1. Viewmode优势
ViewModel旨在以注重生命周期的方式存储和管理界面相关的数据(配合它里面的livedata)。
1.1 将Activity的UI处理和数据处理分离,分开管理,解耦且高效。
1.2 ViewModel在屏幕旋转等系统配置更改后被继续保留,避免再次请求数据,浪费网络资源。重建该 Activity时,它接收的ViewModel实例与之前的Activity持有的ViewModel相同。
只有当Activity真正销毁时,框架才会调用getViewModelStore().clear()清除所有的ViewModel。
1.3 避免页面销毁后,数据返回后刷新界面导致crash,例如页面发起请求后,数据还没返回就关闭activity,数据返回后,刷新界面,因view不存在而crash。
1.4 两个Fragment可以使用其Activity的ViewModel来处理通信。
1.5 和onSaveInstanceState()对比,onSaveInstanceState()仅适合可以序列化再反序列化的少量数据,而不适合数量可能较大的数据,如用户列表或位图。
1.6 ViewModelScope,为应用中的每个ViewModel定义了ViewModelScope。如果ViewModel已清除,则在此范围内启动的协程都会自动取消。
2. 传递数据到ViewModel中
// 创建一个ViewModel类,带有参数
class ViewModelDemo(var str:String) : ViewModel() {
var liveData = MutableLiveData<Int>(4)
}
//通过ViewModelProvider的工厂类创建一个带有参数的ViewModel
var viewModelDemo = ViewModelProvider(this, object : ViewModelProvider.Factory {
override fun <T : ViewModel?> create(modelClass: Class<T>): T {
return ViewModelDemo("test") as T
}
}).get(ViewModelDemo::class.java)
3. ViewModelProvider获取ViewModel
3.1 ViewModelProvider构造方法
//通过构造方法的调用链,我们可以看到最终都是调用了第三个构造方法
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
ContextAware,
LifecycleOwner,
ViewModelStoreOwner,
HasDefaultViewModelProviderFactory,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner,
ActivityResultRegistryOwner,
ActivityResultCaller
ViewModelStoreOwner:是一个接口,ComponentActivity和Fragment实现了这个接口,所以我们在Activity或者Fragment中使用ViewModelProvider传入的this就可以了。
ViewModelStore:ViewModelStore主要是用来存储ViewModel对象的,内部有一个HashMap集合用来存储ViewModel对象。
ComponentActivity持有一个ViewModelStore,可以通过ViewModelStoreOwner中的getViewModelStore()方法获取。(为啥不直接在Activity获取ViewModelStore,再获取ViewModel呢?因为ViewModel要通过ViewModelProvider.Factory创建)
Factory:是一个接口,用来创建ViewModel的
3.2 ViewModelProvider #get()
@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");
}
//构造了一个key
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)) {
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;
}
调用get方法后,会调用第二个get方法,传递key(DEFAULT_KEY + ":" + canonicalName)给第二个get方法
首先根据提供的key从ViewModelStore中获取一个ViewModel对象
如果这个获取到的ViewModel对象实例存在,那么就将其返回
如果该ViewModel对象不存在,就通过Factory创建一个ViewModel对象,并将其存储到ViewModelStore中,并将这个新创建的ViewModel对象返回。
这里面存在三个Factory:Factory,KeyedFactory和OnrequeryFactory,keyedFactory和Factory相比就是create方法中多了一个key参数。
ViewModelStore获取到ViewModel时,会判断当前mFactory是否是OnRequeryFactory类型的,是的话会回调onRequery方法
那么OnRequeryFactory回调onRequery有什么用呢?其实ViewModel不仅可以因为配置改变可以恢复Activity数据,也能恢复因为系统资源紧张而回收掉的Activity数据,只不过后者需要依靠SaveStateHandler
总结:ViewModelProvider获取到ViewModel:
1,首先创建ViewModelProvider传入ViewModelStoreOwner和Factory
2,调用ViewModelProvider的get方法,从ViewModelStore中获取ViewModel,有则直接返回,没有就通过反射创建一个,加入ViewModelStore中后,返回。
4. ViewModel的恢复
ViewModel是从ViewModelStore中获取
ViewModelStore是通过ViewModelStoreOwner.getViewModelStore方法获取
ComponentActivity实现了ViewModelStoreOwner接口和HasDefaultViewModelProviderFactory
4.1 ComponentActivity # getViewModelStore()
@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.");
}
ensureViewModelStore();
return mViewModelStore;
}
void ensureViewModelStore() {
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
}
getViewModelStore()通过两种方法获取到ViewModelStore
1,从NonConfigurationInstances中拿到
2,new一个出来
NonConfigurationInstances,用来包装不受配置更改影响的数据
Activity的NonConfigurationInstances在系统配置改变时保存了ViewModelStore和fragments等
4.2 ActivityThread #handleRelaunchActivity
系统配置发生改变时,AMS会调用ActivityThread的handleRelaunchActivity,并且通过当前Activity对应的ActivityRecord构建一个ActivityClientRecord传递过来
Activity的生命周期方法是在ActivityThread中执行的
private void handleRelaunchActivity(ActivityClientRecord tmp) {
ActivityClientRecord r = mActivities.get(tmp.token);
...
//注意最后一个参数getNonConfigInstance为true,如果是正常退出Activity的走到onDestory的该参数为false
handleDestroyActivity(r.token, false, configChanges, true);
...
handleLaunchActivity(r, currentIntent, "handleRelaunchActivity");
}
private void handleDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance) {
ActivityClientRecord r = performDestroyActivity(token, finishing,
configChanges, getNonConfigInstance);
}
4.2 ActivityThread #performDestroyActivity
ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance, String reason) {
....
//注意此时ActivityClientRecord并未从mActivities中移除,只有执行完Destroy才会移除
ActivityClientRecord r = mActivities.get(token);
performPauseActivityIfNeeded(r, "destroy");
if (!r.stopped) {
//执行Activity的onStop()方法
callActivityOnStop(r, false /* saveState */, "destroy");
}
/此时为true,正常退出的为false
if (getNonConfigInstance) {
try {
//调用对应Activity的retainNonConfigurationInstances方法
//返回值NonConfigurationInstance赋给ActivityClientRecord内的lastNonConfigurationInstances持有
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
...
}
}
}
/最终回调到onDestroy方法
mInstrumentation.callActivityOnDestroy(r.activity);
...
}
Activity的retainNonConfigurationInstances 调用了onRetainNonConfigurationInstance
4.3 ComponentActivity # onRetainNonConfigurationInstance()
public final Object onRetainNonConfigurationInstance() {
// Maintain backward compatibility.
Object custom = onRetainCustomNonConfigurationInstance();
ViewModelStore viewModelStore = mViewModelStore;
if (viewModelStore == null) {
// No one called getViewModelStore(),从lastNonConfigurationInstance取出viewModelStore
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
viewModelStore = nc.viewModelStore;
}
}
if (viewModelStore == null && custom == null) {
return null;
}
//创建一个NonConfigurationInstances,将此时的mViewModelStore设置进去
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
在调用onDestory()方法前,会创建一个NonConfigurationInstances对象,将viewModelStore存储在NonConfigurationInstances,然后将NonConfigurationInstances存储在ActivityClientrecord中。
4.4 ActivityThread #performLaunchActivity
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
}
...
ActivityThread的handleLaunchActivity最终会调用
performLaunchActivity,最终调用到activity.attach,传入了lastNonConfigurationInstances
这样对于新的Activity来说,获取到的就是之前Activity的NonConfigurationInstance,其中的ViewModelStore也是之前的,ViewModel自然也是之前的。
这样就保证了在系统配置改变时,ViewModel不变了。
总结:
1,系统配置改变时,构建一个NonConfigurationInstance,将ViewModelStore保持到NonConfigurationInstance,再将NonConfigurationInstance保存到ActivityClientrecord的lastNonConfigurationInstances
2,恢复时,将ActivityClientrecord的lastNonConfigurationInstances传递给新的Activity,再通过getViewModelStore()获取时就能从新的Activity的lastNonConfigurationInstances获取ViewModelStore,进而获取之前的ViewModel
(1,系统配置发生改变时,调用handleRelaunchActivity,在这个方法中会先取出activity对应的ActivityClientrecord,然后调用handleDestroyActivity,在这个方法中,会调用ComponentActivity 的onRetainNonConfigurationInstance(),在这个方法中,会构建一个NonConfigurationInstance,将ViewModelStore保持到NonConfigurationInstance,再将NonConfigurationInstance保存到ActivityClientrecord的lastNonConfigurationInstances
2,恢复时,调用到handleLaunchActivity,在这个方法中会调用activity.attach方法,在这个方法中,将ActivityClientrecord的lastNonConfigurationInstances传递给新的Activity的lastNonConfigurationInstances变量。获取ViewModelStore时,会从Activity的lastNonConfigurationInstances去获取(如果没有就新创建一个)如果有的话,自然就是之前的ViewModelStore,里面的ViewModel自然也是之前的。)
5. Activity正常销毁
public ComponentActivity() {
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
// Clear out the available context
mContextAwareHelper.clearAvailableContext();
// And clear the ViewModelStore
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
}
Activity正常销毁时,会通过getViewModelStore().clear()清理所有的ViewModel。
6. 参考
1,https://www.jianshu.com/p/e2cc680d5829
2,https://developer.android.com/topic/libraries/architecture/viewmodel?hl=zh-cn
3,https://juejin.cn/post/6873356946896846856
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