java-架构组件：ViewModelProvider如何知道要调用哪个构造函数？

概述我一直在阅读有关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publicclassUserProfileViewModelextendsViewModel{

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> 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 extends viewmodel> T create(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);        }

在viewmodelProvIDers.java内部的DefaultFactory类内部.但是,这使我更加困惑.当viewmodel对象没有以Application为参数的构造函数时,getConstructor(Application.class)怎么工作？

解决方法:

在代码段中,有一个条件可以检查modelClass是否为AndroIDviewmodel类型(继承自viewmodel),该构造函数采用Application参数.这更像是排他性的案例,它使Factory无需查找与特定参数匹配的构造函数.
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 extends viewmodel> T create(Class<T> 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 < mConstructorParams.length; i++) {                        classes[i] = mConstructorParams[i].getClass();                    }                    return modelClass.getConstructor(classes).newInstance(mConstructorParams);                }            } catch (InstantiationException e) {                e.printstacktrace();            } catch (illegalaccessexception e) {                e.printstacktrace();            } catch (NoSuchMethodException e) {                e.printstacktrace();            } catch (InvocationTargetException e) {                e.printstacktrace();            }            return null;        }    }}

这是kotlin version.
这是more read on the subject

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