Kotlin:无法调用表达式，因为找不到函数 invoke()

Question

我正在尝试构建一个实现 Google map 的应用程序。由于某种原因，我收到无法调用表达式的错误，因为找不到函数 invoke() 。我不知道如何解决这个问题，也许你们中的一个可以帮助？

    package com.example.maxs.kotlinnearbyv2

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.Toast
import com.example.maxs.kotlinnearbyv2.Common.Common
import com.example.maxs.kotlinnearbyv2.Model.MyPlaces
import com.example.maxs.kotlinnearbyv2.Remote.IGoogleAPIService
import com.google.android.gms.maps.*
import com.google.android.gms.maps.model.BitmapDescriptorFactory

import com.google.android.gms.maps.model.LatLng
import com.google.android.gms.maps.model.MarkerOptions
import kotlinx.android.synthetic.main.activity_maps.*
import retrofit2.Call
import retrofit2.Callback
import retrofit2.Response

class MapsActivity : AppCompatActivity(), OnMapReadyCallback {

private lateinit var mMap: GoogleMap

private var latitude:Double=0.toDouble()
private var longitude:Double=0.toDouble()

lateinit var mService:IGoogleAPIService

internal var currentPlace: MyPlaces?=null

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_maps)
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    val mapFragment = supportFragmentManager
        .findFragmentById(R.id.map) as SupportMapFragment
    mapFragment.getMapAsync(this)

    //Init Service
    mService = Common.googleApiService

    bottom_navigation_view.setOnNavigationItemReselectedListener {item ->
        when(item.itemId)
        {
            R.id.action_hospital -> nearByPlace("hospital")
            R.id.action_restaurant -> nearByPlace("restaurant")
            R.id.action_market -> nearByPlace("market")
            R.id.action_school -> nearByPlace("school")
        }
    }
}

private fun nearByPlace(typePlace: String) {

    //Clear all marker on Map
    mMap.clear()
    //build URL request base on location
    val url = getUrl(latitude,longitude, typePlace)

    mService.getNearByPlaces(url)
        .enqueue(object : Callback<MyPlaces>{
            override fun onResponse(call: Call<MyPlaces>, response: Response<MyPlaces>) {

                currentPlace = response.body()

                if(response!!.isSuccessful)
                {
                    for(i in 0 until response!!.body()!!.results!!.size)
                    {
                        val markerOptions=MarkerOptions()
                        val googlePlace = response.body().results!!(i)
                        val lat = googlePlace.geometry!!.location!!.lat
                        val lng = googlePlace.geometry!!.location!!.lng
                        val placeName = googlePlace.name
                        val latLng = LatLng(lat, lng)

                        markerOptions.position(latLng)
                        markerOptions.title(placeName)
                        if (typePlace.equals("hospital"))
                            markerOptions.icon(BitmapDescriptorFactory.fromResource(R.drawable.ic_local_hospital_black_24dp))
                        else if (typePlace.equals("market"))
                            markerOptions.icon(BitmapDescriptorFactory.fromResource(R.drawable.ic_shopping_cart_black_24dp))
                        else if (typePlace.equals("restaurant"))
                            markerOptions.icon(BitmapDescriptorFactory.fromResource(R.drawable.ic_restaurant_black_24dp))
                        else if (typePlace.equals("school"))
                            markerOptions.icon(BitmapDescriptorFactory.fromResource(R.drawable.ic_school_black_24dp))
                        else
                            markerOptions.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_BLUE))

                        markerOptions.snippet(i.toString())

                        //add marker to map
                        mMap!!.addMarker(markerOptions)


                    }
                    //move camera
                    mMap!!.moveCamera(CameraUpdateFactory.newLatLng(LatLng(latitude, longitude)))
                    mMap!!.animateCamera(CameraUpdateFactory.zoomTo(15.0f))
                }
            }

            override fun onFailure(call: Call<MyPlaces>, t: Throwable) {
                Toast.makeText(baseContext, ""+t!!.message,Toast.LENGTH_SHORT).show()
            }

        })
}

private fun getUrl(latitude: Double, longitude: Double, typePlace: String): String {

    val googlePlaceUrl = StringBuilder("https://maps.googleapis.com/maps/api/place/nearbysearch/json")
    googlePlaceUrl.append("?location=$latitude,$longitude")
    googlePlaceUrl.append("&radius=10000") //10 km
    googlePlaceUrl.append("&type=$typePlace")
    googlePlaceUrl.append("&key=")

    Log.d("URL_DEBUG", googlePlaceUrl.toString())
    return googlePlaceUrl.toString()
}

/**
 * Manipulates the map once available.
 * This callback is triggered when the map is ready to be used.
 * This is where we can add markers or lines, add listeners or move the camera. In this case,
 * we just add a marker near Sydney, Australia.
 * If Google Play services is not installed on the device, the user will be prompted to install
 * it inside the SupportMapFragment. This method will only be triggered once the user has
 * installed Google Play services and returned to the app.
 */
override fun onMapReady(googleMap: GoogleMap) {
    mMap = googleMap

    // Add a marker in Sydney and move the camera
    val barbier = LatLng(52.391274, 6.449712)
    mMap.addMarker(MarkerOptions().position(barbier).title("Marker in Barbier"))
    mMap.moveCamera(CameraUpdateFactory.newLatLng(barbier))
    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(barbier, 15.0f))
}
}

我似乎找不到任何解决方案，而且我可能觉得很难……
错误是在 response.body()!!.results!!(i)

val googlePlace = response.body().results!!(i)

它现在肯定让我发疯。

Answer 1

要访问数组或列表中的元素，请使用方括号，例如:

array[i]
list[i] // or list.get(i)
results!![i]

关于错误消息:Kotlin 假设 invoke operator ，你没有提供。您可能想了解调用运算符的用途。有时它会派上用场。但是对于您的问题，方括号就足够了。

作为(进一步的)旁注:不要用大量 !! 编写代码，而是尝试确定什么是 null首先，如果它不适合您的需要，则省略其余部分，例如:

response?.also {
  if (it.isSuccessful) {
    it.body()?.results?.forEach {
      //...
    }
  }
}

只是一个开始...然后您可能想进一步简化事情...只需省略 !!只要有可能...您可能还想了解 null safety in Kotlin也许还有关于 smart casts .

您的 typePlace.equals(... -条件也可以完美地替换为 when ，例如:

when(typePlace) {
  "hospital" -> ...
  "market" -> ...

这结合了 let或 also甚至可以进一步减少您的代码，但这可能是另一个故事，更适合 code review .