体系结构组件：ViewModelProvider如何知道要调用哪个构造函数？

我一直在阅读有关Android引入的新架构组件的信息，但我无法弄清楚其工作原理：

ViewModelProviders.of(Activity).get(Class)

最初，我认为它调用默认构造函数并返回一个ViewModel对象，然后使用该对象实例化。一个init（）方法

public class UserProfileViewModel extends ViewModel {
    private String userId;
    private User user;

    public void init(String userId) {
        this.userId = userId;
    }
    public User getUser() {
        return user;
    }
}

摘录自指南：https : //developer.android.com/topic/libraries/architecture/guide.html

但是，在指南的后面有以下代码段：

public class UserProfileViewModel extends ViewModel {
    private LiveData user;
    private UserRepository userRepo;

    @Inject // UserRepository parameter is provided by Dagger 2
    public UserProfileViewModel(UserRepository userRepo) {
        this.userRepo = userRepo;
    }

    public void init(String userId) {
        if (this.user != null) {
            // ViewModel is created per Fragment so
            // we know the userId won't change
            return;
        }
        user = userRepo.getUser(userId);
    }

那么ViewModelProvider如何知道调用提供的构造函数？还是看到只有1个构造函数并调用它？例如，如果有两个构造函数，将会发生什么？

我尝试挖掘代码，发现的是：

@Override
        public  T create(Class 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);
        }

在DefaultFactory班级里面ViewModelProviders.java。但是，这使我更加困惑。getConstructor(Application.class)当ViewModel对象没有使用Application作为参数的构造函数时，它怎么工作？

在代码段中，有一个条件可以检查modelClass是否为AndroidViewModel（inherits ViewModel）类型，而该构造函数采用Application参数。这更像是排他性的情况，它省去了Factory查找与特定参数匹配的构造函数的过程。该提供程序在创建时会查找与提供程序参数匹配的构造函数：

public class ViewModelParameterizedProvider {

    private AtomicBoolean set = new AtomicBoolean(false);

    private ViewModelStore viewModelStore = null;


    static ViewModelParameterizedProvider getProvider() {
        return new ViewModelParameterizedProvider();
    }

    @MainThread
    public static ViewModelProvider ofSupportFragment(Fragment fragment, Object... params) {
        return getProvider().of(fragment).with(params);
    }

    @MainThread
    public static ViewModelProvider ofActivity(FragmentActivity fragmentActivity, Object... params) {
        return getProvider().of(fragmentActivity).with(params);
    }

    @MainThread
    public static ViewModelProvider ofFragment(android.app.Fragment fragment, Object... params) {
        return getProvider().of(fragment).with(params);
    }

    private ViewModelParameterizedProvider of(Fragment fragment) {
        checkForPreviousTargetsAndSet();
        viewModelStore = ViewModelStores.of(fragment);
        return this;
    }

    private ViewModelParameterizedProvider of(android.app.Fragment fragment) {
        FragmentActivity fragAct = (FragmentActivity) fragment.getActivity();
        return of(fragAct);
    }

    private ViewModelParameterizedProvider of(FragmentActivity activity) {
        checkForPreviousTargetsAndSet();
        viewModelStore = ViewModelStores.of(activity);
        return this;
    }


    private ViewModelProvider with(Object... constructorParams) {
        return new ViewModelProvider(viewModelStore, parametrizedFactory(constructorParams));
    }


    private void checkForPreviousTargetsAndSet() {
        if (set.get()) {
            throw new IllegalArgumentException("ViewModelStore already has been set. Create new instance.");
        }
        set.set(true);
    }

    private ViewModelProvider.Factory parametrizedFactory(Object... constructorParams) {
        return new ParametrizedFactory(constructorParams);
    }


    private final class ParametrizedFactory implements ViewModelProvider.Factory {
        private final Object[] mConstructorParams;

        ParametrizedFactory(Object... constructorParams) {
            mCOnstructorParams= constructorParams;
        }

        @Override
        public  T create(Class modelClass) {
            if (modelClass == null) {
                throw new IllegalArgumentException("Target ViewModel class can not be null")
            }
            Log.w("ParametrizedFactory", "Don't use callbacks or Context parameters in order to avoid leaks!!")
            try {
                if (mCOnstructorParams== null || mConstructorParams.length == 0) {
                    return modelClass.newInstance();
                } else {
                    Class[] classes = new Class[mConstructorParams.length];
                    for (int i = 0; i 

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