Java项目转Kotlin后RecyclerView智能转换失败与mUploads未识别问题求助

原因 #

Kotlin的空安全机制限制：postRecyclerView、shimmerFrameLayout这类属性是用var声明的可变可空属性，编译器无法保证你调用方法的瞬间，该属性没有被其他线程修改为null，因此禁止直接调用成员方法，抛出对应错误。

解决方案 #

有几种常用处理方式，可按需选择：

• 空安全调用：如果属性为空时不需要执行对应逻辑，用?.调用，空值时会自动跳过执行不会崩溃

  postRecyclerView?.layoutManager = staggeredGridLayoutManager
  postRecyclerView?.addOnScrollListener(/* 滚动监听实例 */)
  shimmerFrameLayout?.startShimmer()
  

• 非空断言：如果你能100%确认调用时属性一定不为空，用!!强制认定非空，注意空值时会抛出空指针异常

  postRecyclerView!!.layoutManager = staggeredGridLayoutManager
  

• 局部变量缓存：先把可变属性赋值给不可变局部变量，编译器确认值不会变更后就允许直接调用，适合需要连续调用多个方法的场景

  val recyclerView = postRecyclerView
  if (recyclerView != null) {
      recyclerView.layoutManager = staggeredGridLayoutManager
      recyclerView.addOnScrollListener(/* 滚动监听实例 */)
  }
  

• 根治方案：如果是视图属性，建议开启View Binding，自动生成的视图属性均为非空不可变类型，不会触发该报错；也可将确定会提前初始化的属性声明为lateinit var，规避空安全检查。

原因 #

你转换后的Kotlin代码中，companion object（对应Java的静态域）为空，没有定义mUploads属性，因此调用时会报未找到引用的错误。另外你当前的Kotlin代码构造函数也缺失了原Java代码的列表入参，和原有逻辑不匹配。

解决方案 #

这里不推荐保留静态声明的实现，静态变量生命周期和类绑定，容易引发内存泄漏，建议改为实例成员，修改后完整Adapter核心代码如下：

class PostAdapter(
    val mcontext: Context,
    private var mUploads: List<Upload> = emptyList()
) : RecyclerView.Adapter<PostAdapter.PostViewHolder>() {
    private var mListener: OnItemClickListener? = null

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): PostViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.post_item_container_profile, parent, false)
        return PostViewHolder(view)
    }

    override fun onBindViewHolder(holder: PostViewHolder, position: Int) {
        val uploadCurrent = mUploads[position]
        // 原有加载图片等逻辑保持不变
        val shimmer = ColorHighlightBuilder()
            .setBaseColor(Color.parseColor("#F3F3F3"))
            .setBaseAlpha(1f)
            .setHighlightColor(Color.parseColor("#E7E7E7"))
            .setHighlightAlpha(1f)
            .setDropoff(50f)
            .build()
        val shimmerDrawable = ShimmerDrawable()
        shimmerDrawable.setShimmer(shimmer)
        Glide.with(mcontext)
            .load(uploadCurrent.getmImageUrl())
            .diskCacheStrategy(DiskCacheStrategy.AUTOMATIC)
            .placeholder(shimmerDrawable)
            .centerCrop()
            .fitCenter()
            .into(holder.imageView)
    }

    override fun getItemCount(): Int = mUploads.size

    fun setOnItemClickListener(listener: OnItemClickListener?) {
        mListener = listener
    }

    fun setUploads(uploads: List<Upload>) {
        mUploads = uploads
        notifyDataSetChanged() // 数据更新后刷新列表，原有Java代码缺失该逻辑
    }

    interface OnItemClickListener {
        fun onClick(view: View?)
        fun onItemClick(position: Int)
        fun onDeleteClick(position: Int)
    }

    // 去掉inner关键字，对应原Java的静态内部类逻辑
    class PostViewHolder(itemView: View) :
        RecyclerView.ViewHolder(itemView), View.OnClickListener, View.OnCreateContextMenuListener,
        MenuItem.OnMenuItemClickListener {
        val imageView: ShapeableImageView = itemView.findViewById(R.id.imagePost)
        private var mListener: OnItemClickListener? = null

        override fun onClick(v: View) {
            val position = adapterPosition
            if (position != RecyclerView.NO_POSITION) {
                mListener?.onItemClick(position)
            }
        }

        override fun onCreateContextMenu(menu: ContextMenu, v: View, menuInfo: ContextMenu.ContextMenuInfo?) {
            val delete = menu.add(Menu.NONE, 2, 2, "Delete")
            delete.setOnMenuItemClickListener(this)
        }

        override fun onMenuItemClick(item: MenuItem): Boolean {
            val position = adapterPosition
            if (position != RecyclerView.NO_POSITION && item.itemId == 2) {
                mListener?.onDeleteClick(position)
                return true
            }
            return false
        }

        init {
            itemView.setOnClickListener(this)
            itemView.setOnCreateContextMenuListener(this)
        }
    }
}

如果确实需要保留原有静态逻辑，只需要在空的companion object中添加mUploads声明即可：

companion object {
    var mUploads: List<Upload> = emptyList()
}

内容的提问来源于stack exchange，提问作者Vasant Raval