overscanIndicesGetter doesnt know if its a horizontal or vertical scroll

[offsky]

If you implement overscanIndicesGetter, it gets called twice on every scroll event. Once to get the horizontal value and again to get the vertical value. However, nothing is passed in to let us know which of these it is. If we want to return different values for horizontal vs vertical scrolling, it isn't possible.

This is what the code is

      const overscanColumnIndices = overscanIndicesGetter({
        cellCount: columnCount,
        overscanCellsCount: overscanColumnCount,
        scrollDirection: scrollDirectionHorizontal,
        startIndex: this._renderedColumnStartIndex,
        stopIndex: this._renderedColumnStopIndex
      })

      const overscanRowIndices = overscanIndicesGetter({
        cellCount: rowCount,
        overscanCellsCount: overscanRowCount,
        scrollDirection: scrollDirectionVertical,
        startIndex: this._renderedRowStartIndex,
        stopIndex: this._renderedRowStopIndex
      })

I propose passing another value for horiz/vertical scrolling. Something like this:

      const overscanColumnIndices = overscanIndicesGetter({
        cellCount: columnCount,
        overscanCellsCount: overscanColumnCount,
        scrollDirection: scrollDirectionHorizontal,
        horizontal: true,
        startIndex: this._renderedColumnStartIndex,
        stopIndex: this._renderedColumnStopIndex
      })

      const overscanRowIndices = overscanIndicesGetter({
        cellCount: rowCount,
        overscanCellsCount: overscanRowCount,
        scrollDirection: scrollDirectionVertical,
        horizontal: false,
        startIndex: this._renderedRowStartIndex,
        stopIndex: this._renderedRowStopIndex
      })

What do you think?

[bvaughn]

Good observation. I probably prefer direction: 'vertical' | 'horizontal' but I don't feel strongly. Since this would be a backwards compatible change, I'd be happy to accept a PR for it.

[offsky]

I'll do this. One additional thing to consider. When using a List, the overscanIndexGetter is being called for both Horizontal and Vertical, even though there is no horizontal. Should I also optimize this away so that Lists don't ask the overscanner for horizontal?

[bvaughn]

Hm...if it doesn't add much complexity to Grid to do this for List and Table then...sure. If it does then I'd say no.