Kotlin 抽象类二级构造函数答案

【问题标题】：Kotlin abstract class secondary constructorKotlin 抽象类二级构造函数
【发布时间】：2019-06-02 21:31:36
【问题描述】：

我无法为这个抽象类创建辅助构造函数。

@SuppressLint("StaticFieldLeak")
abstract class BaseAndroidViewModel(
        application: Application,
        private val creditsGetByIdUseCase: CreditsGetByIdUseCase,
        private val videosGetByIdUseCase: VideosGetByIdUseCase,
        private val reviewGetByIdUseCase: ReviewGetByIdUseCase,
        private val addToFavoritesUseCase: AddToFavoritesUseCase,
        private val getFavoriteByIdUseCase: GetFavoriteByIdUseCase
) : AndroidViewModel(application) {

constructor(application: Application) : this(application) // There's a cycle in the delegation calls chain error

【问题讨论】：

标签： android kotlin constructor class-constructors

【解决方案1】：

当你创建一个 secondary 构造函数，并且一个类有非空的 primary 构造函数时，你应该传递 primary 构造函数具有的所有参数，例如：

abstract class BaseAndroidViewModel(
        application: Application,
        private val creditsGetByIdUseCase: String,
        private val videosGetByIdUseCase: String
) : AndroidViewModel(application) {

    constructor(application: Application) : this(application, 
            "creditsGetByIdUseCase", "videosGetByIdUseCase") // here we pass other necessary parameters
}

在您的情况下，可能如下：

@SuppressLint("StaticFieldLeak")
abstract class BaseAndroidViewModel(
    application: Application,
    private val creditsGetByIdUseCase: CreditsGetByIdUseCase?,
    private val videosGetByIdUseCase: VideosGetByIdUseCase?,
    private val reviewGetByIdUseCase: ReviewGetByIdUseCase?,
    private val addToFavoritesUseCase: AddToFavoritesUseCase?,
    private val getFavoriteByIdUseCase: GetFavoriteByIdUseCase?
) : AndroidViewModel(application) {

    constructor(application: Application) : this(application, null, null, null, null, null)
}

或者您可以使用默认参数创建 primary 构造函数：

abstract class BaseAndroidViewModel(
        application: Application,
        private val creditsGetByIdUseCase: CreditsGetByIdUseCase? = null,
        private val videosGetByIdUseCase: VideosGetByIdUseCase? = null,
        private val reviewGetByIdUseCase: ReviewGetByIdUseCase? = null ,
        private val addToFavoritesUseCase: AddToFavoritesUseCase? = null,
        private val getFavoriteByIdUseCase: GetFavoriteByIdUseCase? = null
    ) : AndroidViewModel(application) { ... }

【讨论】：

【解决方案2】：

你可以像这样调用二级构造函数-

@SuppressLint("StaticFieldLeak") abstract class BaseAndroidViewModel : AndroidViewModel{
constructor(application: Application,
            creditsGetByIdUseCase: CreditsGetByIdUseCase,
            videosGetByIdUseCase: VideosGetByIdUseCase,
            reviewGetByIdUseCase: ReviewGetByIdUseCase,
            addToFavoritesUseCase: AddToFavoritesUseCase,
            getFavoriteByIdUseCase: GetFavoriteByIdUseCase) : super(application)

constructor(application: Application) : super(application) }

【讨论】：

【解决方案3】：

您收到cycle in the delegation calls chain error 的原因是因为

constructor(application: Application): this(application)

相当于java：

public MyClass(Application application){
    this(application)
}

这表示你正在递归调用你的构造函数。

如Alok Mishra's answer 所述，您应该改为调用超级构造函数。

constructor(application: Application): super(application)

这实际上相当于你的主构造函数：

BaseAndroidViewModel(application: Application): AndroidViewModel(application)

您可以声明这只是将关键字 constructor 替换为 BaseAndroidViewModel 和 super 替换为 AndroidViewModel

【讨论】：