- Watch Kit was introduced in November 2014.
- Apple Watch is just an external display for program running on a connected
- Watch Kit development requires the iOS 8.2 SDK or later.
- Two separate bundles:
+ WatchKit app (runs on Apple Watch)
+ WatchKit extension (runs on user’s iPhone)
3. Watch Kit Solution
Has 3 projects:
- The Watch Extension: runs on the
connected iPhones, contains the code
and large resource storage for the Watch
App but has no visible representation
beyond what the Watch App provides.
- The Watch App: small project which
runs on the Apple Watch, generally
consisting of only a storyboard, small
resources, and link references to files in
the Watch Extension.
- The iOS Parent App: a project whose
primary purpose is to bundle the Watch
App and Extension.
4. Application Lifecycle
- Handle each Storyboard scene by a subclass of WKInterfaceController.
- WKInterfaceController ≈ UIViewController (do not have the same level of access to the view
- Can: + hide and reveal controls
+ change size, transparency, appearance options of some controls
- Can’t: + dynamically add controls
+ restructure UI
The lifecycle of a WKInterfaceController object involves the
• Awake: should perform most of initialization here.
• WillActive: Call shortly before the Watch App appears.
Use this method to perform last-moment initialization,
start animation, etc.
• At this point, the Watch App appears and the Extension
begins responding to user input and updating the Watch
App’s display per application logic.
• DidDeactivate: is called after the Watch App has been
dismissed by the user or the connection to the iPhone is
broken. After this method returns, user interface controls
can’t be modified until the next time WillActive is
• After the Extension has been deactivated, it is
inaccessible to program. Pending asynchronous
functions will not be called.
6. Types of User Interface
• Normal Interaction: the majority of Watch App/Extension
interaction will be with sub-class of WKInterfaceController (by
custom controllers: LabelDetailController,
ButtonDetailController, SwitchDetailController, etc.)
• Glances: non-interactive, single-page UIs that are intended to
provide summary or crucial data.
• Notifications: are a major user-case for the Apple Watch. Both
local and remote notifications are supported.
8. User Interface: Glances
When a Glance is tapped, it
launches the main Watch App.
Glances may deep-link into the
Watch App by passing context
info in a call to
9. User Interface: Notifications
- Occurs in two stages: Short Locks and Long Lock
• Short Locks: displayed briefly and show the Watch App icon, its name and the Title.
• Long Lock: combines a system provided sash area and Dismiss button with custom
- React to notification event by overriding two methods
DidReceiveLocalNotification and DidReceiveRemoteNotification
10. Screen Sizes
The Apple Watch has two face sizes: 38mm & 42mm, both are 5:4 display ratio, Retina screen.
• 38mm: 136 x 170 logical pixels ( 272 x 340 physical pixels)
• 42mm: 156 x 195 logical pixels (312 x 390 physical pixels)
Use WKInterfaceDevice.ScreenBounds to determine on which display Watch App is running.
(*): Should develop text and layout design with 38mm display then scale up because scaling
down from large environment may lead to ugly overlap or text truncation.
11. Limit of Watch Kit
• No access to the sensors on the Apple Watch.
• The App/Extension will not run without an active connection to an iPhone. Notifications will
not appear on a disconnected Watch.
• The Watch App has a 20Mb image cache
WKInterfaceDevice.CurrentDevice.WeakCachedImages). This limit can’t be
changed and developers must manage the cache contents themselves.
• Different classes ( WKInterfaceButton vs UIButton, WKInterfaceSwitch vs
UISwitch, etc) and have a limited set of methods compared to their UIKit equivalents.
• No access to the display graphic context on the Watch.
• Apple will not allow 3rd-party custom watch faces.
• Doesn’t support background execution. Any pending asynchronous functions will not
execute after Watch Kit Extensions are being deactivated.
• Can’t play sound on the Apple Watch.
• No access to the Taptic (Vibration) API.
• Cannot route notifications to the Watch only or the iPhone only.