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“没有足够的信息来推断参数T”与Kotlin和Android

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我正在尝试使用Kotlin在我的Android应用程序中复制以下ListView：https://github.com/bidrohi/KotlinListView .

不幸的是我收到了一个错误，我无法解决自己 . 这是我的代码：

MainActivity.kt:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    val listView = findViewById(R.id.list) as ListView
    listView.adapter = ListExampleAdapter(this)
}

private class ListExampleAdapter(context: Context) : BaseAdapter() {
    internal var sList = arrayOf("Eins", "Zwei", "Drei")
    private  val mInflator: LayoutInflater

    init {
        this.mInflator = LayoutInflater.from(context)
    }

    override fun getCount(): Int {
        return sList.size
    }

    override fun getItem(position: Int): Any {
        return sList[position]
    }

    override fun getItemId(position: Int): Long {
        return position.toLong()
    }

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View? {
        val view: View?
        val vh: ListRowHolder

        if(convertView == null) {
            view = this.mInflator.inflate(R.layout.list_row, parent, false)
            vh = ListRowHolder(view)
            view.tag = vh
        } else {
            view = convertView
            vh = view.tag as ListRowHolder
        }

        vh.label.text = sList[position]
        return view
    }
}

private class ListRowHolder(row: View?) {
    public val label: TextView

    init {
        this.label = row?.findViewById(R.id.label) as TextView
    }
}
}

布局完全如下：https://github.com/bidrohi/KotlinListView/tree/master/app/src/main/res/layout

我得到的完整错误消息是： Error:(92, 31) Type inference failed: Not enough information to infer parameter T in fun findViewById(p0: Int): T! Please specify it explicitly.

我很感激我能得到任何帮助 .

android kotlin android-8.0-oreo

5 回答

  • 0

    您必须使用API级别26（或更高级别） . 此版本更改了 View.findViewById() 的签名 - 请参阅此处https://developer.android.com/preview/api-overview.html#fvbi-signature

    所以在你的情况下， findViewById 的结果是不明确的，你需要提供类型：

    1 /改变

    val listView = findViewById(R.id.list) as ListView 来

    val listView = findViewById<ListView>(R.id.list)

    2 /改变

    this.label = row?.findViewById(R.id.label) as TextView 来

    this.label = row?.findViewById<TextView>(R.id.label) as TextView

    请注意，在2 /中只需要强制转换，因为 row 可以为空 . 如果 label 也可以为空，或者如果您使 row 不可为空，则不需要它 .

    回复于
  • 6

    Andoid O改变了findViewById api

    public View findViewById（int id）;

    至

    public final T findViewById（int id）

    所以，如果你是API 26的目标，你可以改变

    val listView = findViewById（R.id.list）作为ListView

    至

    val listView = findViewById（R.id.list）

    要么

    val listView：ListView = findViewById（R.id.list）

    回复于
  • 2

    Its Working

    API Level 25 or below use this

    var et_user_name = findViewById(R.id.et_user_name) as EditText

    API Level 26 or Above use this

    val et_user_name: EditText = findViewById(R.id.et_user_name)

    快乐的编码！

    回复于
  • 3

    将您的代码更改为此 . 发生主要变化的地方标有星号 .

    package com.phenakit.tg.phenakit

import android.content.Context
import android.os.Bundle
import android.support.design.widget.BottomNavigationView
import android.support.v7.app.AppCompatActivity
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.BaseAdapter
import android.widget.ListView
import android.widget.TextView

public class MainActivity : AppCompatActivity() {

    private var mTextMessage: TextView? = null

    private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
        when (item.itemId) {
            R.id.navigation_home -> {
                mTextMessage!!.setText(R.string.title_home)
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_dashboard -> {
                mTextMessage!!.setText(R.string.title_dashboard)
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_notifications -> {
                setContentView(R.layout.activity_list_view)
                return@OnNavigationItemSelectedListener true
            }
        }
        false
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        mTextMessage = findViewById(R.id.message) as TextView?
        val navigation = findViewById(R.id.navigation) as BottomNavigationView
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)


        **val listView = findViewById<ListView>(R.id.list)**



        **listView?.adapter = ListExampleAdapter(this)**
    }

    private class ListExampleAdapter(context: Context) : BaseAdapter() {
        internal var sList = arrayOf("Eins", "Zwei", "Drei")
        private  val mInflator: LayoutInflater

        init {
            this.mInflator = LayoutInflater.from(context)
        }

        override fun getCount(): Int {
            return sList.size
        }

        override fun getItem(position: Int): Any {
            return sList[position]
        }

        override fun getItemId(position: Int): Long {
            return position.toLong()
        }

        override fun getView(position: Int, convertView: View?, parent: ViewGroup): View? {
            val view: View?
            val vh: ListRowHolder

            if(convertView == null) {
                view = this.mInflator.inflate(R.layout.list_row, parent, false)
                vh = ListRowHolder(view)
                view.tag = vh
            } else {
                view = convertView
                vh = view.tag as ListRowHolder
            }

            vh.label.text = sList[position]
            return view
        }
    }

    private class ListRowHolder(row: View?) {
        public var label: TextView

        **init { this.label = row?.findViewById<TextView?>(R.id.label) as TextView }**
    }
}
    回复于
  • 181

    我建议你使用 synthetics kotlin Android扩展程序：

    https://kotlinlang.org/docs/tutorials/android-plugin.html

    https://antonioleiva.com/kotlin-android-extensions/

    在您的情况下，代码将是这样的：

    init {
   this.label = row.label
}

    就如此容易 ;）

    回复于

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