tvos, the focus engine, and swift

Evan Maloney Distinguished Engineer, Gilt Groupe

tvOS, the Focus Engine

& Swift

Discoveries from building “Gilt on TV”

Demoing “Gilt on TV”

tvOS vs. iOS: Similarities

• Most of tvOS is a direct subset of iOS

• Familiarity with UIKit will serve you well with tvOS

• But, some of the things you may expect won’t be there

• No web views

• Certain frameworks like MultipeerConnectivity aren’t available

• Several UIControls (pickers, steppers, sliders, etc.)

• Can use Interface Builder (or not), Swift or Objective-C

tvOS tvOS vs. iOS

tvOS vs. iOS: Differences

• User interaction is indirect touch

• Unlike an iPhone or iPad, the user doesn’t directly manipulate the UI — uses indirect touch like a trackpad

• Directionality is reversed from iOS

• Relies on the Focus Engine for determining UI state

• Only on tvOS — adds extensions to UIViewController, UIView, UITableView, UICollectionView, etc.

tvOS tvOS vs. iOS

Focus Engine Philosophy

• The Focus Engine is intended to be a black box

• You provide inputs that determine the initially focused view

• You define where the user can go

• Provide some visual indication of focusability

• Provide an animation or visual state change upon becoming focused (often handled by the tvOS SDK)

• Once your view controller is onscreen and the initial view has become focused, the user should be in control of all state changes

• It is considered rude to forcibly change focus on the user

tvOS The Focus Engine

What is “Focus”?• A focusable view is one that the user can select with the touch

remote

• Being focusable implies that the user can perform an action on the view

• Focusable views are often “pressable,” like UIButtons

• A focused view renders itself to stand out from the others onscreen, usually with one or more of the following attributes:

• A larger size

• A drop shadow

• A highlight color or border

tvOS Focus

How does the Focus Engine work?

• When the view hierarchy changes, the Focus Engine creates a map of the visible focusable views

• If the user navigates in a given direction, the Focus Engine looks for a focusable view in that direction, taking into account each candidate view’s focus frame size & relative position

• A candidate for receiving next focus is selected

• If the candidate is a table or collection cell, the relevant delegate is given an opportunity to reject focus and another candidate may be selected



Gilt on TV 1.0 — Women’s Store


Focus Frames

How does a view become “Focusable”?

• By default, the following views are focusable:

• UIControls

• Table cells and collection cells

• Can be overridden by UICollectionViewDelegate

• UIViews that are not otherwise focusable can be made so by overriding canBecomeFocused()

• The focused property of a UIView indicates whether the view is currently in focus

tvOS Focus

How does a view become “Focused”?

• When a view controller is displayed, the focus engine asks for the preferredFocusedView from the relevant views and view controllers to determine which view gets initial focus

• From there, the user controls focus using the remote

• When the user swipes in a given direction, the Focus Engine selects an appropriate focusable view to get next focus

• Developers can use the UIFocusUpdateContext to coordinate animations when views go into and out of focus

tvOS Focus


Gilt on TV 1.0 — Men’s Store (scrolled)


Focus Frames


Changing Focus — Unambiguous Move


Ambiguous Move — Focus Engine Chooses


Sale View


Focus Frames


Navigating to “Buy Now” — A Near Miss

The Dilemma

tvOS Focus Guides

How could you solve it?

You can’t assume the Focus Engine “sees” focusable views that aren’t in the direct path of the swipe direction

tvOS solution

Allow a UIView to report to a focus frame that’s larger than the view’s native frame

1. Call parentView.addLayoutGuide(UIFocusGuide)

2. Set the focus guide’s preferredFocusedView

3. Add Auto Layout constraints using the focus guide as you would any UIView

tvOS Focus Guides

A focus guide defines a screen rectangle

tvOS Focus Guides

Focus is forwarded to the preferredFocusedView

tvOS Focus Guides

Allowing a small button to respond to a large frame

Handling scrolling content

• You will not be able to present scrolling content in the same way you do in iOS

• User can only scroll by navigating to something focusable within a scroll view

• The Focus Engine only pays attention to focusable non-hidden views with non-zero alphas that have at least one pixel within the screen bounds

• Focusable views and cells that are offscreen won’t be visible to the Focus Engine, so the user can’t navigate to them


tvOS Focus Guides

Product View

tvOS Focus Guides

Focus Frames

Other challenges for tvOS developers

• Apparently a lot of people rely on web views for mission-critical screens within their apps — they will be forced to rethink

• Restrictions on App Store binaries:

• Applications need to be submitted as Bitcode binaries

• This includes embedded frameworks, which makes many third-party closed-source frameworks unusable in tvOS

• On-device testing — Very few people have development devices, and simulator testing may not reveal all problems

tvOS tvOS Development

Fun things about tvOS development

• Some of the iOS pain points don’t exist on tvOS

• Apple TVs will often be used to stream video — unlike with mobile devices, tvOS developers can assume the presence of low-latency, always-on, high-bandwidth networks

• Don’t need to think about screen rotation or size class changes

• Only need to develop for a single screen resolution — for now

tvOS tvOS Development

A few words about Swift

and finally…

Swift — Pain Points• The language is still changing, so major releases of Xcode

often require a bit of code rewriting

• Usually minimal, and the language is improving as a result

• Developer tooling still needs work

• Debugger sometimes refuses to cough up variable values

• Compiler errors can be vague or misleading

• Compiler stability is sometimes an issue

• Heavy use of Swift frameworks sometimes doesn’t play nicely with “whole module optimization”

• Binaries may not be compatible across compiler versions

tvOS Swift

Swift — Advantages

• Swift may not be mature, but it is ready for prime-time

• Swift absolutely fulfills the promise of more efficient development

• An entire class of errors that used to require runtime debugging is now caught by the compiler

• Building a stable app takes much less work than Objective-C

• I probably spent about ¼ the time debugging as I would have had to do in ObjC

• Coding in Swift is an absolute pleasure

tvOS Swift

Swift — The Verdict

tvOS Swift

Do it!You know you’ll “go Swift” eventually…

…and the longer you wait, the more you’ll be kicking yourself for not having done it sooner once you finally do!

Evan Maloney Distinguished Engineer, Gilt Groupe

A parting gift for you

https://github.com/emaloney/CleanroomLogger

CleanroomLogger — A pure Swift logging engine

Simple, lightweight & performant

Code goodies!

tvos, the focus engine, and swift

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