LifecycleOwner implemented RecyclerView ViewHolder & Adapter (concept design)

HowToUseActivity.kt

import android.os.Bundle
import android.util.Log
import android.view.ViewGroup
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleObserver
import androidx.lifecycle.OnLifecycleEvent
import app.keima.android.recyclerviewsandbox.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)

        binding.recyclerView.apply {
            adapter = MyAdapter(
                arrayOf(
                    "A", "B", "C", "D",
                    "A", "B", "C", "D",
                    "A", "B", "C", "D",
                    "A", "B", "C", "D"
                )
            )
        }
    }
}

class MyAdapter(private val dataset: Array<String>) :
    LifecycleRecyclerAdapter<MyAdapter.MyViewHolder>() {

    class MyViewHolder(private val textView: TextView) : LifecycleViewHolder(textView) {
        private val observer = MyObserver()

        init {
            lifecycle.addObserver(observer)
        }

        fun bindData(data: String) {
            textView.text = data
            observer.data = data
        }
    }

    class MyObserver() : LifecycleObserver {
        var data: String = "?"

        @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
        fun onCreate() {
            Log.d("MyObserver", "appear: $data")
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        fun onDestroy() {
            Log.d("MyObserver", "disappear: $data")
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        return MyViewHolder(TextView(parent.context).apply {
            setPadding(8, 40, 8, 40)
        })
    }

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        holder.bindData("${dataset[position]} $position")
    }

    override fun getItemCount() = dataset.size

}

Licenses.md

This gist snippet is Public Domain.

LifecycleRecyclerAdapter.kt

import androidx.recyclerview.widget.RecyclerView

abstract class LifecycleRecyclerAdapter<T : LifecycleViewHolder> : RecyclerView.Adapter<T>() {

  override fun onViewAttachedToWindow(holder: T) {
    super.onViewAttachedToWindow(holder)
    holder.onAppear()
  }

  override fun onViewDetachedFromWindow(holder: T) {
    super.onViewDetachedFromWindow(holder)
    holder.onDisappear()
  }

}

LifecycleViewHolder.kt

import android.view.View
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.LifecycleRegistry
import androidx.recyclerview.widget.RecyclerView

abstract class LifecycleViewHolder(itemView: View) :
    RecyclerView.ViewHolder(itemView), LifecycleOwner {

    private val lifecycleRegistry = LifecycleRegistry(this)

    init {
        lifecycleRegistry.currentState = Lifecycle.State.INITIALIZED
    }

    fun onAppear() {
        lifecycleRegistry.currentState = Lifecycle.State.CREATED
    }

    fun onDisappear() {
        lifecycleRegistry.currentState = Lifecycle.State.DESTROYED
    }

    override fun getLifecycle(): Lifecycle {
        return lifecycleRegistry
    }

}

My implementation

class SimpleLifeCycleOwnerImpl : SimpleLifeCycleOwner {
    private var lifecycleRegistry: LifecycleRegistry? = null

    override fun getLifecycle() =
        lifecycleRegistry ?: run {
            initialize()
            lifecycleRegistry!!
        }

    override fun initialize() {
        // The object can be revived, create a new lifecycle
        lifecycleRegistry?.run {
            if (currentState != Lifecycle.State.DESTROYED)
                throw IllegalStateException("can be revived only after destroyed, get $currentState")
        }
        lifecycleRegistry = LifecycleRegistry(this)
    }

    override fun handleLifecycleEvent(event: Lifecycle.Event) {
        lifecycleRegistry!!.handleLifecycleEvent(event)
    }
}

// kotlin cannot inherit from type parameter like C++, we must use this class as base
// otherwise we need to copy the code for each adapter
abstract class LifecycleAdapter<VH : RecyclerView.ViewHolder>(lifecycleOwner: LifecycleOwner) :
    RecyclerView.Adapter<VH>() {
    private var recyclerView: RecyclerView? = null

    init {
        val observer = object : LifecycleEventObserver {
            private fun visibleChildApply(block: SimpleLifeCycleOwner.() -> Unit) =
                recyclerView?.run {
                    for (i in 0 until childCount)
                        getChildAt(i)?.let {
                            (getChildViewHolder(it) as? SimpleLifeCycleOwner)?.block()
                        }
                } ?: Unit

            private fun allChildApply(block: SimpleLifeCycleOwner.() -> Unit) =
                recyclerView?.run {
                    for (i in 0 until itemCount)
                        (findViewHolderForAdapterPosition(i) as? SimpleLifeCycleOwner)?.block()
                } ?: Unit

            override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) =
                when (event) {
                    Lifecycle.Event.ON_DESTROY -> allChildApply { handleLifecycleEvent(event) }
                    else -> visibleChildApply { handleLifecycleEvent(event) }
                }
        }
        lifecycleOwner.lifecycle.addObserver(observer)
    }

    override fun onAttachedToRecyclerView(recyclerView: RecyclerView) {
        if (this.recyclerView != null)
            throw IllegalStateException("can be attached to only one RecyclerView")
        this.recyclerView = recyclerView
    }

    override fun onDetachedFromRecyclerView(recyclerView: RecyclerView) {
        throw IllegalStateException("must not be detached")
    }

    override fun onViewAttachedToWindow(holder: VH) =
        (holder as? SimpleLifeCycleOwner)?.run {
            handleLifecycleEvent(Lifecycle.Event.ON_START)
            handleLifecycleEvent(Lifecycle.Event.ON_RESUME)
        } ?: Unit

    override fun onViewDetachedFromWindow(holder: VH) =
        (holder as? SimpleLifeCycleOwner)?.run {
            handleLifecycleEvent(Lifecycle.Event.ON_PAUSE)
            handleLifecycleEvent(Lifecycle.Event.ON_STOP)
        } ?: Unit

    override fun onBindViewHolder(holder: VH, position: Int) =
        (holder as? SimpleLifeCycleOwner)?.run {
            initialize()
            handleLifecycleEvent(Lifecycle.Event.ON_CREATE)
            // logical views are created here
        } :? Unit

    override fun onViewRecycled(holder: VH) =
        (holder as? SimpleLifeCycleOwner)?.run {
            handleLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        } ?: Unit
}

It will cause memory leak, when RecyclerView detach from window, the onViewDetachedFromWindow callback will not invoked.