ViewModel源码解析

作者: 雯艺雪 | 来源:发表于2019-08-16 17:37 被阅读0次

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<Observable<TaskBean>>()//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<Observable<TaskBean>>{
            override fun onChanged(observable: Observable<TaskBean>?) {
                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<TaskBean>() {
                            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<Activity, HolderFragment>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<String, ViewModel>，实现get,put，clear方法，这里就不贴代码了，把它当做Map的一部分就行，主要用于存放ViewModel。
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 extends ViewModel> T get(@NonNull Class<T> 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 extends ViewModel> T get(@NonNull String key, @NonNull Class<T> 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

ViewModel源码解析

0.前言

1.使用

2.源码分析

总结

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