SpanGroupIndex and SpanIndex Recycler View

Question

I was looking at the documentation of GridLayoutManager and its SpanSizeLookUp Class for RecyclerView. But I cant get to know about the getSpanGroupIndex and getSpanIndex methods of the SpanSizeLookUp Class. I also experimented by overriding and returning arbitary number(less than span count). But I cant get to know what is it used for.

I am not able to understand the Documentation instructions. I would like to clear this.

Answer 1

By default, each item in a grid layout takes 1 span. If Grid is horizontal, span means a row and if Grid is vertical, span means a column. If your items may need to layout themselves into multiple spans (e.g. a header item in a 3 column grid would have spanSize=3), you can provide a SpanSizeLookup which returns 3 if item is a header item and 1 if it is a regular item.

When GridLayoutManager (GLM) needs to decide the span for an item, it calls getSpanIndex in SpanSizeLookup. That method traverses every item before the given position to decide which span this item should be placed at.

For example, if you call glm.scrollToPosition(n), SpanSizeLookup goes through every item from 0 to N, checks how many spans each of them consume and decide where the item at N would be positioned. Lets say you have a grid w/ a header row at index 0 and 3, and rest of the items take 1 span. If GLM calls spanSizeLookup.getSpanIndex(4) it will call getSpanIndex for 0 1 2 3, realize that after 3rd item, next item should be placed at span 0 and return it.

Although it caches these results, it is inefficient to traverse all items if you can calculate it faster. For instance, maybe you know that all items take 1 span, then you can override getSpanIndex and return n % spanCount . This is why that method is public.

getSpanGroupIndex is the group of the item. If GLM is vertical, group is "row" and if GLM is horizontal, group is "column". It is used mainly for accessibility. For the example above where each item consumes 1 span, you can override this method and return n / spanCount. I agree that this method is not clear, will create a ticket to improve documentation.