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，没有就创建后返回。

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) {
    ...
    //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

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。