【Android学习】Adapter中使用Context

参考文章

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需求： Glide加载图片需要用到Context

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1. 通过 Adapter 构造函数传入 Context

class MyAdapter(
    private val context: Context,
    private val dataList: List<MyData>
) : RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
​
    class MyViewHolder(val binding: RcyItemViewBinding) :
        RecyclerView.ViewHolder(binding.root) {
    }
​
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = RcyItemViewBinding.inflate(
            LayoutInflater.from(parent.context),
            parent,
            false
        )
        return MyViewHolder(binding)
    }
​
    override fun getItemCount(): Int = dataList.size
​
    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val data = dataList[position]
        holder.binding.apply {
            Glide.with(context).load(data.imageUrl).into(holder.binding.imageIv)
            holder.binding.contentTv.text = data.content
        }
    }
}

2. 通过 Parent.context 获取

class MyAdapter(private val dataList: List<MyData>) :
    RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
​
    private lateinit var mContext: Context  //
​
    class MyViewHolder(val binding: RcyItemViewBinding) :
        RecyclerView.ViewHolder(binding.root) {
​
    }
​
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        mContext = parent.context //
        val binding = RcyItemViewBinding.inflate(
            LayoutInflater.from(parent.context),
            parent,
            false
        )
        return MyViewHolder(binding)
    }
​
    override fun getItemCount(): Int = dataList.size
​
    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val data = dataList[position]
        holder.binding.apply {
            Glide.with(mContext).load(data.imageUrl).into(holder.binding.imageIv)
            holder.binding.contentTv.text = data.content
        }
    }
}
​

3. 通过 onAttachedToRecyclerView() 方法获取

onAttachedToRecyclerView() 是 RecyclerView.Adapter 类中的一个回调方法，主要用于在 RecyclerView 适配器与 RecyclerView 关联时执行一些初始化操作。这个方法在适配器绑定到 RecyclerView 时被调用，通常用来执行需要依赖于 RecyclerView 的上下文的初始化逻辑，例如设置布局管理器、添加 ItemAnimator 或进行数据初始化等。

class MyAdapter(private val dataList: List<MyData>) :
    RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
​
    private lateinit var mContext: Context//
​
    override fun onAttachedToRecyclerView(recyclerView: RecyclerView) {
        super.onAttachedToRecyclerView(recyclerView)
        mContext = recyclerView.context//
    }
​
    class MyViewHolder(val binding: RcyItemViewBinding) :
        RecyclerView.ViewHolder(binding.root) {
​
    }
​
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = RcyItemViewBinding.inflate(
            LayoutInflater.from(parent.context),
            parent,
            false
        )
        return MyViewHolder(binding)
    }
​
    override fun getItemCount(): Int = dataList.size
​
    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val data = dataList[position]
        holder.binding.apply {
            Glide.with(mContext).load(data.imageUrl).into(holder.binding.imageIv)
            holder.binding.contentTv.text = data.content
        }
    }
}

4. 通过 ImageView 获取 context (局限于本例子中)

class MyAdapter(private val dataList: List<MyData>) :
    RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
​
    class MyViewHolder(val binding: RcyItemViewBinding) :
        RecyclerView.ViewHolder(binding.root) {
​
    }
​
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = RcyItemViewBinding.inflate(
            LayoutInflater.from(parent.context),
            parent,
            false
        )
        return MyViewHolder(binding)
    }
​
    override fun getItemCount(): Int = dataList.size
​
    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val data = dataList[position]
        holder.binding.apply {
            Glide.with(imageIv.context).load(data.imageUrl).into(holder.binding.imageIv)
            holder.binding.contentTv.text = data.content
        }
    }
}

5. 四种方法对比分析

• 这四种方法虽然看着写法很不一样，但是其获取到的 context 其实都是同一个，那就是 MainActivity。
• 方法1：既然可以在类内部直接获取到的参数，完全就没有必要再写一个参数从外部导入
• 方法2：不会导致内存泄漏，但是我也不推荐这样的写法，毕竟 onCreateViewHolder() 方法的执行次数是由 itemCount
  所决定的，所以也就意味着 mContext = parent.context 也会执行 itemCount次，明明一次就行，其它多余的操作，会造成不必要的开销
• 方法3：通过 onAttachedToRecyclerView() 方法，只会进行一次赋值操作，很OK啊，但毕竟他又是定义全局变量，又是覆写方法

6. 作者推荐的方法

class MyAdapter(private val dataList: List<MyData>) :
    RecyclerView.Adapter<MyAdapter.MyViewHolder>() {
​
    class MyViewHolder(
        private val context: Context,
        private val binding: RcyItemViewBinding
    ) :
        RecyclerView.ViewHolder(binding.root) {
​
        fun bind(data: MyData) {
            Glide.with(context).load(data.imageUrl).into(binding.imageIv)
            binding.contentTv.text = data.content
        }
    }
​
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val binding = RcyItemViewBinding.inflate(
            LayoutInflater.from(parent.context),
            parent,
            false
        )
        return MyViewHolder(parent.context, binding)
    }
​
    override fun getItemCount(): Int = dataList.size
​
    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        holder.bind(dataList[position])
    }
}

我选择在 onCreateViewHolder() 通过 parent.context 方法获取 context，但是，不是赋值给全部变量 mContext，而是直接传给 ViewHolder作为一个私有变量供其使用。