Kotlin's Android Extensions and variables

Question

Before Kotlin, Android developers supposed to save reference to the Activity's Views in a variable like this:

Button fooBtn = (Button) findViewById(R.id.btn_foo)

to reduce the amount of the boiler-plate code and the number of findViewById calls.

With the introduction of the Kotlin's Android Extensions we can reference the same Button by simply using:

btn_foo

---

Questions:

1. Does the btn_foo have a reference to the Button saved, or does it call findViewById every time?
2. Do developers still suppose to use variables to store btn_foo to improve app's performance, or just use it directly in the code?

---

Edit: there is an explanation how Extensions work, however it is still a bit unclear.

Answer 1

It's cached, so findViewById isn't called every time you need it. Storing the variable won't definitely improve the app's performance

Answer 2

One of the Kotlin Android Extension (KAE) developers Ihor Kucherenko confirmed that:

• KAE will keep a reference to the view after the first call, instead of using findViewById all the time. That works only for Activities and Fragments.

• KAE will not cache data and will use findViewById every time for any other element (except for an Activity/Fragment).

So in case you are going to init a ViewHolder:

class FooViewHolder(view: View): RecyclerView.ViewHolder(view) {
    fun bind(day: FooItem.Day) {
        btn_foo.text = day.title
    }
}

Decompile into Java call will look like:

((Button)this.itemView.findViewById(R.id.btn_foo)).setText((CharSequence)day.getTitle());

which is exactly what you want to avoid.

The developers might be aware of this.

---

Conclusion: fill free to use KAE without any additional variables, but only for your Activitiies/Fragments.