Android ViewModel 没有零参数构造函数 [英] Android ViewModel has no zero argument constructor
问题描述
我正在关注 this 文档以了解 LiveData 和 ViewModel.在文档中,ViewModel 类具有构造函数,
public class UserModel extends ViewModel {私有 MutableLiveData<用户>用户;@Inject UserModel(MutableLiveData<用户>用户){this.user = 用户;}公共无效初始化(){如果(this.user != null){返回;}this.user = new MutableLiveData<>();}public MutableLiveData<用户>获取用户(){返回用户;}}
但是,当我运行代码时,出现异常:
final UserViewModelviewModel = ViewModelProviders.of(this).get(UserViewModel.class);
<块引用>
Caused by: java.lang.RuntimeException: Cannot create an instance of class UserViewModel引起:java.lang.InstantiationException:类没有零参数构造函数
当使用 ViewModelProviders
初始化 ViewModel
的子类时,默认情况下它需要你的 UserModel代码> 类具有零参数构造函数.在您的情况下,您的构造函数具有参数
MutableLiveData
.
解决此问题的一种方法是为您的 UserModel
设置一个默认的无参数构造函数.
否则,如果您想为您的 ViewModel 类使用非零参数构造函数,您可能需要创建一个自定义的 ViewModelFactory
类来初始化您的 ViewModel 实例,该实例实现了 ViewModelProvider.Factory
接口.
我还没有尝试过,但这里有一个优秀的谷歌示例链接:github.com/googlesamples/android-architecture-components.具体看这个类/GithubViewModelFactory.java" rel="nofollow noreferrer">GithubViewModelFactory.java 用于 Java 代码和此类 GithubViewModelFactory.kt 对应的 Kotlin 代码.
I am following this documentation to learn about LiveData and ViewModel. In the doc, the ViewModel class has constructor as such,
public class UserModel extends ViewModel {
private MutableLiveData<User> user;
@Inject UserModel(MutableLiveData<User> user) {
this.user = user;
}
public void init() {
if (this.user != null) {
return;
}
this.user = new MutableLiveData<>();
}
public MutableLiveData<User> getUser() {
return user;
}
}
However, when I run the code, I get exception:
final UserViewModelviewModel = ViewModelProviders.of(this).get(UserViewModel.class);
Caused by: java.lang.RuntimeException: Cannot create an instance of class UserViewModel Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor
While initializing subclasses of ViewModel
using ViewModelProviders
, by default it expects your UserModel
class to have a zero argument constructor.
In your case your constructor has the argument MutableLiveData<User> user
.
One way to fix this is to have a default no arg constructor for your UserModel
.
Otherwise, if you want to have a non-zero argument constructor for your ViewModel class, you may have to create a custom ViewModelFactory
class to initialise your ViewModel instance, which implements the ViewModelProvider.Factory
interface.
I have not tried this yet, but here's a link to an excellent Google sample for this: github.com/googlesamples/android-architecture-components. Specifically, check out this class GithubViewModelFactory.java for Java code and this class GithubViewModelFactory.kt for the corresponding Kotlin code.
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