0.前言
viewModel是什么？
官方解释：
The ViewModel class is designed to store and manage UI-related data in a lifecycle conscious way. The ViewModel class allows data to survive configuration changes such as screen rotations.
ViewModel类旨在以生命周期有意识的方式存储和管理ui相关的数据。ViewModel类允许数据在配置更改(如屏幕旋转)之后存活。
这里明确指出，ViewModel可以感知activity生命周期，并在配置更改后允许数据存活（原理是内部类HolderFragment调用了setRetainInstance(true)，从而当屏幕旋转时，activity重绘但Fragment不重绘），与viewModel绑定的Activity必须继承于FragmentActivity。
官方网址
以及关于ViewModel与Activity生命周期的关联图：
viewmodel-lifecycle.png
1.使用
ViewModel一般与LiveData结合使用：
添加依赖
// ViewModel and LiveData
implementation "android.arch.lifecycle:extensions:1.1.1"
// alternatively, just ViewModel
implementation "android.arch.lifecycle:viewmodel:1.1.1"
// alternatively, just LiveData
implementation "android.arch.lifecycle:livedata:1.1.1"
package foolish
import android.arch.lifecycle.LifecycleOwner
import android.arch.lifecycle.MutableLiveData
import android.arch.lifecycle.Observer
import android.arch.lifecycle.ViewModel
import beans.response.TaskBean
import com.uber.autodispose.AutoDispose
import com.uber.autodispose.android.lifecycle.AndroidLifecycleScopeProvider
import comment.LogUit
import io.reactivex.Observable
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.observers.DisposableObserver
import io.reactivex.schedulers.Schedulers
class TestModel:ViewModel(){
val taskLiveData= MutableLiveData>()//ViewModel内部持有LiveData
fun initData( lifecycleOwner: LifecycleOwner){
//此处选择在TestModel内部处理LiveData（网上教程很多是把LiveData放在View中处理）
taskLiveData.observe(lifecycleOwner,object : Observer>{
override fun onChanged(observable: Observable?) {
observable!!.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.`as`(AutoDispose.autoDisposable(AndroidLifecycleScopeProvider.from(lifecycleOwner)))//使用AutoDispose绑定lifecycleOwner，自动取消订阅
.subscribe(object : DisposableObserver() {
override fun onComplete() {
}
override fun onNext(t: TaskBean) {
LogUit.printI("TaskPresentPrint", "t=${t.toString()}")
}
override fun onError(e: Throwable) {
//处理网略错误
}
})
}
})
}
}
Activity中调用：
private val registerOrLoginModel by lazy { ViewModelProviders.of(this).get(TestModel::class.java) }
2.源码分析
先来看看of做了什么
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
//如果没有传递factory过来，默认null
return of(activity, null);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
//如果没有传递factory过来，就自己新建一个
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
//从ViewModelStores里面取出对应Activity的ViewModelProvider
return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
在查看ViewModelProvider源码之前，先来看看ViewModelStores：
- ViewModelStores
/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.arch.lifecycle;
import static android.arch.lifecycle.HolderFragment.holderFragmentFor;
import android.support.annotation.MainThread;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
/**
* Factory methods for {@link ViewModelStore} class.
*/
@SuppressWarnings("WeakerAccess")
public class ViewModelStores {
private ViewModelStores() {
}
/**
* Returns the {@link ViewModelStore} of the given activity.
*
* @param activity an activity whose {@code ViewModelStore} is requested
* @return a {@code ViewModelStore}
*/
@NonNull
@MainThread
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
//如果已经存在ViewModelStore，直接返回它
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
//否则，新建一个
return holderFragmentFor(activity).getViewModelStore();
}
/**
* Returns the {@link ViewModelStore} of the given fragment.
*
* @param fragment a fragment whose {@code ViewModelStore} is requested
* @return a {@code ViewModelStore}
*/
@NonNull
@MainThread
public static ViewModelStore of(@NonNull Fragment fragment) {
if (fragment instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) fragment).getViewModelStore();
}
return holderFragmentFor(fragment).getViewModelStore();
}
}
只有两个of方法和一个构造方法，只接受FragmentActivity 和Fragment，没骗你吧。
这里，holderFragmentFor是包内定义好的一个Fragment类，继承于Fragment实现ViewModelStoreOwner接口（里面包含ViewModelStore getViewModelStore();方法，主要就是为了实现这个方法，符合接口隔离原则），HolderFragment的构造方法里面：
public HolderFragment() {
setRetainInstance(true);
}
调用 setRetainInstance(true)了！，没骗你吧
- holderFragmentFor方法
/**
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
//调用静态内部类HolderFragmentManager的holderFragmentFor
return sHolderFragmentManager.holderFragmentFor(activity);
}
......
/*省略部分代码，在HolderFragment 里面：*/
HolderFragment holderFragmentFor(FragmentActivity activity) {
FragmentManager fm = activity.getSupportFragmentManager();
HolderFragment holder = findHolderFragment(fm);//获取holder（holder是Fragment）
//非空立刻返回holder
if (holder != null) {
return holder;
}
//mNotCommittedActivityHolders是HolderFragmentManager内部用于存储键值对的变量
holder = mNotCommittedActivityHolders.get(activity);
//非空返回，用于防止重复执行
if (holder != null) {
return holder;
}
if (!mActivityCallbacksIsAdded) {
mActivityCallbacksIsAdded = true;
activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
}
holder = createHolderFragment(fm);//新建一个
mNotCommittedActivityHolders.put(activity, holder);
return holder;
}
//新建HolderFragment
private static HolderFragment createHolderFragment(FragmentManager fragmentManager) {
HolderFragment holder = new HolderFragment();
fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
return holder;
}
private static HolderFragment findHolderFragment(FragmentManager manager) {
if (manager.isDestroyed()) {
throw new IllegalStateException("Can't access ViewModels from onDestroy");
}
Fragment fragmentByTag = manager.findFragmentByTag(HOLDER_TAG);
if (fragmentByTag != null && !(fragmentByTag instanceof HolderFragment)) {
throw new IllegalStateException("Unexpected "
+ "fragment instance was returned by HOLDER_TAG");
}
return (HolderFragment) fragmentByTag;
}
findHolderFragment和createHolderFragment代码都比较简单，就不解释了。
这一步主要就是为了获得HolderFragment，回到ViewModelStores.of方法，接着主要还是要获取它的ViewModelStore，ViewModelStore内部维护了一个HashMap
如此一来，就获取到ViewModelStore了，然后再回到 ViewModelProvider.of里面，将它传递给ViewModelProvider
return new ViewModelProvider(ViewModelStores.of(activity), factory);
- ViewModelProvider
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
this.mViewModelStore = store;
}
ViewModelProvider把这两个家伙给存储起来以供调用，如此一来，刚才那段代码
ViewModelProviders.of(this).get(TestModel::class.java)
of到这里就结束了，接下来看get：
@NonNull
@MainThread
public T get(@NonNull Class modelClass) {
String canonicalName = modelClass.getCanonicalName();//获取ViewModel的名字
if (canonicalName == null) {
//如果为空，抛异常
throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
}
//否则获取ViewModel
//这里DEFAULT_KEY 是默认值，
return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
@NonNull
@MainThread
public T get(@NonNull String key, @NonNull Class modelClass) {
ViewModel viewModel = mViewModelStore.get(key);//从mViewModelStore里面获取viewModel
//如果已存在，返回
if (modelClass.isInstance(viewModel)) {
//noinspection unchecked
return (T) viewModel;
} else {
//noinspection StatementWithEmptyBody
if (viewModel != null) {
// TODO: log a warning.
}
}
//从工厂里面创建出ViewModel并返回
viewModel = mFactory.create(modelClass);
mViewModelStore.put(key, viewModel);
//noinspection unchecked
return (T) viewModel;
}
这里的工厂就是我们传递进来的那个啦，如果不传，则默认会新建一个， ViewModelProvider.of已经说明。
由此就可以获取到ViewModel啦！
ViewModel.png
以上！
总结
获取ViewModel的方法
ViewModelProviders.of(this).get(TestModel::class.java)
分为两个步骤：
1.of获取到ViewModelProvider,而获取ViewModelProvider需要先获取ViewModelStores
2.从ViewModelProvider中获取ViewModel
原创文章，转载请附上https://www.jianshu.com/p/123051a328ba