1.What?

The above is only a personal summary of the official Chinese New Year.

2. Usage

1. Create the ViewModel to be saved

public class BoyiKiaViewModel extends ViewModel {

}

2. Create a provider and obtain the ViewModel instance saved in the ViewModelStore through the provider

     ViewModelProvider provider=new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory());
     
        BoyiKiaViewModel boyiKiaViewModel = provider.get(BoyiKiaViewModel.class);

3.ViewModel

When the viewModel is destroyed, the onCleared method is executed

public abstract class ViewModel {
    /**
     * This method will be called when this ViewModel is no longer used and will be destroyed.
     * <p>
     * It is useful when ViewModel observes some data and you need to clear this subscription to
     * prevent a leak of this ViewModel.
     */
    @SuppressWarnings("WeakerAccess")
    protected void onCleared() {
    }
}

1. ViewModel associated with application

public class AndroidViewModel extends ViewModel {
    @SuppressLint("StaticFieldLeak")
    private Application mApplication;

    public AndroidViewModel(@NonNull Application application) {
        mApplication = application;
    }

    /**
     * Return the application.
     */
    @SuppressWarnings("TypeParameterUnusedInFormals")
    @NonNull
    public <T extends Application> T getApplication() {
        //noinspection unchecked
        return (T) mApplication;
    }
}

4.ViewModelProvider

Constructor

Binding the constructor and ViewStore of ViewModel

//1.
 public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        this.mViewModelStore = store;
    }

//2. Get the viewstore through ViewModelStoreOwner (a modelstoreowner is implemented by default when the activity is created)

public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}
 public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
        this(owner.getViewModelStore(), factory);
    }

Factory

ViewModel instance builder

 public interface Factory {
        /**
         * Creates a new instance of the given {@code Class}.
         * <p>
         *
         * @param modelClass a {@code Class} whose instance is requested
         * @param <T>        The type parameter for the ViewModel.
         * @return a newly created ViewModel
         */
        @NonNull
        <T extends ViewModel> T create(@NonNull Class<T> modelClass);
    }

Default implemented constructor

public static class NewInstanceFactory implements Factory {

        @SuppressWarnings("ClassNewInstance")
        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            //noinspection TryWithIdenticalCatches
            try {
                return modelClass.newInstance();
            } catch (InstantiationException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            }
        }
    }

 /**
     * {@link Factory} which may create {@link AndroidViewModel} and
     * {@link ViewModel}, which have an empty constructor.
     */
    public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

        private static AndroidViewModelFactory sInstance;

        /**
         * Retrieve a singleton instance of AndroidViewModelFactory.
         *
         * @param application an application to pass in {@link AndroidViewModel}
         * @return A valid {@link AndroidViewModelFactory}
         */
        @NonNull
        public static AndroidViewModelFactory getInstance(@NonNull Application application) {
            if (sInstance == null) {
                sInstance = new AndroidViewModelFactory(application);
            }
            return sInstance;
        }

        private Application mApplication;

        /**
         * Creates a {@code AndroidViewModelFactory}
         *
         * @param application an application to pass in {@link AndroidViewModel}
         */
        public AndroidViewModelFactory(@NonNull Application application) {
            mApplication = application;
        }

        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
                //noinspection TryWithIdenticalCatches
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
                } catch (NoSuchMethodException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InstantiationException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InvocationTargetException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                }
            }
            return super.create(modelClass);
        }
    }

Get ViewModel instance

Get the ViewModel from the viewModelStore. If not, instance a ViewModel and put it into the viewModelStore

//ViewModelProvider.java

    private static final String DEFAULT_KEY =
            "androidx.lifecycle.ViewModelProvider.DefaultKey";
     @NonNull
    @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");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }



  @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(viewModel)) {
            //noinspection unchecked
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }

        viewModel = mFactory.create(modelClass);
        mViewModelStore.put(key, viewModel);
        //noinspection unchecked
        return (T) viewModel;
    }

5.ViewModelStore

A hash table is maintained by default

public class ViewModelStore {

    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);
    }

    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
     //Destroy all stored viewmodels
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.onCleared();
        }
        mMap.clear();
    }
}

6. Component architecture of activity and fragment

A ViewModelStore is maintained by default in activity

//FragmentActivity.java

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


 //Destroy viewModelStore
  @Override
    protected void onDestroy() {
        super.onDestroy();

        if (mViewModelStore != null && !isChangingConfigurations()) {
            mViewModelStore.clear();
        }

        mFragments.dispatchDestroy();
    }

ViewModelStore held by fragment by default

//fragment.java
   @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        if (getContext() == null) {
            throw new IllegalStateException("Can't access ViewModels from detached fragment");
        }
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
        return mViewModelStore;
    }
 
 //Destroy viewModelStore
 @CallSuper
    public void onDestroy() {
 
        if (mViewModelStore != null && !isChangingConfigurations) {
            mViewModelStore.clear();
        }
    }

7 Summary

1. Different components, as long as they hold the same ViewModelStore, can share data by acquiring ViewModel
2. Activities and fragments, or fragments under the same host, can share arrays. You can use the host's viewModelStore
3. viewModelStore in activity and fragment is destroyed only when the component is destroyed.