BasicViewHolder.md

BasicViewHolder

BasicViewHolder makes it easier to define ViewHolders and minimises the need for creating ViewHolder classes. Usually, in a ViewHolder class you associate the views of your item view so that they can be accessed and populated in onBindViewHolder method of RecyclerView.Adapter. A ViewHolder needs a View object from which its views can be defined. A typical ViewHolder looks like this.

inner class SampleViewHolder(view: View): RecyclerView.ViewHolder(view) {
    internal val icon: ImageView = view.findViewById(R.id.viewIcon)
    internal val title: TextView = view.findViewById(R.id.viewTitle)
    internal val description: TextView = view.findViewById(R.id.viewDescription)
}

(In Java, to do the same thing you would need to write few more lines). The ViewHolder is created in RecyclerView.Adapter's onCreateViewHolder method like this.

override fun onCreateViewHolder(parent: ViewGroup,
                                    viewType: Int): MyAdapter.SampleViewHolder {
    // create a new view
    val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.sample_item, parent, false)

    return SampleViewHolder(view)
}

And then in onBindViewHolder method you access the ViewHolder.

override fun onBindViewHolder(holder: SampleViewHolder, position: Int) {
    holder.icon.setImageResource(R.drawable.icon)
    holder.title.text = "Title"
    holder.description.text = "Description"
}

The process of creating BasicViewHolder is similar as well. You don't need to define a ViewHolder class. You need to instantiate a BasicViewHolder object in onCreateViewHolder method.

override fun onCreateViewHolder(parent: ViewGroup,
                                    viewType: Int): MyAdapter.BasicViewHolder {
    // create a new view
    val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.sample_item, parent, false)

    return BasicViewHolder(view, listOf(R.id.viewIcon, R.id.viewTitle, R.id.viewDescription))
}

As you can see, we are passing the inflated view and the list of view ids present in the item as parameters while creating a BasicViewHolder. The list of view ids is needed by BasicViewHolder to find and map them to Views which can be used later. And now in onBindViewHolder method, you access them like this.

override fun onBindViewHolder(holder: BasicViewHolder, position: Int) {
    holder.getViewById<ImageView>(R.id.viewIcon)?.setImageResource(R.drawable.icon)
    holder.getViewById<TextView>(R.id.viewTitle)?.text = "Title"
    holder.getViewById<TextView>(R.id.viewDescription)?.text = "Description"
}

BasicViewHolder provides a getViewById method for accessing the views. All you need to do is pass the view id you want to access and specify the type of the View. BasicViewHolder internally uses a HashMap to store the view mappings, which is why the returned views are Nullable. If you are specifying the view ids correctly, you should be fine. Since HashMap is used, getViewById call does not impose any overheads and is efficient.

BasicViewHolder does not support onClick events. To achieve this, please see ClickableViewHolder.