viewmodel

问题, 为什么，这种方式，可以获取application对象

private val myViewModel: MyViewModel by viewModels()

class MyViewModel(application: Application) : AndroidViewModel(application) {

https://developer.android.com/topic/libraries/architecture/viewmodel

Share data between fragments

livedata和viewmodel关系

viewmodel中的数据发生变化时通知页面

ViewModel出现前

1. 瞬间数据丢失
2. 异步调用的内存泄漏
3. 类膨胀提高维护难度和测试难度

Share data between fragments

It’s very common that two or more fragments in an activity need to communicate with each other

class SharedViewModel : ViewModel() {
    val selected = MutableLiveData<Item>()

    fun select(item: Item) {
        selected.value = item
    }
}

class MasterFragment : Fragment() {

    private lateinit var itemSelector: Selector

    // Use the 'by activityViewModels()' Kotlin property delegate
    // from the fragment-ktx artifact
    private val model: SharedViewModel by activityViewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        itemSelector.setOnClickListener { item ->
            // Update the UI
        }
    }
}

class DetailFragment : Fragment() {

    // Use the 'by activityViewModels()' Kotlin property delegate
    // from the fragment-ktx artifact
    private val model: SharedViewModel by activityViewModels()

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        model.selected.observe(viewLifecycleOwner, Observer<Item> { item ->
            // Update the UI
        })
    }
}

https://codelabs.developers.google.com/codelabs/android-lifecycles/#0

使用

public class MyViewModel  extends ViewModel { 
    public MyViewModel(Application application){
        super();
    }

    public int number;

}

public class MyViewModel extends AndroidViewModel { //需要用到application就用AndroidViewModel
    public MyViewModel(Application application) {
        super(application);
    }

    public int number;
}

ViewModel保持数据原理

1. 使用

viewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(MyViewModel.class);

2. owner.getViewModelStore()

   public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
       this(owner.getViewModelStore(), factory);
   }

3. ComponentActivity

   @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.");
       }
       if (mViewModelStore == null) {
           NonConfigurationInstances nc =
                   (NonConfigurationInstances) getLastNonConfigurationInstance(); //拿到上次屏幕的状态
           if (nc != null) {
               // Restore the ViewModelStore from NonConfigurationInstances
               mViewModelStore = nc.viewModelStore; // 获取上次的mViewModelStore，里面保存有HashMap<String, ViewModel> mMap
           }
           if (mViewModelStore == null) {
               mViewModelStore = new ViewModelStore();
           }
       }
       return mViewModelStore;
   }

所以ViewModel横竖屏切换保存数据的秘诀就是

//拿到上次屏幕的状态,获取上次的mViewModelStore，里面保存有HashMap<String, ViewModel> mMap,接着通过.get(MyViewModel.class);获取ViewModel,如果获取不到就反射newInstance()一个。

https://www.bilibili.com/video/BV1Hh411a7LX?p=6&spm_id_from=pageDriver

Unit test

https://simplifiedcoding.in/all-courses

https://simplifiedcoding.in/course/android-testing-tutorial-from-junit-to-espresso/start/1

点 DOWNLOAD CODE OF SPENDS TRACKER 跳转后就有下载地址

spend-tracker

https://www.youtube.com/watch?v=B-dJTFeOAqw

https://www.youtube.com/watch?v=uH6-HNxK32E

SavedStateHandle

https://www.kodeco.com/5212210-jetpack-saved-state-for-viewmodel-getting-started

https://juejin.cn/post/6907121847024746503

VisibleForTesting

通过上面的代码看出， 被测试的testPrivate方法的可见性还是被改成Protected。也就是,VisibleForTesting只是一个注释，一个元数据metadata，它并没有进入程序逻辑，也没有被转化成字节码byte code 从而被JVM执行。

笔者猜测可能是Guava 的 程序员犯懒了, 即不愿意在unit test里直接利用Reflection来测试私有方法。也没有把私有方法写入另一个类中。所以设计了VisibleForTesting的注解来提醒其他程序员: 这里为了测试私有方法把私有方法改成了Protected(受保护的)并放宽了访问限制。

可是就JAVA本身而言，只有通过Reflection才能真正测试私有方法。

https://www.cnblogs.com/yanlongpankow/p/6240563.html

https://jefflin1982.medium.com/android-visiblefortesting%E7%9A%84%E7%94%A8%E9%80%94-5a666a17ba95

viewmodel share

https://github.com/google-developer-training/android-basics-kotlin-cupcake-app/tree/starter