android – 尝试访问Kotlin片段中的视图时出现NullPointerException

如何使用Kotlin Android Extensions with Fragments？
如果我在onCreateView()中使用它们,我会得到这个NullPointerException异常：

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
    }
}
`

解决方法:

Kotlin合成属性不是神奇的,并且以非常简单的方式工作.当你访问btn_K时,它会调用getView().findViewById(R.id.btn_K).

问题是您过早访问它. getView()在onCreateView中返回null.尝试在onViewCreated方法中执行此操作：

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