I was already well into developing Runtime when the iPhone 5s was announced
and we learned about the new M7 “motion co-processor” from Apple. There
have already been a few good articles talking about what the M7 does and
how we believe it works, but essentially from a developer’s perspective
the M7 provides a great way to track a user’s steps and type of activity
while they are moving. Instead of writing about what the M7 is or how it
works, I wanted to write about what its like to use as a developer.
The M7 API is part of the Core Motion framework. Tracking a users steps and
activity has always been possible by using Core Motion, but it was much
more difficult, and required much more power. Instead of trying to
calculate this information ourselves using data directly from the
accelerometer and gyroscope, we interact with two new classes that give us
this data directly.
The first one, CMStepCounter, provides us with the number of steps the user
has taken while carrying the device. There are only a few methods here.
There’s a class method to tell you whether or not the device supports step
counting, aka whether or not the M7 is installed. There are two methods for
starting and stopping step updates. And then there is a method to query the
history of steps taken with a start and end date.
Lets talk about getting step updates first. While your app is running you
can ask iOS to execute a block every time a certain threshold number of
steps is reached. Runtime uses this method to update the Stopwatch screen
while the user is running. From my experience, the updates are delivered
about when you would expect them to be.
There is also a query method to look up the number of steps during a
certain time frame. The M7 stores 7 days worth of data, so the window can
be any period during that 7 days. The most surprising thing to me about
this API was how fast it is. Querying even an entire week worth of step
data takes virtually no time. Despite this, you still have the option to
specify a specific queue to have the block executed on. You can specify the
main queue if you want the callback to be synchronous. If you’re going to
be updating the UI with the result, you might as well just do that. If
you’re performing some other type of calculation with the result, then
perhaps you might want to use a background queue.
Next up with activity tracking are two new classes,
CMMotionActivityManager and CMMotionActivity. The activity manager follows
the same pattern as the step counter, with a class method to determine
availability, and block-based methods for updates and queries.
In this case though, the query and update callback blocks behave slightly
differently. The query block returns an ordered array of CMMotionActivity
objects. The activities are ordered by time based on when they occurred in
the specified window of time. This is very similar to the new CoreLocation
deferred location updates method, which returns a list of location updates
in a similarly ordered fashion. The update callback block instead returns a
single CMMotionActivity object, and gets called repeatedly each time the
CMMotionActivity objects encapsulate what type of activity has taken place,
be it running, walking, standing, driving, or an unknown type of activity,
as well as the system’s confidence level that it has correctly identified
that activity. One thing that can be kind of funny when you start looking
at the data is when you see an Unknown activity type with a low or high
degree of confidence. That means that iOS is either sort of sure, or
absolutely sure, that it has no idea what you are doing :)
One pattern I’ve noticed with the data is how it transitions from low, to
medium, to high degree of confidence for something like walking or running.
There tends to be about 5 seconds worth of low confidence, about 5 seconds
worth of medium confidence, and then an extended period of high confidence
if you maintain the same type of activity for a long time. Below is a
screenshot of a test app I wrote to take a look at the at a being returned
when running a query for activities over a certain period of time. Red
represents low confidence and green represents high confidence. The period
of time below is me shuffling through the throng of people at Circuit of
the Americas after the US Grand Prix last Sunday, which is why its slightly
Overall I feel like the activity data is extremely accurate. I’ve tested it
out pretty thoroughly with Runtime on a few runs here in Austin, and out in
New York’s Central Park. I’ve stuck with the low thresholds for running and
walking, because even that seems to be pretty accurate for my needs. Here’s
a screenshot from Runtime showing the different activity types during one
of my runs. The time I spent running is highlighted orange, while the time
spent walking is highlighted yellow.
To build this feature in Runtime I used the query API, and simply query the
activity type for the start and end time of a user’s run. I can then
iterate through the returned activities to determine how to highlight the
route the user took out on the trail.
Both APIs are very nicely designed block-based interfaces. In some ways I
look at this as the next evolution of Apple’s API design patterns. A class
method to determine whether or not access is available. Update methods with
a callback block. And query methods with a callback block. They’re very
clean, functional, and easy interfaces to use.
The data also appears to be highly accurate. The activity detection in
particular is basically dead on for distinguishing between walking and
running. I think the accuracy may vary slightly based on how you hold your
phone, but with it in my pocket or in an arm band I have noticed very high
If you’re considering adding support to the M7 to your app, hopefully this
will help point you in the right direction. I think its great that more
apps beyond fitness apps are beginning to use the M7. One example is Day
One, the excellent journalling app for iOS and Mac, which lets you add your
step data to your journal entries in their latest update. I desperately
wish I’d had an iPhone 5s during my John Muir Trail hike this summer, so
that I could have used this feature!
The M7 is a great new feature for iOS and something that can help build a
better experience in your app by giving the user access to move information
about their physical activity. Its a great new feature, and a fun API to