Android ViewModel

2022-08-18  本文已影响0人  大聪明的博客

ViewModel注重以生命周期的方法存储和管理界面相关的数据,ViewModel类可在屏幕发生旋转等配置更改后仍然保存。在没有ViewModel之前我们保存应用是通过onSaveInstanceState()和onCreate()方法来完成的,但是此方法不适合大量数据的保存和恢复。另外一个问题是,有些数据是异步获取的,还夹杂着业务逻辑,这样可能会使View层逻辑比较复杂,比较容易引发各种问题,而ViewModel的引入,向开发者隐藏了数据的保存和恢复逻辑,节省了模板代码的编写,提高了开发效率。所以接下来我们来看下如果使用ViewModel吧

public class MyViewModel extends ViewModel {
    private MutableLiveData<List<User>> users;
    public LiveData<List<User>> getUsers() {
        if (users == null) {
            users = new MutableLiveData<List<User>>();
            loadUsers();
        }
        return users;
    }

    private void loadUsers() {
        // Do an asynchronous operation to fetch users.
    }
}

public class MyActivity extends AppCompatActivity {
    public void onCreate(Bundle savedInstanceState) {
        // Create a ViewModel the first time the system calls an activity's onCreate() method.
        // Re-created activities receive the same MyViewModel instance created by the first activity.

        MyViewModel model = new ViewModelProvider(this).get(MyViewModel.class);
        model.getUsers().observe(this, users -> {
            // update UI
        });
    }
}

以上代码来自AndroidDeveloper官网。那么ViewModel是怎么实现在Activity销毁后重建的呢?首先我们来看ViewModel是如何获取到的

 ViewModelProvider viewModelProvider = new ViewModelProvider(this);
 userViewModel = viewModelProvider.get(UserViewModel.class);

以上两行代码为我们解答了疑问,首先ViewModelProvider 是Activity持有的一个成员变量,而UserViewModel则是ViewModelProvider中保存的某个缓存。接着我们打开ViewModelProvider源码,

    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

我们发现Activity被当成了ViewModelStoreOwner传递进来,第二行代码通过viewModelProvider.get(UserViewModel.class)获取到了ViewModel对象方法,点开get方法源码

    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);
    }

    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;
    }

通过以上代码我们发现ViewModel在这里被创建,并且被存在了ViewModelStore里面了。鉴于我们现在是在研究ViewMode在设备配置发生改变时是如果被保存和恢复的,所以我们就不分析ViewModel的创建过程了,我们只需要知道它被创建后保存在了ViewModelStore中就行了,那么ViewMdoelStore是哪里来的呢?就是通过ComponentActivity的LifecylceOwner接口返回的,所以我们的ViewModel数据还是保存在Activity中的。那么ViewModelStore是哪里来的?请看以下代码

    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) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
    }

通过以上代码我们发现ViewModel优先取自NonConfigurationInstances,如果没有,就需要重新new一个,所以我们猜测,如果Activity是全新构建的这个时候NonConfigurationInstances中的ViewModel是空的,因为旋转重建后就不会是空的。所以我们弄清楚NonConfigurationInstances的来源就可以知道ViewModel是如何恢复的了。查看Activity的源码我们发现NonConfigurationInstances是attch方法传递进来的,所以我测试和Activity启动有关。所以我去ActivityThread中搜索了activity.attach字符串还真的有,就在android.app.ActivityThread#performLaunchActivity中,这个方法的调用源头在android.app.ActivityThread#handleRelaunchActivity方法中,从名字就可以看出这个方法是处理Activity的重启的。而NonConfigurationInstances就保存在该方法的入参ActivityClientRecord中;我们看下ActivityClientRecord类,他是记录Activity的一个辅助类.

public static final class ActivityClientRecord {
        public IBinder token;
        int ident;
        Intent intent;
        String referrer;
        IVoiceInteractor voiceInteractor;
        Bundle state;
        PersistableBundle persistentState;
        Activity activity;
        Window window;
        Activity parent;
        String embeddedID;
        Activity.NonConfigurationInstances lastNonConfigurationInstances;
}

所以我们的ViewModel在Activity的重建过程中被传进来了。就实现了ViewModel的恢复;但是ViewModel是什么时候被存在NonConfigurationInstances中的呢?答案在android.app.ActivityThread#handleRelaunchActivityInner里,这个方法在Activity需要销毁重建时调用。

handleDestroyActivity(r.token, false, configChanges, true, reason);
  performDestroyActivity(token, finishing,configChanges, getNonConfigInstance, reason)
    activity.retainNonConfigurationInstances()

通过以上方法轨迹我们最后发现activity.retainNonConfigurationInstances()返回了NonConfigurationInstances对象并且赋值给了ActivityClientRecord 。所以谜题解开了。是ActivityThread和AMS通过ActivityClientRecord完成了ViewModel的恢复和保存。最后我们看下activity.retainNonConfigurationInstances方法

NonConfigurationInstances retainNonConfigurationInstances() {
        Object activity = onRetainNonConfigurationInstance();
        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;
    }

在代码段后面我们发现返回了NonConfigurationInstances对象实例,其中的nci.activity对象保存了onRetainNonConfigurationInstance()方法返回的对象,这个方法返回的对象就保存了ViewModel.

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