【Android Jetpack】- ViewModel(源码分析)

简介

ViewModel类旨在以生命周期感知的方式存储和管理与UI相关的数据。

使用场景1

• 定义ViewModel类

class ViewModelData : ViewModel(){
    private val viewModel : VMData by lazy {
        VMData().also {
            it.age = 26
            it.name = "丘比特"
        }
    }
}

class VMData{
    var name = ""
    var age = 21
}

• 使用ViewModelProvider创建ViewModel对象

val vmData = ViewModelProvider.AndroidViewModelFactory(application).create(ViewModelData::class.java)
// 这个区别是AndroidViewModelFactory是单例
val  vmData1 = ViewModelProvider.AndroidViewModelFactory.getInstance(application).create(ViewModelData::class.java)

这是最基本的时候方式，使用ViewModelProvider来提供ViewModel对象。通过查看源码可以得知。这种方式每使用一次就会产生一个ViewModel对象。

这样看上去ViewModel的用处并不是很大，根据官网的建议，我们可以把加载数据的逻辑放在ViewModel里面，替代以前的Loaders模块。

1. 不使用ViewModel

   
   
   屏幕快照 2019-10-20 下午8.58.50.png

2. 使用ViewModel

   
   
   屏幕快照 2019-10-20 下午8.58.56.png

使用场景2 — Fragment之间的数据共享

通过查看ViewModelProvider源码，发现ViewModelProvider可以通过get方法来获取ViewModel。

public  T get(@NonNull String key, @NonNull Class modelClass) {
       ViewModel viewModel = mViewModelStore.get(key);
       if (modelClass.isInstance(viewModel)) {
           return (T) viewModel;
       } else {
          ...
       }
       ...
      if (mFactory instanceof KeyedFactory) {
           viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
       } else {
           viewModel = (mFactory).create(modelClass);
       }
       mViewModelStore.put(key, viewModel);
       return (T) viewModel;
}

如果想不同Fragment直接拿到是相同的ViewModel，那么就得保证mViewModelStore唯一，mViewModelStore是ViewModelStore实例，源码很简单，自行查看。

• 使用

class ViewModelActivity: AppCompatActivity(){
    ...
   // Fragment直接的数据共享
    private fun fragmentShare(){
        val vm = ViewModelProvider(this).get(ViewModelData::class.java)
        // ViewModelProviders已经被抛弃
        val vm1 = ViewModelProviders.of(this)[ViewModelData::class.java]
    }
}

看一下构造体

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

owner就是AppCompatActivity对象，而AppCompatActivity实现了HasDefaultViewModelProviderFactory和ViewModelStoreOwner接口。那这里得到的Factory和ViewModelStore就是AppCompatActivity返回的。

public ViewModelStore getViewModelStore() {
       ...
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
        return mViewModelStore;
}

public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
        ...
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
}

如果是同一个AppCompatActivity，那么Factory和ViewModelStore便是唯一的，自然就可以在Fragment之间共享数据了。

使用场景3 - 状态的备份和恢复

如“保存UI状态”文章中所述，ViewModel对象可以处理配置更改，因此您不必担心轮换或其他情况下的状态。但是，如果需要处理系统启动的进程终止，则可能要使用onSaveInstanceState作为备份。

在场景2中，创建的ViewModelProvider是一个SavedStateViewModelFactory对象，那么我们来看看SavedStateViewModelFactory实现了什么功能。SavedStateViewModelFactory继承了AbstractSavedStateViewModelFactory抽象类，所以这里集中分析这两个类。

• AbstractSavedStateViewModelFactory
  改类主要有三个变量

  private final SavedStateRegistry mSavedStateRegistry;
  private final Lifecycle mLifecycle;
  private final Bundle mDefaultArgs;
  

  SavedStateViewModelFactory构造体

   public SavedStateViewModelFactory(@NonNull Application application,
           @NonNull SavedStateRegistryOwner owner,
           @Nullable Bundle defaultArgs) {
       super(owner, defaultArgs);
       mApplication = application;
       mFactory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
   }
  

  AbstractSavedStateViewModelFactory构造体

  public AbstractSavedStateViewModelFactory(@NonNull SavedStateRegistryOwner owner,
            @Nullable Bundle defaultArgs) {
        mSavedStateRegistry = owner.getSavedStateRegistry();
        mLifecycle = owner.getLifecycle();
        mDefaultArgs = defaultArgs;
  }
  

  mSavedStateRegistry，mLifecycle都是AppCompatActivity对象返回，mDefaultArgs是AppCompatActivity的持有的Bundle对象。mFactory是ViewModelProvider.AndroidViewModelFactory单例。

  • 创建ViewModel 对象
    调用AbstractSavedStateViewModelFactory的create方法

    public final  T create(@NonNull String key, @NonNull Class modelClass) {
        // 消耗先前提供的保存状态，之前存储的Bundle对象
        Bundle restoredState = mSavedStateRegistry.consumeRestoredStateForKey(key);
        // 将之前的保存状态和现在进行合并，放到SavedStateHandle中的一个数组里
        SavedStateHandle handle = SavedStateHandle.createHandle(restoredState, mDefaultArgs);
        // 创建持有key和handle的SavedStateHandleController对象
        SavedStateHandleController controller = new SavedStateHandleController(key, handle);
        // 添加监听和注册SavedStateProvider到SavedStateRegistry里
        controller.attachToLifecycle(mSavedStateRegistry, mLifecycle);
        // 创建viewmodel对象
        T viewmodel = create(key, modelClass, handle);
       // 保存controller到viewmodel实例中
      viewmodel.setTagIfAbsent(TAG_SAVED_STATE_HANDLE_CONTROLLER, controller);
        // 创建拥有组件重新启动时执行类的对象
        mSavedStateRegistry.runOnNextRecreation(OnRecreation.class);
        return viewmodel;
    }
    

    runOnNextRecreation方法

    public void runOnNextRecreation(@NonNull Class clazz) {
       ...
        if (mRecreatorProvider == null) {
            mRecreatorProvider = new Recreator.SavedStateProvider(this);
        }
        ...
        mRecreatorProvider.add(clazz.getName());
    }
    

    SavedStateProvider构造体

    SavedStateProvider(final SavedStateRegistry registry) {
            registry.registerSavedStateProvider(COMPONENT_KEY, this);
    }
    

    向SavedStateRegistry对象注册直接，key = COMPONENT_KEY。

  • AppCompatActivity调用

    protected void onCreate(@Nullable Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       mSavedStateRegistryController.performRestore(savedInstanceState);
       ...
     }
    

    protected void onSaveInstanceState(@NonNull Bundle outState) {
      ...
      mSavedStateRegistryController.performSave(outState);
    }
    

  • SavedStateRegistryController

    public void performRestore(@Nullable Bundle savedState) {
      Lifecycle lifecycle = mOwner.getLifecycle();
     ...
      lifecycle.addObserver(new Recreator(mOwner));
      mRegistry.performRestore(lifecycle, savedState);
    }
    
    public void performSave(@NonNull Bundle outBundle) {
        mRegistry.performSave(outBundle);
    }
    

  • 使用
    定义ViewModel

    class SavedStateViewModel(private val state: SavedStateHandle) : ViewModel(){
        private val viewModel : VMData by lazy {
            VMData().also {
                it.age = 26
                it.name = "丘比特"
            }
        }
    }
    

  • 创建ViewModel

    private fun savedStateModel(){
       val vm = ViewModelProvider(this, SavedStateViewModelFactory(application,this)).get(SavedStateViewModel::class.java)
       val vm1 = ViewModelProvider(this, defaultViewModelProviderFactory).get(SavedStateViewModel::class.java)
    }