Drawing chart with lots of points efficiently on iOS

Question

I'm looking for a way to draw a line chart with lots of points (up to 10 000) efficiently on iOS. The graph gets real-time data and needs to be updated multiple times a second. I'm trying to figure out a way to draw the graph efficiently so it doesn't max out the CPU and block the main thread while drawing.

I'm currently creating a UIBezierPath for the line chart, in a background thread, adding all the points and drawing it in a CALayer (that has asynchronous drawing enabled). It's not very quick, maxes out the CPU and the drawing is so slow the UI becomes laggy. I am plotting real time data, and in theory I could reuse the same UIBezierPath every time and just append the new points, however older values are discarded after a while, meaning the points for the deleted values would have to be removed from the bezier path, which isn't possible.

I haven't found anything on efficient plotting large data sets on iOS, but I'm hoping there are ways to use the GPU to increase the performance.

Answer 1

I had a situation like this a few years ago (back in iPhone 4 days).

I used a CGMutablePathRef to which I added points using CGPathMoveToPoint for the first point, and CGPathAddLineToPoint for subsequent points.

To get acceptable performance, I stored the data points in a C array (not an NSArray). Also, when plotting a point, if its display coordinates were the same as the last plotted point, I just skipped the call to CGPathAddLineToPoint. (This will often be the case when you have more data points than pixels).

I don't recall the exact drawing time, but it was surprisingly fast.