Tuesday, January 14, 2014

Building a native iOS graphing engine for Pocket Weather 4



Specifically, my scope was to build a new, modern graphing engine for them, to replace the old legacy graphing code they had been using for years. An app like Pocket Weather lives or dies based on (a) Forecast data quality; and (b) Forecast data visualisations with rich graphs. Pocket Weather does both really well, visualising forecast data in a multitude of forms to cover many use cases. I couldn't help with their forecast data quality (which was already very high and even better in PW4) but I was happy to build a modern new high performant interactive graphing engine to cover their visualisation needs.

The graphing engine needed to support both static graphs as well as smooth scrolling interactive graphs, to cover all the visualisations that Pocket Weather 4 required. On iPhone, five main graphs were needed:
  • "Today": a static graph charting both air and "feels like" temperature forecasts, with "chance of rain" overlays.
  • "Detailed Outlook": a static graph charting forecast for the next two days, similar to the Today graph.
  • "Tide Times": a line chart of tide low/high forecast times, with edges nicely smooth out.
  • "Interactive Timeline": a scrolling graph charting air and "feels like" temperature forecasts, plus rain overlays, for the next 7 days.
  • "Interactive Tide Times": a scrolling graph of tide low/high forecast tides for the next few weeks.
Screenshots of two of the static graphs can be seen above, near the top. The interactive graphs are shown below.


There are a few iOS graphing libraries around that produce good results, both commercial and open source. However, third party library dependencies are always a trade off between convenience and managing extra baggage. Plus the ability and/or cost to customise them to meet requirements varies quite a bit. Based on the requirements, my preference was to hand-roll a graph rendering engine using native Core Graphics and Core Animation frameworks, rather than bring in a third party graphing library. Having had already done this a few times for previous projects, I was confident that we could get good results in a short time. This matched Shifty Jelly's preference also, as they would end up with modern code that they could own, manage and continue to customise themselves.


A common factor between all the required graphs was that they were all line graphs, charting either one or two lines of data, drawn with various styling options. As such it made sense to encapsulate line chart rendering in a class configurable enough to be shared by all the graph views. As some views would be drawing into UIView contexts (for static graphs) and others would be drawing into contexts at the CALayer level (for scrolling graphs, more on this below) that meant that the chart renderer could be a shared NSObject subclass, without any knowledge of the UIKit or CALayer levels. The chart renderer would talk Core Graphics, accepting a CGContext to render in to.

Above is an abstract diagram of the class architecture. Each graph view is a UIView subclass, with common elements abstracted into shared base classes, down to one base class for static graphs, and one for interactive graphs. Each graph view contains a chart renderer instance, configured appropriately for the particular graph type. Chart renderer objects were ultimately responsible for actually drawing the line graphs into specified CGContexts.

The static graph views were relatively straight forward. On "needs display" their drawRect: method delegates drawing to the chart renderer, passing down the CGContext to draw into.

The interactive graph views were a little more complex. They were very wide scrollable graphs. Rendering a large scrollable view is not possible, and would be very inefficient even if it was. The answer was to use a CATiledLayer to render the graph into tiles, on demand as the view is scrolled. CATiledLayer works great for this. Or would have, if it wasn't severely broken in iOS 7. In the end I had to roll my own CATiledLayer clone to get the job done, but the result was the same (more on this below).

Shown here are two images of an example scrollable graph view. The first image shows what the graph looks like rendered on screen. The second shows the view explosion, with all the subviews Revealed. You can see the large scrollable view, which cannot be snapshot by Reveal, but at runtime is rendered as horizontal tiles. The tiling layer renders tiles on demand, only drawing tiles as they become visible. Even then, rendering is performed asynchronously so as not to interrupt scrolling performance. Rendered tiles are then cached so they don't have to be drawn again unless necessary.


As mentioned above, CATiledLayer is unfortunately broken in iOS 7.0 (rdar://15402066). It suffers a severe problem of not redrawing purged tiles in some cases. Easily reproducible and not shippable. After a discussion with Apple engineers via a TSI I concluded that the only practical current solution was to avoid CATiledLayer and write my own equivalent. I did that and it worked well for our case.

Another interesting challenge was the requirement to smooth the curves of the tide graphs, while also tracking the curved line path with a circular overlay when the graph is being scrolled. See the video below for an example of what I mean.

The tide data was a simple data set of low tide/high tide forecasts, in time order. For example: [low tide, high tide, low tide, high tide, ...]. By default the data would be graphed with sharp edges, as shown below in the first image. Shifty Jelly's rockstar designers understandably requested that the line be smoothed out, as shown in the second image below.
Techniques to smooth curves in a line are fairly well known and I used a Catmull-Rom spline to smooth the line.

The Catmull-Rom spline generated enough points to create a CGPath for Core Graphics to draw a smooth line. However, it didn't generate enough points to calculate the exact Y position needed for the circular overlay for every X position of the graph. The solution here was to interpolate between the curved line points to generate a set of (X, Y) points with just enough granularity for every X position of the graph. This calculation is done once for a data set + graph layout configuration and cached. Then, while scrolling, a lookup of overlay Y position for any X position is direct and instant, allowing the circular overlay to track the graph line.


While hand-rolling a graph engine may sound like a lot of work, in practice with a good knowledge of Core Graphics and Core Animation to do all the heavy lifting, the amount of code required is not terribly large.

We were all very pleased with the results we got from building a native graph engine ourselves. With the engine now built and shipped, modifications are fast and easy to do as the library is clean and well understood.

The requirements for the iPad interface were not known to me while building the graph engine. In fact, I didn't even know how the iPad build would look until PW4 shipped. I was pleased to see that the iPad interface was built around the scrolling forecast graph, very cleverly designed. The graphing engine scaled up to the iPad interface with little extra effort.

A big thanks to Shifty Jelly for bringing me on-board for this part of the project, it was a pleasure to work with them.