JetPack之ViewModel源码解析

我们认识中的ViewModel是什么

Viewmodel其实就是activity中一个普通的实体。Activity持有viewmodel的引用，业务逻辑在viewmodel进行，关于界面和ui的操作在activity中进行，而添加和释放资源和viewmodel有关的那么就在viewmodel中设置函数，然后在activity中调用。ViewModel作为Activity/Fragment与其他组件的连接器，负责转换和聚合Model中返回的数据，使这些数据易于显示，并把这些数据改变及时的通知给Activity/Fragment。

ViewModel的生命周期

从图中看出，Activity的生命周期不断变化，经历了被销毁重新创建，而ViewModel的生命周期没有发生变化。

从创建一个viewModel入手分析源码:

MyViewModel myViewModel = ViewModelProviders.of(this).get(MyViewModel.class);

进入ViewModelProviders.of(this)方法：

    @NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull FragmentActivity activity) {
        return of(activity, null);
    }

    @NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) {
        Application application = checkApplication(checkActivity(fragment));
        if (factory == null) {
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(fragment.getViewModelStore(), factory);
    }

    @NonNull
    @MainThread
    public static ViewModelProvider of(@NonNull FragmentActivity activity,
            @Nullable Factory factory) {
        Application application = checkApplication(activity);
        if (factory == null) {
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(activity.getViewModelStore(), factory);
    }

checkApplication(activity)会获取该activity所属的application。
ViewModelProvider.AndroidViewModelFactory.getInstance(application)会创建一个单例的AndroidViewModelFactory，并从activity或fragment中获取viewmodelstore，作为两个个参数传入ViewModelProvider构造方法，最后会新建一个ViewModelProvider作为返回。

ViewModelStore代码实现如下：

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);
    }

    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();
    }
}

ViewModelStore类通过hashmap来存储和移除Viewmodel。

AndroidViewModelFactory类代码如下：

    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);
        }
    }

ViewModel是抽象类，AndroidViewModel继承于ViewModel，AndroidViewModelFactory从它的create方法可以看出，通过反射生成ViewModel的实现类。

get(MyViewModel.class)部分

进入到了ViewModelProvider进行执行，ViewModelProvider代码如下：

public class ViewModelProvider {

    private static final String DEFAULT_KEY =
            "androidx.lifecycle.ViewModelProvider.DefaultKey";

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

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        String canonicalName = modelClass.getCanonicalName();    //注释1
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);  //注释2
    }

    @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.
            }
        }
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
        } else {
            viewModel = (mFactory).create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        //noinspection unchecked
        return (T) viewModel;
    }
}

过程分析：注释1处得到类的名称，对这个名称进行字符串拼接，拼接上前缀androidx.lifecycle.ViewModelProvider.DefaultKey，作为注释2处方法的参数，作为key值去ViewModelStore中获取viewmodel实例。如果ViewModel能转换为modelClass类的对象，直接返回该ViewModel。否则就会通过AndroidViewModelFactory的create方法通过反射创建一个ViewModel，并将其存储到ViewModelStore中。