1

I am in the process of writing an Apple Watch Complication for WatchOS 2. The particular data I am trying to show is given (via web request) in intervals of time ranging from 3-6 minutes. I have a predictive algorithm that can predict what the data values will look like. This presents a problem to me.

Because I want to display the data my predictive algorithm has to offer in time travel, I would like to use getTimelineEntriesForComplication (the version that asks for data after a certain date) to supply the future values that my algorithm believes will be true to the timeline. However, when time moves forward (as it tends to do) and we reach the time that one of these predicted data points was set to occur at, the predicted value is no longer accurate.

For instance, lets say it is 12:00 PM, and I currently have an (accurate) data value of A. The predictive algorithm might predict the following data values of the next two hours:

  • 12:30 PM | B
  • 1:00 PM | C
  • 1:30 PM | D
  • 2:00 PM | E

However, when 12:30 PM actually comes around, the actualy data value might be F. In addition, the algorithm will generate a new set of predictions all the way to 2:30 PM. I understand I can use updateTimelineForComplication to indicate that the timeline has to be rebuilt, but I have two problems with this method:

  1. I fear I will exceed the execution time limit rather quickly
  2. updateTimelineForComplication flushes the entire timeline, which seems wasteful to me considering that all the past data is perfectly valid, its simply the next 4 or so values that need to be updated.

Is there a better way to handle this problem?

ericmarkmartin
  • 717
  • 1
  • 8
  • 19

1 Answers1

3

At present, there's no way to alter a specific timeline entry, without reloading the entire timeline. You could submit a feature request to Apple.

Summary

To summarize the details that follow, even though your server updates its predictions every 3-6 minutes, the complication server will only update itself at 10 minute intervals, starting at the top of an hour. Reloading the timeline is your only option, as it will guarantee that all your predictions are updated and accurate within 10 minutes.

Specific findings

What I've found in past tests involving extendTimelineForComplication: using the minimum 10-minute update interval, is that the dataSource is asked for 100 entries before and after a sliding window based on the current time.

The sliding window isn't centered on the current time. For watchOS 2.0.1, it appears to be skewed to ask for more recent future entries (after ~14-27 minutes in the future), and less recent past entries (before ~100 minutes in the past).

Reloading is the only way to update any entries that fall within the ~two hour sliding window.

Issues

In my experience, extendTimelineForComplication has been less reliable than reloading the timeline, as a timeline that that is never reloaded needs to be trimmed to discard entries. The fewer number of entries per hour, the less frequently this occurs, but once the timeline cache grows large enough, the SDK appears to aggressively discard entries from the head and tail of the cache, even if those entries fall within the 72-hour time-travel window. The worst I've seen is only being able to time-travel forward 30 entries, instead of 100.

Having provided those details, I wouldn't suggest that anyone try to take advantage of any behaviors that may change in the future.

Daily budget and battery life

As for the daily budget, it sound more ominous than it is, but I think you'd have to do some intense calculations before the complication server cuts you off. Even with ten minute updates, I never exceeded the budget. The real issue is battery use. You'll find that frequent updates can drain your battery before the day is over. This is probably the most significant reason for Apple's recommendation:

Complications should provide as much data as possible during each update cycle, so specify a date as far into the future as you can manage. Do not ask the system to update your complication within minutes. Provide data to last for many hours or for an entire day.

  • Thank you so much for your detailed answer @PetahChristian. There's one more thing I'm curious about though. They ask that you not update your complication within minutes, yet for a complication for which no future data is known (like a stock market complication), how do you avoid using stupendous amounts of battery? – ericmarkmartin Dec 01 '15 at 04:31
  • Offload as much work as possible using Watch Connectivity so the complication controller merely has to provide predetermined data. Consider less frequent reloads if the majority of your users won't need 10-minute updates. [Optimize the experience for the watch.](https://realm.io/news/more-watchkit-mistakes/) –  Dec 01 '15 at 05:46