Get scroll position of dragable line chart

[delsner]

Probably a very simple question: I have created a simple dragable/scrollable linechart and need to get the current scroll position to determine whether I have reached the end of the chart. For more details see: http://stackoverflow.com/questions/38606896/get-scroll-position-of-dragable-linechart

[delsner]

it doesn't necessarily need to be the scroll position but more importantly an indicator telling that I have reached the newest (most right) value in the line chart.

[liuxuan30]

would highestVisibleXIndex solve your problem? If the highest visible xIndex is the the last data entry xIndex.

[delsner]

Unfortunately not as the highestVisibleXIndex is the same for one entire screen width of chart entries. I tried reverse calculating the total possible translation (max transX) and subtract it by the current transX, but I do not get the exact value for the possible translation. Note that I add more data when scrolling to the left which changes the total possible translation while scrolling.

[liuxuan30]

Why highestVisibleXIndex does not work for you? The entry contains xIndex, so you just have to compare highestVisibleXIndex and your max xIndex value.

I couldn't find a more dedicated API for this right now. But I think what we could try here is to manually calculate the current offset, like contentOffset concept, though we don't have contentOffset property.

contentRect is the rect that defines the area of the chart content, excluding the axis stuff. By computing the matrix and contentRect, we should know when it reaches last entry.

Sorry it has been some time for me doing the matrix calculation, it's a little rusty. You can take a look at ChartViewPortHandler and ChartTransformer for the APIs that handles the data value coordinate space and pixel value coordinates.

One example I can come up right now:

We know the last entry, so we know the highest xIndex; this xIndex can be project to pixel coordinate system via pointValueToPixel(inout point: CGPoint), so we know the edge of the last data x coordinate, and we can figure out a way to calculate when user scrolls to last?

[liuxuan30]

I think I have seen such questions before, but couldn't find the old issues now

Also, please take a look at moveViewToX(xIndex: CGFloat) and centerViewTo(), these are examples for the above calculation I mentioned.

[delsner]

It doesn't work because I am using a dragOffset of half a screen width. Thus, when scrolling the highestVisibleXIndex stays the same for half a screen width when reaching the end, even though I can keep scrolling until the dragOffset is fully scrolled through. Sounds complicated, see http://stackoverflow.com/questions/38606896/get-scroll-position-of-dragable-linechart for a screenshot. I will have a look at pointValueToPixel. Thanks so far. By the way apparently the issue is common also in the android library: http://stackoverflow.com/questions/28036212/how-to-get-the-scroll-offset-on-mpandroidchart

[liuxuan30]

you implemented your own dragOffset? Seems I can't find it.

[delsner]

like this: self.chartView.setDragOffsetX(self.view.frame.width / 2)
It enables the user to scroll half of a screen size over the edge of the line chart. Thus, the highestVisibleXIndex stays the same for this offset.

[liuxuan30]

hmm.. seems I can't provide more help here.. maybe ask @danielgindi

[danielgindi]

Well if you're adding a dragoffset - then of course highestVisibleXIndex won't be good for what you want, as it does not "bounce".

But you need to work with the viewport here.
Check out limitTransAndScale and canZoomOutMore. These do what you want, but not exactly:
limitTransAndScale calculates the new position in the given constraints - and you can reverse it to fetch the min/max values and determine if you're at the beginning/end.
canZoomOutMore is a convenience function that tells, well, if you can zoom out more...

So what I'm saying is- if you make a PR with the counterparts for canZoomOutMore, I'll be happy to merge it in :-)