kotlin-android-extensions in ViewHolder

Question

class MyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {

       fun bindata(text: SingleText){
            itemView.title.text = text.title
            itemView.desc.text = text.desc
       }
}

like this code, Kotlin has any cache in android-extensions?

when i decompile kotlin bytecode

public final void bindata(@NotNull SingleText text) {

  Intrinsics.checkParameterIsNotNull(text, "text");
  ((AppCompatTextView)this.itemView.findViewById(id.title)).setText((CharSequence)text.getTitle());
  ((AppCompatTextView)this.itemView.findViewById(id.desc)).setText((CharSequence)text.getDesc());

}

it means when i called binData in Adapter.onBindViewHolder(), it will called findViewById each time

This significantly increases the loss of performance，and It does not achieve the purpose of layout reuse

Kotlin has any cache logic in android-extensions with ViewHolder?

`findViewById` finds already created view. So that means you are reusing it. There is no performance issue that your are talking about. — Sachin Chandil, Aug 30 '17 at 04:58

Bob · Accepted Answer · 2017-08-30T12:58:41.220

View caching in a ViewHolder or any custom class is possible only from Kotlin 1.1.4 and it's currently in the experimental stage.

Update your Kotlin version in your root level build.gradle file

classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.1.4-3"

Add these lines in your app build.gradle:

androidExtensions { experimental = true }

Inherit your ViewHolder class from LayoutContainer. LayoutContainer is an interface available in kotlinx.android.extensions package.
Add the below imports, where view_item is the layout name.

import kotlinx.android.synthetic.main.view_item.* import kotlinx.android.synthetic.main.view_item.view.*

The entire ViewHolder class looks like:

class ViewHolder(override val containerView: View) : RecyclerView.ViewHolder(containerView),
        LayoutContainer {

    fun bind(title: String) {
        itemTitle.text = "Hello Kotlin!" // itemTitle is the id of the TextView in the layout 
    }
}

The decompiled Java code shows that this class uses cache for the Views:

    public final void bind(@NotNull String title) {
          Intrinsics.checkParameterIsNotNull(title, "title");
          ((TextView)this._$_findCachedViewById(id.itemTitle)).setText((CharSequence)"Hello Kotlin!");
    }

Further readings: KEEP proposal, an awesome tutorial.

score 13 · Answer 2 · answered Aug 30 '17 at 06:42

13

From my understanding, kotlin-android-extensions is just a statically imported extension(generated codes) to replace View.findViewById.

I would recommend you to store the references in your view holder.

class MyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
   val title: TextView = itemView.title
   val desc: TextView = itemView.desc

   fun bindata(text: SingleText){
       title.text = text.title
       desc.text = text.desc
   }
}

One of the benefits of this approach is any type mismatch will be discovered in compile time!

answered Aug 30 '17 at 06:42

Gary LO

1,003
6
11

If you add a line of code for each View, you might as well not use Kotlin Android extensions. If you wrap findViewById() in an extension function it will be type safe as well. `val title = itemView.find(R.id.title)` – Frank Harper Sep 18 '18 at 21:23
@FrankHarper if `R.id.title` is not pointing to a TextView, let say pointed to ImageView. `itemView.find(R.id.title)` will still compile and will cause run-time crash while using Kotlin Android extensions will throw error at compile time. – Gary LO Oct 07 '18 at 17:55

abhilasha Yadav · Answer 3 · 2018-05-23T07:36:50.993

In your project gradle:-

 buildscript {
        ext.kotlin_version = '1.2.41'

        repositories {
            google()
            jcenter()
        }

and in your app gradle:

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android {}
dependencies {
  compile "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

After these you can use in you activity :

import kotlinx.android.synthetic.main.unit_preferences_activity.*

and:

  private fun setClickListener() {
            rlCaloriesBurnt.setOnClickListener{
                launchActivity(CaloriesBurntPrefreenceUnit::class.java)
            }

            rlDistanceTravelled.setOnClickListener{

           tvDistanceUnit.text=resources.getString(R.string.km)

                launchActivity(DistanceTravelledUnit::class.java)
            }

            rlWaterConsumption.setOnClickListener{
                launchActivity(WaterPreferenceUnit::class.java)
            }
            backArrow.setOnClickListener{
                finish()
            }

        }

kotlin-android-extensions in ViewHolder

3 Answers3