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Cleaning up resources in RecyclerView ViewHolder

 4 years ago
source link: https://androidexplained.github.io/android/ui/2020/11/09/cleaning-up-resources-in-view-holder.html
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Cleaning up resources in RecyclerView ViewHolder

Nov 9, 2020

About 4 mins

In your journey you might have often wondered when exactly to clean up resources in a RecyclerView ViewHolder.
The topic isn’t easy and like most you probably tried to use onViewAttachedToWindow & onViewDetachedFromWindow, only to find it’s not working and gave up hoping you’ll get away with no resource freeing mechanism.
Sure there is this 1% of users of your app who get OutofMemoryException from time to time and on older phones scrolling through a list of items looks more like playing Tetris.
You didn’t really care until you get a negative review with 500 upvotes saying that your app is almost unusable.

But worry not, in this article we will look at how to cleanup resources in RecyclerView ViewHolder properly.

Release the… resources

There are couple of methods that will be important to us in this article

  • RecyclerView.Adapter#onBindViewHolder - called to display data at specified position, should update the contents of ViewHolder to reflect an item at position
  • RecyclerView.Adapter#onViewRecycled - called when View is recycled and put into the pool, since the View might sit in the pool for a while it might be a good idea to release heavy resources like bitmaps in this method

Lets look at a situation when user is scrolling downwards in a RecyclerView.
We will assume that there are two ViewHolders of the same type displayed (1 & 2).

  • items are displayed on the screen - onBindViewHolder will be called for both ViewHolder 1 & 2
  • user scrolls down and the top ViewHolder 1 disappears, it will be put into the pool, after which the onViewRecycled method will be called on that ViewHolder
  • at the same time new ViewHolder appears on the bottom - since we have ViewHolder 1 in the pool it will be reused and onBindViewHolder will be called again to reflect the new position
  • and so on and so on…

Therefore we can basically utilize these two above methods to initialise resources (onBindViewHolder) and cleanup them up (onViewRecycled) accordingly.

The below code can serve as an example of how this would look in practice

class MovieAdapter(private val movies: Array<String>) :
    RecyclerView.Adapter<MovieAdapter.ViewHolder>() {

    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) =
        viewHolder.onBind(movies[position])

    override fun getItemCount() = movies.size

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.movie_item, parent, false)

        return ViewHolder(view)
    }

    override fun onViewRecycled(holder: ViewHolder) {
        super.onViewRecycled(holder)

        holder.cleanup()
    }

    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {

        fun onBind(movie: String) {
            // initialise resources and adjust the view to reflect position in the RecyclerView
        }

        fun cleanup() {
            // release heavy resources like bitmaps etc.
        }
    }
}

In this example we could for instance download a movie cover image and display it in the onBindViewHolder after which we would release the downloaded Bitmap in onViewRecycled method to cleanup this heavy object so it does not sit unused in the pool.

In this article we have learned how to clean up resources in RecyclerView.ViewHolder!


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