Where and When to get data for Watch Complication

Question

For watchOS 3, Apple recommends that you switch from using the complication datasource getNextRequestedUpdateDate scheduled update to update your complication.

The old way for watchOS 2

requestedUpdateDidBegin() is really only designed to update the complication. Keeping your complication (and watch app) up to date usually involves far more than reloading the timeline (and asynchronously retrieving data never fit in well with the old approach).

The new way for watchOS 3

The new and better approach is to use background refresh app tasks. You can use a series of background tasks to schedule and handle your app extension being woken in the background to:

• Fetch new data

  • using WKWatchConnectivityRefreshBackgroundTask to obtain data from the phone, or
  • using WKURLSessionRefreshBackgroundTask to download data from a server
• update your model once the data arrives,
• update your complication from the model (by reloading or extending the timeline), and finally
• update your app's dock snapshot to show the data on the dock

Call each tasks’s setTaskCompleted method as soon as the task is complete.

Other benefits of using app tasks

One of the key features about this design is that the watch extension can now handle a variety of foreground and background scenarios which cover:

• initially loading data when your app/complication starts,
• updating data in the background, when the extension is woken by a background task, and
• updating data in the foreground, when the user resumes your app from the dock.

Apple recommends that you use each opportunity you are given regardless of whether your app is in the foreground or background to keep your complication, app, and dock snapshot up to date.

Are there any limitations?

The number of total available tasks per day is divided among the number of apps in the dock. The fewer apps in the dock, the more tasks your app could utilize. The more apps in the dock, the fewer you can utilize.

• If your complication is active, your app can be woken up at least four times an hour.

• If your complication is not active, your app is guaranteed to be woken at least once an hour.

Since your app is now running in the background, you're expected to efficiently and quickly complete your background tasks.

Background tasks are limited by the amount of CPU time and CPU usage allowed them. If you exceed the CPU time (or use more than 10% of the CPU while in the background), the system will terminate your app (resulting in a crash).

For more information

• A good introduction explaining when and why to update your watch app is covered in Designing Great Apple Watch Experiences.

• For specifics, the Keeping Your Watch App Up to Date session covers everything you need to know to keep your complication, app, and dock snapshot up to date.

• WatchBackgroundRefresh sample code demonstrates how to use WKRefreshBackgroundTask to update WatchKit apps in the background.

Edit: El Tea (op) has posted a good answer at https://stackoverflow.com/a/32994055/630614

This is an interesting question/problem, and I've been wondering about a lot of the same!

For the most part, it seems that when I'm working on a new complication I need to step back and see when I really want to update it. A "countdown" complication could set all future timeline entries at one time, when the "end date" is set. An app that shows the current status of a web service could have relevant data stored in NSUserDefaults when an APNS comes through.

If you don't have access to APNS, don't want to run your iOS app in a background mode, and don't want to make HTTP requests from Apple Watch, I can think of 2 other options.

1) Schedule local notifications. The good part is that your Apple Watch should run didReceiveLocalNotification, but the bad part is that the user will get a notification when you're simply trying to check the status without a disruption.

2) Send a message to iOS via sendMessage(_:replyHandler:errorHandler:) in your reloadTimelineForComplication method, setting nil for the replyHandler to make it as quick as possible:

Calling this method from your WatchKit extension while it is active and running wakes up the corresponding iOS app in the background and makes it reachable.

Your iOS app could perform whatever network requests are needed and then store the information or push it to Apple Watch. Unfortunately, I don't think the watch extension will have it's session.didReceive... called until you run it, but you could access the data on the next call to requestedUpdateDidBegin.

As I said, I'm very interested in this same thing, so post some thoughts back and maybe we can extrapolate on some best practices here.