Where and When to get data for Watch Complication

Question

After working with complications for a few days, I feel confident saying the following about the update process for updates that happen at a prescribed interval:

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Answer 1

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.