Jetpack已经出了很久很久了，近几年的GDD几乎每次都会介绍新的组件，说来惭愧，一直没有好好学习，看近年的Google 的很多Demo中其实都有所体现，之前都是大概的了解了一遍。最近决定，好好梳理一遍，既学习其用法，也尝试学习下其设计思想。也是时候该补充一下了。进入正题--ViewModel
首先都是看官方的例子，ViewModel官方的的例子是会和另一个架构库LiveData写在一起，很多的博客也是照官方的例子来说明，开始接触时甚至给了我一种假象：ViewModel都是和LiveData一起使用的。后来阅读才了解，ViewModel和LiveData职责分工还是很明显的，使用LiveData Demo主要使用其observe功能，LiveDate的使用及原理之后再分析，甚至在appcompat-v7:27.1.1中直接单独集成了ViewModel.所以，故为排除干扰，今天不会使用官方的主流Demo用法，先来看ViewModel。
Android的UI控制器（Activity和Fragment）从创建到销毁拥有自己完整的生命周期，当系统配置发生改变时（(Configuration changes)），系统就会销毁Activity和与之关联的Fragment然后再次重建（可通过在AndroidManifast.xml中配置android:configChanges修改某些行为，这里不讨论）,那么存储在当前UI中的临时数据也会被清空，例如，登陆输入框，输入账号或密码后旋转屏幕，视图被重建，输入过的数据也清空了，这无疑是一种不友好的用户体验。对于少量的可序列化数据可以使用onSaveInstanceState()方法保存然后在onCreate()方法中重新恢复，正如所说onSaveInstanceState对于大量的数据缓存有一定的局限性，大量的数据缓存则可以使用Fragment.setRetainInstance(true)来保存数据。ViewModel也是提供了相同的功能，其实和“RetainInstance”也有关联，用来存储和管理与UI相关的数据，允许数据在系统配置变化后存活，我们一起看一下这个ViewModel的缓存是怎么实现的呢？
使用方式
首先先看下使用方式，先上效果图
public class MyViewModel extends ViewModel {
String name;
public String getName(){
return name;
}
public void setName(String name){
this.name = name;
}
@Override
protected void onCleared() {
super.onCleared();
name = null;
}
}
public class ViewModelActivity extends AppCompatActivity implements View.OnClickListener {
private static final String TAG = "ViewModelActivity";
TextView textView;
private MyViewModel myViewModel;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_viewmodel);
textView = findViewById(R.id.textView);
textView.setOnClickListener(this);
ViewModelProvider.Factory factory = ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication());
/*
*这里的this是ViewModelStoreOwner接口在appcompat-v7:27.1.1支持库中AppCompatActivity已经实现了，
*如果是较低版本，需要更新支持包或者参考其实现对本来继承的Activity做对应实现。
*/
ViewModelProvider provider = new ViewModelProvider(this, factory);//
myViewModel = provider.get(MyViewModel.class);
Log.e(TAG, "onCreate: " + myViewModel.getName() );
if (myViewModel.getName() != null) {
textView.setText(myViewModel.getName());
}
}
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.textView:
myViewModel.setName("MyViewModel Test");
textView.setText(myViewModel.getName());
break;
}
}
}
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:text="default"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
非常简单的一个例子，这就是ViewModel最简单的使用了，就是TextView中显示ViewModel的数据。ViewModel需要由ViewModelProvider.get(Class)来取得，旋转屏幕销毁后，之前改变的数据还在。
发现的一些疑问
接下来就是进入主题分析下ViewModel到底是怎么实现的呢？
带着问题看源码：
- ViewModelProvider是干啥的？
- AndroidViewModelFactory 这命名一看就是应该是工厂模式，工厂创建了什么？
- provider.get(MyViewModel.class) 这里直接使用的get命名就得到了需要的唯一数据
- 注释中ViewModelStoreOwner又是什么角色？
源码分析 先看ViewModel类，没什么说的，就是一个么有任何真正实现的抽象类，只有一个抽象方法onCleared()
public abstract class ViewModel {
/**
* This method will be called when this ViewModel is no longer used and will be destroyed.
* <p>
* It is useful when ViewModel observes some data and you need to clear this subscription to
* prevent a leak of this ViewModel.
*/
@SuppressWarnings("WeakerAccess")
protected void onCleared() {
}
}
接着看下ViewModelFactory，顾名思义就是制造ViewModel的。
AndroidViewModelFactory的继承关系如下:
android.arch.lifecycle.ViewModelProvider.Factory
android.arch.lifecycle.ViewModelProvider.NewInstanceFactory
android.arch.lifecycle.ViewModelProvider.AndroidViewModelFactory
Factory是一个只包含一个create的interface，NewInstanceFactory实现了该方法传入Class会利用ViewModel的默认无参构造器创建一个对应ViewModel的实例，而AndroidViewModelFactory增加了一个属性就是应用的Applicaion,同时重写create方法，查看ViewModel是否有包含Applicaion参数的构造方法从而使用，对应的其实还有一个AndroidViewModel是ViewModel的子类，默认已经实现了带有Application参数的构造方法，需要使用在ViewModel中使用application的直接继承AndroidViewModel就可以，看到这里其实最上面的例子有个不是问题的问题，其实上面的Factory直接使用NewInstanceFactory就可以创建出对应的ViewModel实例了。
/**
* Implementations of {@code Factory} interface are responsible to instantiate ViewModels.
*/
public interface Factory {
/**
* Creates a new instance of the given {@code Class}.
* <p>
*
* @param modelClass a {@code Class} whose instance is requested
* @param <T> The type parameter for the ViewModel.
* @return a newly created ViewModel
*/
@NonNull
<T extends ViewModel> T create(@NonNull Class<T> modelClass);
}
/**
* Simple factory, which calls empty constructor on the give class.
*/
public static class NewInstanceFactory implements Factory {
@SuppressWarnings("ClassNewInstance")
@NonNull
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
//noinspection TryWithIdenticalCatches
try {
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);
}
}
}
/**
* {@link Factory} which may create {@link AndroidViewModel} and
* {@link ViewModel}, which have an empty constructor.
*/
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);
}
}
之后通过ViewModelStoreOwner和刚刚创建的Factory创建出ViewModelPrivider实例
/**
* Creates {@code ViewModelProvider}, which will create {@code ViewModels} via the given
* {@code Factory} and retain them in a store of the given {@code ViewModelStoreOwner}.
*
* @param owner a {@code ViewModelStoreOwner} whose {@link ViewModelStore} will be used to
* retain {@code ViewModels}
* @param factory a {@code Factory} which will be used to instantiate
* new {@code ViewModels}
*/
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
/**
* Creates {@code ViewModelProvider}, which will create {@code ViewModels} via the given
* {@code Factory} and retain them in the given {@code store}.
*
* @param store {@code ViewModelStore} where ViewModels will be stored.
* @param factory factory a {@code Factory} which will be used to instantiate
* new {@code ViewModels}
*/
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
this.mViewModelStore = store;
}
/**
* A scope that owns {@link ViewModelStore}.
* <p>
* A responsibility of an implementation of this interface is to retain owned ViewModelStore
* during the configuration changes and call {@link ViewModelStore#clear()}, when this scope is
* going to be destroyed.
*/
@SuppressWarnings("WeakerAccess")
public interface ViewModelStoreOwner {
/**
* Returns owned {@link ViewModelStore}
*
* @return a {@code ViewModelStore}
*/
@NonNull
ViewModelStore getViewModelStore();
}
ViewModelStoreOwner 也是一个接口是FragmentActivity实现了该接口并实现了其中的getViewModelStore()方法
public class FragmentActivity extends BaseFragmentActivityApi16 implements
ViewModelStoreOwner...{
private ViewModelStore mViewModelStore;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
mFragments.attachHost(null /*parent*/);
super.onCreate(savedInstanceState);
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
mViewModelStore = nc.viewModelStore;
}
/**
* Returns the {@link ViewModelStore} associated with this activity
*
* @return a {@code ViewModelStore}
*/
@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) {
mViewModelStore = new ViewModelStore();
}
return mViewModelStore;
}
}
这个ViewModelStore又是什么呢，其实就是真正利用HashMap存储ViewModel的地方了，看下代码在存储和clear同时会调用ViewModel需要实现的抽象方法onClear()
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);
}
/**
* Clears internal storage and notifies ViewModels that they are no longer used.
*/
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.onCleared();
}
mMap.clear();
}
}
这样ViewModelProvider就是有了一个ViewModel的容器，这时去调用ViewModelProvider的get(Class)方法就是去调用mViewModelStore 的get()方法取出对应的ViewModel所以这里只要持有的ViewModelStore是有缓存的，那么取出的ViewModel就是相同的缓存了。
/**
* Returns an existing ViewModel or creates a new one in the scope (usually, a fragment or
* an activity), associated with this {@code ViewModelProvider}.
* <p>
* The created ViewModel is associated with the given scope and will be retained
* as long as the scope is alive (e.g. if it is an activity, until it is
* finished or process is killed).
*
* @param modelClass The class of the ViewModel to create an instance of it if it is not
* present.
* @param <T> The type parameter for the ViewModel.
* @return A ViewModel that is an instance of the given type {@code T}.
*/
@NonNull
@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");
}
return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
@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.
}
}
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
}
看到这里就会发现ViewModelStore的缓存其实是通过NonConfigurationInstances的缓存来实现的，这样就完成了Activity销毁重建后ViewModel还保存原来的数据的过程，那么NonConfigurationInstances 是什么呢？如果有了解过使用在Activity中使用onRetainNonConfigurationInstance()保存缓存数据，在onCreate()中通过getLastNonConfigurationInstance()恢复之前的数据状态的同学可能会很熟悉这里的写法，是的，这里FragmentActivity就是使用的这种方式来保存之前的ViewModelStore,看下FragmentActivity的onRetainNonConfigurationInstance()方法。
/**
* Retain all appropriate fragment state. You can NOT
* override this yourself! Use {@link #onRetainCustomNonConfigurationInstance()}
* if you want to retain your own state.
*/
@Override
public final Object onRetainNonConfigurationInstance() {
if (mStopped) {
doReallyStop(true);
}
Object custom = onRetainCustomNonConfigurationInstance();
FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
if (fragments == null && mViewModelStore == null && custom == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = mViewModelStore;//就是这里了，会把之前的VeiwmodelStroe存储到NonConfigurationInstances中以供后续恢复使用
nci.fragments = fragments;
return nci;
}
这里其实再次出现了一个问题 onRetainNonConfigurationInstance()和getLastNonConfigurationInstance()又是怎么恢复数据呢?...这个其实和Activity的启动流程相关，这里也介绍一下吧，之后的内容其实是Activity的内容了，趁这次看ViwModel也跟着看了一遍，有了解过Activity启动流程的同学更容易理解的多，大家酌情观看。
也不能从头开始说起，再从头就要越扯越远了，就从ActivityThread.java中的scheduleLaunchActivity开始
@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;
r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;
r.startsNotResumed = notResumed;
r.isForward = isForward;
r.profilerInfo = profilerInfo;
r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
从ActivityThread.java中H（extents Handler）接收到LAUNCH_ACTIVITY，并且会接收ActivityClientRecord，其中会调用ActivityThread的handleLaunchActivity方法：
//ActivityThread.java
//没有前后文的H中的handleMessage~~~
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
//ActivityClientRecord 是apk进程中一个Activity的代表，这个对象的activity成员引用真正的Activity组件,后面的都和它有关系
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");///这里~这里~
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
...
Activity a = performLaunchActivity(r, customIntent);
...
}
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
if (activity != null) {
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (r.overrideConfig != null) {
config.updateFrom(r.overrideConfig);
}
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
appContext.setOuterContext(activity);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config, //看到这个r.lastNonConfigurationInstances 就是在Activity方法中调用getLastNonConfigurationInstance()获取到的Object了。
r.referrer, r.voiceInteractor, window, r.configCallback);
...
}
注释中的地方就是lastNonConfigurationInstances的赋值的地方，可能会发现在scheduleLaunchActivity并没有对lastNonConfigurationInstances赋值，因为第一次启动Activity时，这里其实就是null的，那么赋值的地方在哪里呢，既然是销毁后会恢复数据，追踪发现在performDestroyActivity()也就是在调用onDestroy生命周期之前有这样一段代码
private ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance) {
ActivityClientRecord r = mActivities.get(token);
...无关代码省略
if (getNonConfigInstance) {
try {
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();///就是这里出现了想要找的NonConfigurationInstances
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to retain activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
}
try {
r.activity.mCalled = false;
mInstrumentation.callActivityOnDestroy(r.activity);
...无关代码省略
return r;
}
在performDestroyActivity()调用了Activity.retainNonConfigurationInstances()方法了，所以逻辑切换回Activity中...
/**
* This method is similar to {@link #onRetainNonConfigurationInstance()} except that
* it should return either a mapping from child activity id strings to arbitrary objects,
* or null. This method is intended to be used by Activity framework subclasses that control a
* set of child activities, such as ActivityGroup. The same guarantees and restrictions apply
* as for {@link #onRetainNonConfigurationInstance()}. The default implementation returns null.
*/
@Nullable
HashMap<String,Object> onRetainNonConfigurationChildInstances() {
return null;
}
NonConfigurationInstances retainNonConfigurationInstances() {
Object activity = onRetainNonConfigurationInstance();///熟悉的代码，原来的配方，和分析ActivityThread之前联系起来了，在Activity中是空实现，这里就是获取子类的NonConfigurationInstance()，之前的例子就是的得FragmentActivity中的具体实现，上文中已经在分析ActivityThread.java已经指出。
HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
// We're already stopped but we've been asked to retain.
// Our fragments are taken care of but we need to mark the loaders for retention.
// In order to do this correctly we need to restart the loaders first before
// handing them off to the next activity.
mFragments.doLoaderStart();
mFragments.doLoaderStop(true);
ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();
if (activity == null && children == null && fragments == null && loaders == null
&& mVoiceInteractor == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.activity = activity;
nci.children = children;
nci.fragments = fragments;
nci.loaders = loaders;
if (mVoiceInteractor != null) {
mVoiceInteractor.retainInstance();
nci.voiceInteractor = mVoiceInteractor;
}
return nci;//这里返回的是Activity中的NonConfigurationInstances就保存在了ActivityClientRecord中了
}
至此，ActivityClientRecord就不再深入了，可以看到在Activity中是以一个ArrayMap来保存Activity的记录，记录的就是Activity的状态，所以这里就实现了对NonConfigurationInstances的保存。
结语：至此就基本看完了ViewModel在Activity中的使用和原理，在Fragment中的实现主要是使用setRetainInstance(true)的方式去保存，跟今天的分析也有关联，分析源码的过程总是看着就有新的问题，再次带着问题去解决会再次有不同的收获，本文的理解也可能有偏差，如有错误和想要交流的也欢迎指正沟通。
