
public T get(@NonNull String key, @NonNull Class 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;
}
可以我们传递的是一个
ViewModelStoreOwner对象
Fragment或者Activity就是这样一个对象,因为他们实现了`
ViewModelStoreOwner接口。
就拿Activity的父类ComponentActivity来举例。
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
HasDefaultViewModelProviderFactory,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
它的getViewModelStore方法实现如下:
@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;
}
看下ViewModelStore的实现
public class ViewModelStore {
// 一个存储ViewModel的hashMap.
private final HashMap<string, viewmodel=""> mMap = new HashMap<>();
//如果当前已经包含ViewModel了,替换就的ViewModel,并回调其onCleared方法。
//这个方法在只在ViewModelProvieder的get方法中调用,当ViewModelProvieder从自己的ViewModelScore中无法获取ViewModel对象时,它会new一个新的,然后加入自己的ViewModelScore中。
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 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();
}
}