Kotlin Android 中 ViewHolder 中的 getActivity() / 上下文

Question

我正在为片段构建 ViewHolder 和适配器，当我尝试为 ViewHolder 创建 OnClick 时，我传递的上下文均无效。 我无法使用getActivity()中的任何activity ，并且p0.!.context和itemView.context都可以工作。 我应该从哪里获取上下文，如何引用它？

package com._________.criminalintent

import android.os.Bundle
import android.support.v4.app.Fragment
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import android.widget.Toast

class CrimeListFragment: Fragment() {
    private var mCrimeRecyclerView: RecyclerView? = null
    private var mAdapter: CrimeAdapter? = null

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
    // fragment_crime_list.xml has a RecyclerView element = crime_recycler_view
    // inflate the fragment into the activity
        val view = inflater!!.inflate(R.layout.fragment_crime_list, container, false)

        // grab the recyclerView and give it a required layoutManager
        mCrimeRecyclerView = view.findViewById(R.id.crime_recycler_view)
        mCrimeRecyclerView!!.layoutManager = LinearLayoutManager(activity)
        updateUI()
        return view
    }

    private fun updateUI() {
        val crimeLab = CrimeLab.get(activity)
        val crimes = crimeLab.getCrimes()
        mAdapter = CrimeAdapter(crimes)
        // Connect the adapter to the recyclerView
        mCrimeRecyclerView!!.adapter = mAdapter
    }

    /**
     * in Kotlin, we must give the view passed into the constructor directly
     * as a substitute for a super() call
     *
     * create a ViewHolder that holders the crime list item's view
     *
     * super(itemView) = super(inflater!!.inflate(R.layout.list_item_crime, parent, false))
     * MUST give it the direct value in Kotlin
     */
    private class CrimeHolder(inflater: LayoutInflater?, parent: ViewGroup):
        RecyclerView.ViewHolder(inflater!!.inflate(R.layout.list_item_crime, parent, false)),
        View.OnClickListener {

        private var mCrime: Crime? = null

        /**
         * When given a crime, this CrimeHolder will update the title and date for this Crime
         */
        fun bind(crime: Crime) {
           mCrime = crime
            val titleTextView = itemView.findViewById<TextView>(R.id.crime_title)
            val dateTextView = itemView.findViewById<TextView>(R.id.crime_date)
            titleTextView.text = mCrime!!.mTitle
            dateTextView.text = mCrime!!.mDate.toString()
        }

        override fun onClick(p0: View?) {

            Toast.makeText(WHAT_TO_PUT_HERE, "${mCrime!!.mTitle} clicked!", Toast.LENGTH_SHORT / 2)
                .show()
    }
}


    private class CrimeAdapter(private var mCrimes: MutableList<Crime>):
        RecyclerView.Adapter<CrimeHolder>() {

        /**
         * - Calls our CrimeHolder to make our custom ViewHolders
         * - Called by RecyclerView when it needs a new view to display
         * - Gets the layoutInflater from the ViewGroup and returns a CrimeHolder of it
         */
        override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): CrimeHolder
        = CrimeHolder(LayoutInflater.from(parent!!.context), parent)


        /**
         * Bind the crime (data) to the CrimeHolder
         */
        override fun onBindViewHolder(holder: CrimeHolder?, position: Int) {
            holder!!.bind(mCrimes[position])
        }

        /**
         * Sees how many items are in the RecyclerView that need to be shown
         */
        override fun getItemCount(): Int = mCrimes.size
    }
}

Answer 1

在您的实现中，您可以安全地使用提供给OnClickListener的View中的Context

override fun onClick(p0: View) {
          Toast.makeText(p0.context, "${mCrime!!.mTitle} clicked!", Toast.LENGTH_SHORT / 2)
                .show()
    }

请记住设置 onclick：

fun bind(crime: Crime) {
           mCrime = crime
            val titleTextView = itemView.findViewById<TextView>(R.id.crime_title)
            val dateTextView = itemView.findViewById<TextView>(R.id.crime_date)
            titleTextView.text = mCrime!!.mTitle
            dateTextView.text = mCrime!!.mDate.toString()
            itemView.setOnClickListener(this)
}

此外，默认情况下，所有 Kotlin 类都是嵌套的（ static的）。 所以你的private class CrimeHolder相当于 Java 中的private static class CrimeHolder 。 这就是为什么您无法从CrimeHolder中访问getActivity()

Answer 2

在您的支架内使用itemView.context属性。

编辑：您的onClick不“工作”（未调用）的原因是因为您尚未注册onClickListener ，例如：

itemView.setOnClickListener(this)

在你的 holder 中init或bind 。

Answer 3

 itemView.setOnClickListener {
                Toast.makeText(itemView.context, "Item is clicked $position", Toast.LENGTH_SHORT).show() }
        }