JetPack组件学习ViewModel

class MyViewModel : ViewModel() {
    //共享数据的核心在于拿到同一个LiveData实例，也就是拿到同一个ViewModel实例，其保存在ViewModelStore中
    //而ViewModelStore是Activity/Fragment提供的（做了屏幕转换的恢复处理，ViewModelStore会保存其数据）
    var progress:MutableLiveData<Int>?=null
    override fun onCleared() {
        //页面销毁回调
        super.onCleared()
        Log.i("wwwwwwwwwwwwwwwww", "onCleared:MyVBiewModel    cleared ")
    }
}

一，传入ViewModelOwner，Activity/Fragment已经实现该接口
ViewModelProvider(this)
.get(MyViewModel::class.java) //默认实现反射创建ViewModel实例
二，创建实例过程自定义返回ViewModel实例即可
ViewModelProvider(this,object :ViewModelProvider.Factory{
            override fun <T : ViewModel?> create(modelClass: Class<T>): T {
                modelClass.constructors
            return    modelClass.getConstructor(Application::class.java).newInstance(application)
            }
        }).get(RoomViewModel::class.java).

public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
    //第一个参数调用其getViewModelStore函数
    //第二个参数是个工厂稍后分析
        this(owner.getViewModelStore(), factory);
    }

private static final String DEFAULT_KEY =
            "androidx.lifecycle.ViewModelProvider.DefaultKey";
 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");
        }
        //传入两个参数，
        //第一个是之后做缓存用的  DEFAULT_KEY 是常量
        //第二个是自定义ViewModel的class
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }

@NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
      //首先从缓存中获取  viewmodelStore可以看成是一个map，保存ViewModel
        ViewModel viewModel = mViewModelStore.get(key);
    //可以看到mFactory 分为两类：
    1.OnRequeryFactory 当缓存命中后该方法会回调并将命中的viewmodel传入
    2.KeyedFactory 继承自OnRequeryFactory 并提供create函数提供class创建实例过程有用户自定义
        if (modelClass.isInstance(viewModel)) {
          //OnRequeryFactory是缓存命中后的回调
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        //调用create函数将class传入，内部使用不同的构造方法创建实例并返回
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
        } else {
            viewModel = (mFactory).create(modelClass);
        }
        //缓存该ViewModel
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

public class ViewModelStore {
//可以看到就是个map缓存
    private final HashMap<String, ViewModel> mMap = new HashMap<>();
    final void put(String key, ViewModel viewModel) {
        ViewModel oldViewModel = mMap.put(key, viewModel);
        if (oldViewModel != null) {
            oldViewModel.onCleared();
        }
    }
    final ViewModel get(String key) {
        return mMap.get(key);
    }
    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }
    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}