Kotlin中通用通配符作为参数 [英] Wildcards generic in Kotlin for parameter

问题描述

我有一个用于侦听api回调的抽象类，如下所示. ApiRs是每个API响应对象都继承自其的父对象

I have an abstract class using for listen api callback like below. ApiRs is a parent object that every API response object inherit from it

abstract class ApiCallback<in T : ApiRs> {

    open fun onSucceed(apiRsModel: T) {}

    open fun onFailed(code: Int,
                      message: String) {
    }
}

我还写了一个函数，在请求api之前检查网络状态，如果网络未连接，它将触发onFailed

and i also write a function to check network status before request api, if network is not connected, then it will trigger onFailed

fun isNetworkAvailable(apiCallback: ApiCallback<ApiRs>?,
                           context: Context): Boolean{
        val connectivityManager = context
                .getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val networkInfo = connectivityManager.activeNetworkInfo

        // device network unreachable
        if (networkInfo == null
                || !networkInfo.isConnected) {
            // callback with network error
            apiCallback?.onFailed(code, message)
            return false
        }

        return true
    }

参数类型ApiCallback<ApiRs>?实际上我想像ApiCallback<? extends ApiRs>?一样写，但是Kotlin没有通配符.如果我将通用替换为ApiCallback<out ApiRs>?，它将显示错误

the parameter type ApiCallback<ApiRs>? actually i want to write it like ApiCallback<? extends ApiRs>?, but Kotlin has no wildcard. If i replace the generic to ApiCallback<out ApiRs>?, it will show an error

投影与ApiCallback的相应类型参数的方差冲突

Projection is conflicting with variance of the corresponding type parameter of ApiCallback

有什么办法可以解决?还是我对Kotlin泛型的概念有误?

Is there any way to fix it? or just i have wrong concept of Kotlin generic?

2017/09/18更新-正确的方法是:

2017/09/18 update - The right way is :

方法:

fun <T : ApiRs> isNetworkAvailable(apiCallback: ApiCallback<T>?,
                                       context: Context): Boolean {
        val connectivityManager = context
                .getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val networkInfo = connectivityManager.activeNetworkInfo

        // device network unreachable
        if (networkInfo == null
                || !networkInfo.isConnected) {
            // callback with network error
            callbackWithFailed(
                    apiCallback,
                    ApiError.API_ERROR_CODE_0,
                    context.getString(R.string.api_error_code_0))
            return false
        }

        return true
    }

推荐答案

我了解您希望您的方法接受apiCallback: ApiCallback<ApiRs>?的方式是ApiCallback的通用类型可以是ApiRs本身，也可以是以下任何一种它的子类型. Kotlin具有通配符，而不是通配符上限.您可以这样声明函数:

I understand that you want to accept apiCallback: ApiCallback<ApiRs>? in your method in a way that the generic type of ApiCallback can be ApiRs itself or any of its subtypes. Instead of wildcards, Kotlin has upper bounds. You can declare the function like this:

fun <T: ApiRs> isNetworkAvailable(apiCallback: ApiCallback<T>?,
                       context: Context)

这篇关于Kotlin中通用通配符作为参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！