Kotlin抽象類二級構造函數

Question

我無法為此抽象類創建輔助構造函數。

@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

Answer 1

您可以像這樣調用輔助構造函數-

@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) }

Answer 2

創建輔助構造函數，並且類具有非空的主要構造函數時，應傳遞主要構造函數具有的所有參數，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
}

在您的情況下，可能是以下情況：

@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，您可以使用默認參數創建主構造函數：

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) { ... }

Answer 3

您cycle in the delegation calls chain error出現cycle in the delegation calls chain error的原因cycle in the delegation calls chain error ，是因為

constructor(application: Application): this(application)

等效於java：

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

這表示您正在遞歸地調用構造函數。

---

如Alok Mishra的回答所述 ，您應該改為調用超級構造函數。

constructor(application: Application): super(application)

實際上，這等效於您的主要構造函數：

BaseAndroidViewModel(application: Application): AndroidViewModel(application)

您可以constate，這只是一個替代關鍵字的constructor由BaseAndroidViewModel和super的AndroidViewModel