How the GridLayoutManager average each item.

Default effect

Expected effect Expect every row like ConstraintLayout chain spread inside

Dash line is GridLayoutManger default each item's left

How to implement？You need know blew point.

• Dash line is spanIndex*spanWidth
• item.left= Dash line location+ItemDecoration.left

So through set itemDecoration.left ,we could implement the average effect.

The Code

private class AverageGridItemDecoration : RecyclerView.ItemDecoration() {

    override fun getItemOffsets(
        outRect: Rect,
        view: View,
        parent: RecyclerView,
        state: RecyclerView.State
    ) {
        super.getItemOffsets(outRect, view, parent, state)
        val layoutManager = parent.layoutManager as? GridLayoutManager ?: return
        val spanCount = layoutManager.spanCount
        val parentWidth = parent.measuredWidth - parent.paddingStart - parent.paddingEnd
        val itemWidth = view.resources.getDimension(R.dimen.course_practice_result_item_size)
        val spanWidth = parentWidth / spanCount
        if (spanCount == 1) return
        val spanMargin = (parentWidth - itemWidth * spanCount) / (spanCount - 1)
        val spanSizeLookup = layoutManager.spanSizeLookup

        for (child in parent) {
            val adapterPosition = parent.getChildAdapterPosition(child)
            val columnIndex = spanSizeLookup.getSpanIndex(adapterPosition, spanCount)
            // Core line:
            // left = expect left- default left
            outRect.left =  ((itemWidth + spanMargin) * columnIndex - spanWidth * columnIndex).toInt()
            if (spanSizeLookup.getSpanGroupIndex(adapterPosition, spanCount) > 0) {
                outRect.top = dp2px(15f).toInt()
            }
        }
    }
}

Demo 地址: https://github.com/pokercc/GridLayoutMangerDemo