如何找出第一次在回收站视图中创建的项目？

Question

I have a recycler view and I want to find out if an item is created in recycler view for the first time or not?

Is there an event handler for it?

Note: I know how to implement it with flag, but I'm looking for another approach.

Answer 1

Why do you want this? To my mind it seems like there is not a good reason to know this information, a ViewHolder should be independent from that knowledge.

Nothing is provided for this by RecyclerView.Adapter , the only callbacks are to manage the ViewHolder instances, which can be recycled and so any particular instance wouldn't know if it is the first instance or not.

You could store a flag in your data model and access it when you set up the ViewHolder in onBindViewHolder , as you seem to know.

Answer 2

You can ovveride onViewAttachedToWindow(holder: ViewHolder) in your recycler adapter. sample code like this:

private var isFirstChildAttached = false // single fire
override fun onViewAttachedToWindow(holder: ViewHolder) {
    super.onViewAttachedToWindow(holder)
    holder.pageView.registerTopLevelTouchListener()


    if (!isFirstChildAttached) {
        isFirstChildAttached = true
        checkAndFirePageDimen(holder.itemView)
    }
}

private fun checkAndFirePageDimen(itemView: View){
    itemView.viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
        override fun onGlobalLayout() {
            itemView.viewTreeObserver.removeOnGlobalLayoutListener(this)
            pagerCallback.onFirstItemAttachedToPager(itemView)
        }
    })
}

"checkAndFirePageDimen" helps you for reliable itemView size.