How are animations implemented in RecyclerView?

RecyclerView uses ItemAnimator to animate changes in the data set. By default, DefaultItemAnimator is used.

When changes occur (adding, removing, moving items), RecyclerView notifies the ItemAnimator. The animator analyzes these changes and triggers the appropriate animations.

The main methods of ItemAnimator for handling changes are:

• animateAdd(): Animation for adding a new item.
• animateRemove(): Animation for removing an item.
• animateMove(): Animation for moving an item.
• animateChange(): Animation for changing the content of an item (e.g., when updating data).

Custom animations are created by inheriting from ItemAnimator or DefaultItemAnimator and overriding these methods. Inside these methods, various animation classes such as ValueAnimator or ObjectAnimator can be used.

Example of a simple custom add animation:

class CustomItemAnimator : DefaultItemAnimator() {

    override fun animateAdd(holder: RecyclerView.ViewHolder): Boolean {
        holder.itemView.alpha = 0f // Initial state: fully transparent
        holder.itemView.animate()
            .alpha(1f) // Final state: fully visible
            .setDuration(addDuration) // Duration of the animation
            .setListener(object : AnimatorListenerAdapter() {
                override fun onAnimationEnd(animation: Animator?) {
                    dispatchAddFinished(holder) // Notify RecyclerView of completion
                }
                override fun onAnimationCancel(animation: Animator?) {
                    dispatchAddFinished(holder)
                }
            })
            .start()
        return true // Return true to indicate the animation has started
    }
}

To apply a custom animator, it needs to be set in the RecyclerView:

recyclerView.itemAnimator = CustomItemAnimator()