尝试访问 Kotlin 片段中的视图时出现 NullPointerException [英] NullPointerException when trying to access views in a Kotlin fragment

问题描述

如何使用带有 Fragment 的 Kotlin Android 扩展?如果我在 onCreateView() 中使用它们，我会得到这个 NullPointerException 异常:

How to use Kotlin Android Extensions with Fragments? If I use them inside onCreateView(), I get this NullPointerException exception:

Caused by: java.lang.NullPointerException: Attempt to invoke方法 'android.view.View android.view.View.findViewById(int)'空对象引用

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference

这是片段代码:

package com.obaied.testrun.Fragment

import android.os.Bundle
import android.support.v4.app.Fragment
import android.util.Log
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.obaied.acaan.R
import kotlinx.android.synthetic.main.fragment_card_selector.*

public class CardSelectorFragment : Fragment() {
    val TAG = javaClass.canonicalName

    companion object {
        fun newInstance(): CardSelectorFragment {
            return CardSelectorFragment()
        }
    }

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        var rootView = inflater?.inflate(R.layout.fragment_card_selector, container, false)
        btn_K.setOnClickListener { Log.d(TAG, "onViewCreated(): hello world"); }

        return rootView
    }
}
`

推荐答案

Kotlin 合成属性并不神奇，而且工作方式非常简单.当您访问 btn_K 时，它会调用 getView().findViewById(R.id.btn_K).

Kotlin synthetic properties are not magic and work in a very simple way. When you access btn_K, it calls for getView().findViewById(R.id.btn_K).

问题是您访问它的速度太快了.getView() 在 onCreateView 中返回 null.尝试在 onViewCreated 方法中执行此操作:

The problem is that you are accessing it too soon. getView() returns null in onCreateView. Try doing it in the onViewCreated method:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    btn_K.setOnClickListener { Log.d(TAG, "onViewCreated(): hello world"); }
}

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