Pro Android with Kotlin

200
CHAPTER 9: User Interface
override
fun onBindViewHolder(holder:ViewHolder,
position:Int) {
// replace the contents of the view with
// the element at this position
holder.v.findViewById(
R.id.firstLine).text =
myDataset[position]
}
override
fun getItemCount() : Int = myDataset.size
}
Here are a couple of notes on that listing:

The class inside the “companion object” is Kotlin’s way of declaring a 
static inner class. This one designates a reference to each data item as 
a UI element. More precisely, the recycler view will internally hold only so 
many view holders as are necessary to represent the visible items.

Only when really needed the function onCreateViewHolder() to create
a view holder gets called. More precisely, it’s called more or less only as 
often as is necessary to render the items visible to the user.

The function onBindViewHolder() connects one of the visible view 
holders with a certain data item. Here we must replace the contents of a 
view holder’s view.
Inside the activity all that is needed to define the recycler view is the following:
with(recycler_view) {
// use this setting to improve performance if you know
// that changes in content do not change the layout
// size of the RecyclerView
setHasFixedSize(true)
// use for example a linear layout manager
layoutManager = LinearLayoutManager(this@MainActivity)
// specify the adapter, use some sample data
val dataset = (1..21).map { "Itm" + it }.toTypedArray()
adapter = MyAdapter(dataset)
}
This will look like Figure 
9-1
. The following are useful extensions to the program:

Add on-click listeners to all items

Make items selectable

Make items or item parts editable

Automatically react to changes in the underlying data

Tailor graphical transition effects

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