ViewModel
ViewModel的职责是为Activity或者Fragment管理数据.
如何获取一个ViewModel?
继承ViewModel或者AndroidViewModel
class MyViewModel : ViewModel(){
}
//如果你需要在ViewModel中使用上下文对象,可以继承AndroidViewModel
class MyViewModel2(var application : Application) : AndroidViewModel(){
}
如果继承AndroidViewModel,构造函数必须有一个Application类型的参数,并且只能是Application,而不是其子类.
在Activity或者Fragment中获取MyViewModel实例对象
val myViewModel = ViewModelProviders.of(activity/fragment).get(MyViewModel::class.java)
ViewModel的唯一性
如果一个Activity/Fragment未被销毁,那么多次获取ViewModel,得到的是同一个实例对象.从源码中去分析这个逻辑. 先看ViewModelPoviders.of()这个方法,返回一个ViewModelProvider对象
public static ViewModelProvider of(@NonNull Fragment fragment) {
return of(fragment, null);
}
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
return of(activity, null);
}
这里只看Activity的
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(ViewModelStores.of(activity), factory);
}
看一下ViewModelProvider这个构造函数
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
this.mViewModelStore = store;
}
当获取到ViewModelProvider对象后,调用其get方法获取到MyViewModel的实例对象看一下get方法
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) {
//从ViewModelStore中获取ViewModel对象
ViewModel viewModel = mViewModelStore.get(key);
if (modelClass.isInstance(viewModel)) {
//noinspection unchecked
return (T) viewModel;
} else {
//noinspection StatementWithEmptyBody
if (viewModel != null) {
// TODO: log a warning.
}
}
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
}
从这里可以看到是从ViewModelStore中获取ViewModel对象.在ViewModelStore中封装了一个HashMap对象.如果HashMap对象中存储了ViewModel对象,就直接复用,如果ViewModel对象未创建,就重新创建后存储在HashMap中.
要保证ViewModel是唯一的,ViewModelStore就必须是唯一的.看一下ViewModelStore是如果获取到的.在给ViewModelProvider传参数时调用了ViewModelStores.of(activity),看一下这一步做了什么?
//ViewModelStores.java
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
return holderFragmentFor(activity).getViewModelStore();
}
查看安卓源码发现,support包下的Fragment和FragmentActivity都继承了ViewModelStoreOwner,自然要实现其方法.
//FragmentActivity
public ViewModelStore getViewModelStore() {
if (this.getApplication() == null) {
throw new IllegalStateException(“Your activity is not yet attached to the Application instance. You can’t request ViewModel before onCreate call.”);
} else {
if (this.mViewModelStore == null) {
FragmentActivity.NonConfigurationInstances nc = (FragmentActivity.NonConfigurationInstances)this.getLastNonConfigurationInstance();
if (nc != null) {
this.mViewModelStore = nc.viewModelStore;
}
if (this.mViewModelStore == null) {
this.mViewModelStore = new ViewModelStore();
}
}
return this.mViewModelStore;
}
}
可以看出,对于一个未销毁的Activity或者Fragment,其ViewModelStore对象是唯一的.那么其存储的ViewModel对象也是唯一的.
ViewModel的特点
由上面可知,Activity或者Fragment未被销毁,ViewModel是唯一的,那么其保存的数据也是不变的,当Activity发生了屏幕旋转等变化时,仍旧可以复用ViewModel中的数据.
不要持有View层的引用
ViewModel内部不要持有View层的引用,比如Activity或者Fragment.可以和LiveData结合使用
生命周期
由图可看出,Activity重建前后,ViewModel保持不变.
LiveData
LiveData持有数据,并可在给定的生命周期内被观测.继承关系如下:[LiveData] ^- [MutableLiveData] ^- [MediatorLiveData MutableLiveData对外暴露setValue和postValue.MediatorLiveData观察别的LiveData.并能将active/inactive状态传递给它所观察的LiveData对象.
postValue与setValue
postValue可以从后台线程更新数据,setValue只能在主线程中更新数据.如果同时调用这两个方法,postValue设置的数据会覆盖code>setValue设置的数据.postValue是通过ArchTaskExecutor来实现在主线程中更新数据.在ArchTaskExecutor中使用主线程的Handler.
observeForever与removeObserver
如果一个观察者通过observeForever被添加,这个观察者会被认为一直处于激活状态,任何数据更新都会立即通知到它.因此需要手动调用removeObserver去移除它.
onActive与onInactive
Lifecycle
v4包下的SupportActivity和Fragment默认都实现了LifecycleOwner(<font color=’#ff0000′ size=’6px’>API27+</font>),并且都默认初始化了lifeCycle实例
//LifecycleOwner
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
//supportActivity
public class SupportActivity extends Activity implements LifecycleOwner, Component {
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
public Lifecycle getLifecycle() {
return this.mLifecycleRegistry;
}
}
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
ViewModelStoreOwner {
LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
public class LifecycleRegistry extends Lifecycle {
//………
}
google官方MVVM架构图
由图可以看出,不管数据从哪里来,最终要汇聚到数据仓库,LiveData观察的是数据仓库的变化.并且ViewModel中可以定义多个LiveData.
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