ViewModel介绍
ViewModel的定义：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。ViewModel本质上是视图（View）与数据（Model）之间的桥梁，想想以前的MVC模式，视图和数据都会写在Activity/Fragment中，导致Activity/Fragment过重，后续难以维护，而ViewModel将视图和数据进行了分离解耦，为视图层提供数据。
ViewModel的特点：
ViewModel生命周期比Activity长
数据可在屏幕发生旋转等配置更改后继续留存。下面是ViewModel生命周期图：
可以看到即使是发生屏幕旋转，旋转之后拿到的ViewModel跟之前的是同一个实例，即发生屏幕旋转时，ViewModel并不会消失重建；而如果Activity是正常finish()，ViewModel则会调用onClear()销毁。
ViewModel中不能持有Activity引用
不要将Activity传入ViewModel中，因为ViewModel的生命周期比Activity长，所以如果ViewModel持有了Activity引用，很容易造成内存泄漏。如果想在ViewModel中使用Application，可以使用ViewModel的子类AndroidViewModel，其在构造方法中需要传入了Application实例：
public class AndroidViewModel extends ViewModel {
private Application mApplication;
public AndroidViewModel(@NonNull Application application) {
mApplication = application;
}
public <T extends Application> T getApplication() {
return (T) mApplication;
}
}ViewModel使用举例
引入ViewModel，在介绍Jetpack系列文章Lifecycle时已经提过一次，这里再写一下：
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0"先看运行效果图：
页面中有两个Fragment，左侧为列表，右侧为详情，当点击左侧某一个Item时，右侧会展示相应的数据，即两个Fragment可以通过ViewModel进行通信；同时可以看到，当屏幕发生旋转的时候，右侧详情页的数据并没有丢失，而是直接进行了展示。
//ViewModelActivity.kt
class ViewModelActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
Log.e(JConsts.VIEW_MODEL, "onCreate")
setContentView(R.layout.activity_view_model)
}
}其中的XML文件:
//activity_view_model.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".viewmodel.ViewModelActivity">
<fragment
android:id="@+id/fragment_item"
android:name="com.example.jetpackstudy.viewmodel.ItemFragment"
android:layout_width="150dp"
android:layout_height="match_parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toLeftOf="@+id/fragment_detail"
app:layout_constraintTop_toTopOf="parent" />
<fragment
android:id="@+id/fragment_detail"
android:name="com.example.jetpackstudy.viewmodel.DetailFragment"
android:layout_width="0dp"
android:layout_height="match_parent"
app:layout_constraintLeft_toRightOf="@+id/fragment_item"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>直接将Fragment以布局方式写入我们的Activity中，继续看两个Fragment：
//左侧列表Fragment
class ItemFragment : Fragment() {
lateinit var mRvView: RecyclerView
//Fragment之间通过传入同一个Activity来共享ViewModel
private val mShareModel by lazy {
ViewModelProvider(activity!!).get(ShareViewModel::class.java)
}
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
val view = LayoutInflater.from(activity)
.inflate(R.layout.layout_fragment_item, container, false)
mRvView = view.findViewById(R.id.rv_view)
mRvView.layoutManager = LinearLayoutManager(activity)
mRvView.adapter = MyAdapter(mShareModel)
return view
}
//构造RecyclerView的Adapter
class MyAdapter(private val sViewModel: ShareViewModel) :
RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
class MyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
val mTvText: TextView = itemView.findViewById(R.id.tv_text)
}
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
val itemView = LayoutInflater.from(parent.context)
.inflate(R.layout.item_fragment_left, parent, false)
return MyViewHolder(itemView)
}
override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
val itemStr = "item pos:$position"
holder.mTvText.text = itemStr
//点击发送数据
holder.itemView.setOnClickListener {
sViewModel.clickItem(itemStr)
}
}
override fun getItemCount(): Int {
return 50
}
}
}//右侧详情页Fragment
class DetailFragment : Fragment() {
lateinit var mTvDetail: TextView
//Fragment之间通过传入同一个Activity来共享ViewModel
private val mShareModel by lazy {
ViewModelProvider(activity!!).get(ShareViewModel::class.java)
}
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
return LayoutInflater.from(context)
.inflate(R.layout.layout_fragment_detail, container, false)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
mTvDetail = view.findViewById(R.id.tv_detail)
//注册Observer并监听数据变化
mShareModel.itemLiveData.observe(activity!!, { itemStr ->
mTvDetail.text = itemStr
})
}
}最后贴一下我们的ViewModel:
//ShareViewModel.kt
class ShareViewModel : ViewModel() {
val itemLiveData: MutableLiveData<String> by lazy { MutableLiveData<String>() }
//点击左侧Fragment中的Item发送数据
fun clickItem(infoStr: String) {
itemLiveData.value = infoStr
}
}这里再强调一下，两个Fragment中ViewModelProvider(activity).get()传入的是同一个Activity，从而得到的ViewModel是同一个实例，进而可以进行互相通信。
上面使用ViewModel的写法有两个好处：
屏幕切换时保存数据
- 屏幕发生变化时，不需要重新请求数据，直接从ViewModel中再次拿数据即可。
Fragment之间共享数据：
- Activity 不需要执行任何操作，也不需要对此通信有任何了解。
- 除了 SharedViewModel 约定之外，Fragment 不需要相互了解。如果其中一个 Fragment 消失，另一个 Fragment 将继续照常工作。
- 每个 Fragment 都有自己的生命周期，而不受另一个 Fragment 的生命周期的影响。如果一个 Fragment 替换另一个 Fragment，界面将继续工作而没有任何问题。
ViewModel与onSaveInstance(Bundle)对比
- ViewModel是将数据存到内存中，而onSaveInstance()是通过Bundle将序列化数据存在磁盘中
- ViewModel可以存储任何形式的数据，且大小不限制(不超过App分配的内存即可)，onSaveInstance()中只能存储可序列化的数据，且大小一般不超过1M（IPC通信数据限制）
示例代码地址
源码解析
ViewModel的存取
我们在获取ViewModel实例时，并不是直接new出来的，而是通过ViewModelProvider.get()获取的，顾名思义，ViewModelProvider意为ViewModel提供者，那么先来看它的构造方法：
//ViewModelProvider.java
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;
}ViewModelProvider构造函数中的几个参数：
- ViewModelStoreOwner：ViewModel存储器拥有者，用来提供ViewModelStore
- ViewModelStore：ViewModel存储器，用来存储ViewModel
- Factory：创建ViewModel的工厂
private final ViewModelStore mViewModelStore;
private static final String DEFAULT_KEY =
"androidx.lifecycle.ViewModelProvider.DefaultKey";
@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");
}
//首先构造了一个key，直接调用下面的get(key,modelClass)
return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
//尝试从ViewModelStore中获取ViewModel
ViewModel viewModel = mViewModelStore.get(key);
if (modelClass.isInstance(viewModel)) {
if (mFactory instanceof OnRequeryFactory) {
((OnRequeryFactory) mFactory).onRequery(viewModel);
}
//viewModel不为空直接返回该实例
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为空，通过Factory创建
viewModel = mFactory.create(modelClass);
}
//将ViewModel保存到ViewModelStore中
mViewModelStore.put(key, viewModel);
return (T) viewModel;
}逻辑很简单，首先尝试通过ViewModelStore.get(key)获取ViewModel，如果不为空直接返回该实例；如果为空，通过Factory.create创建ViewModel并保存到ViewModelStore中。先来看Factory是如何创建ViewModel的，ViewModelProvider构造函数中，如果没有传入Factory，那么会使用NewInstanceFactory：
//接口Factory
public interface Factory {
<T extends ViewModel> T create(@NonNull Class<T> modelClass);
}
//默认Factory的实现类NewInstanceFactory
public static class NewInstanceFactory implements Factory {
private static NewInstanceFactory sInstance;
//获取单例
static NewInstanceFactory getInstance() {
if (sInstance == null) {
sInstance = new NewInstanceFactory();
}
return sInstance;
}
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
try {
//反射创建
return modelClass.newInstance();
} catch (InstantiationException e) {
throw new RuntimeException("Cannot create an instance of " + modelClass, e);
} catch (IllegalAccessException e) {
throw new RuntimeException("Cannot create an instance of " + modelClass, e);
}
}
}可以看到NewInstanceFactory的实现很简单，直接通过传入的class反射创建实例，泛型中限制必须是ViewModel的子类，所以最终创建的是ViewModel实例。
看完Factory，接着来看ViewModelStore：
public class ViewModelStore {
private final HashMap<String, ViewModel> mMap = new HashMap<>();
final void put(String key, ViewModel viewModel) {
ViewModel oldViewModel = mMap.put(key, viewModel);
//如果oldViewModel不为空，调用oldViewModel的onCleared释放资源
if (oldViewModel != null) {
oldViewModel.onCleared();
}
}
final ViewModel get(String key) {
return mMap.get(key);
}
Set<String> keys() {
return new HashSet<>(mMap.keySet());
}
//遍历存储的ViewModel并调用其clear()方法，然后清除所有ViewModel
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.clear();
}
mMap.clear();
}
}ViewModelStore类也很简单，内部就是通过Map进行存储ViewModel的。到这里，我们基本看完了ViewModel的存取，流程如下：
ViewModelStore的存取
上面聊了ViewModel的存取，有一个重要的点没有说到，既然ViewModel的生命周期比Activity长，而ViewModel又是通过ViewModelStore存取的，那么ViewModelStore又是如何存取的呢？在上面的流程图中我们知道ViewModelStore是通过ViewModelStoreOwner提供的：
//接口ViewModelStoreOwner.java
public interface ViewModelStoreOwner {
ViewModelStore getViewModelStore();
}ViewModelStoreOwner中接口方法getViewModelStore()返回的既是ViewModelStore。我们上面例子获取ViewModel时是ViewModelProvider(activity).get(ShareViewModel::class.java)，其中的activity其实就是ViewModelStoreOwner，也就是Activity中实现了这个接口：
//ComponentActivity.java
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,.... {
@Override
public ViewModelStore getViewModelStore() {
//Activity还未关联到Application时，会抛异常，此时不能使用ViewModel
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) {
//从NonConfigurationInstances中恢复ViewModelStore
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
}
@Override
//覆写了Activity的onRetainNonConfigurationInstance方法，在Activity#retainNonConfigurationInstances()方法中被调用，即配置发生变化时调用。
public final Object onRetainNonConfigurationInstance() {
Object custom = onRetainCustomNonConfigurationInstance();
ViewModelStore viewModelStore = mViewModelStore;
if (viewModelStore == null) {
//尝试从之前的存储中获取NonConfigurationInstance
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
//从NonConfigurationInstances中恢复ViewModelStore
viewModelStore = nc.viewModelStore;
}
}
if (viewModelStore == null && custom == null) {
return null;
}
//如果viewModelStore不为空，当配置变化时将ViewModelStore保存到NonConfigurationInstances中
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
//内部类NonConfigurationInstances
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
}NonConfigurationInstances是ComponentActivity的静态内部类，里面包含了ViewModelStore。getViewModelStore()内部首先尝试通过getLastNonConfigurationInstance()来获取NonConfigurationInstances，不为空直接能拿到对应的ViewModelStore；否则直接new一个新的ViewModelStore
跟进去getLastNonConfigurationInstance()这个方法：
//Activity.java
public class Activity extends ContextThemeWrapper {
NonConfigurationInstances mLastNonConfigurationInstances;
public Object getLastNonConfigurationInstance() {
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}
NonConfigurationInstances retainNonConfigurationInstances() {
//onRetainNonConfigurationInstance实现在子类ComponentActivity中实现
Object activity = onRetainNonConfigurationInstance();
.......
if (activity == null && children == null && fragments == null && loaders == null
&& mVoiceInteractor == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.activity = activity;
......
return nci;
}
final void attach(Context context,
.......,
NonConfigurationInstances lastNonConfigurationInstances) {
mLastNonConfigurationInstances = lastNonConfigurationInstances;
}
static final class NonConfigurationInstances {
Object activity;
......
}
}可以看到getLastNonConfigurationInstance()中是通过mLastNonConfigurationInstances是否为空来判断的，搜索一下该变量赋值的地方，就找到了Activity#attach()方法。我们知道Activity#attach()是在创建Activity的时候调用的，顺藤摸瓜就可以找到了ActivityThread：
//ActivityThread.java
final ArrayMap<IBinder, ActivityClientRecord> mActivities = new ArrayMap<>();
Activity.NonConfigurationInstances lastNonConfigurationInstances;
//1、将NonConfigurationInstances存储到ActivityClientRecord
ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance, String reason) {
ActivityClientRecord r = mActivities.get(token);
Class<? extends Activity> activityClass = null;
if (r != null) {
......
if (getNonConfigInstance) {
try {
//retainNonConfigurationInstances
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();
} catch (Exception e) {
......
}
}
}
synchronized (mResourcesManager) {
mActivities.remove(token);
}
return r;
}
//2、从ActivityClientRecord中获取NonConfigurationInstances
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
......
activity.attach(...., r.lastNonConfigurationInstances,....);
}- 在执行performDestroyActivity()的时候，会调用Activity#retainNonConfigurationInstances()方法将生成的NonConfigurationInstances赋值给lastNonConfigurationInstances。
- 在performLaunchActivity()中又会通过Activity#attach()将lastNonConfigurationInstances赋值给Activity.mLastNonConfigurationInstances，进而取到ViewModelStore。
ViewModelStore的存取都是间接在ActivityThread中进行并保存在ActivityClientRecord中。在Activity配置变化时，ViewModelStore可以在Activity销毁时得以保存并在重建时重新从lastNonConfigurationInstances中获取，又因为ViewModelStore提供了ViewModel，所以ViewModel也可以在Activity配置变化时得以保存，这也是为什么ViewModel的生命周期比Activity生命周期长的原因了。
参考
【1】https://developer.android.com/topic/libraries/architecture/viewmodel?hl=zh_cn
