Archive for the ‘Xcode’ Category

Auto Layout, Playgrounds, and Xcode

Today, someone asked what the easiest way was to center a view (specifically a UIImageView) inside a parent view with minimum offsets from the top and sides.

Because you’re working with image views, it’s important that you first set the content mode. For insets, it’s almost always best to go with a “fitting” aspect-scale, which preserves the entire picture even if it has to pillarbox or letterbox. (Pillarboxing adds extra spacing to each side for tall images; letterboxing adds the top and bottom for wide ones.)

// set content mode
imageView.contentMode = .scaleAspectFit

Make sure your view can squish its content by lowering its compression resistance:

[UILayoutConstraintAxis.horizontal, .vertical].forEach {
            UILayoutPriority(rawValue: 100), for: $0)

You must preserve your image’s aspect ratio. Calculate your target ratio by dividing its width by its height:

let size = imageView.image?.size ?? CGSize()
let aspectRatio = size.width / size.height

Add strong constraints that preserve the aspect, and make the view smaller than its parent using inset values you supply:

let insets = CGSize(width: 20.0, height: 32.0)

let constraints = ([
    // Preserve image aspect
        .constraint(equalTo: imageView.heightAnchor, multiplier: aspectRatio),
    // Make view smaller than parent
        .constraint(lessThanOrEqualTo: parentView.widthAnchor,
                    constant: -insets.width * 2),
        .constraint(lessThanOrEqualTo: parentView.heightAnchor,
                    constant: -insets.height * 2),

    // Center in parent
        .constraint(equalTo: parentView.centerXAnchor),
        .constraint(equalTo: parentView.centerYAnchor),

If you want to be super cautious, keep the aspect and two center constraints at 1000 and bring the width and height ones down to 999. You can install the constraints as you create them but I prefer to break that part out so I can tweak the priorities for each constraint group:

constraints.forEach {
    $0.priority = UILayoutPriority(rawValue: 1000)
    $0.isActive = true

I always mess up with the signs (positive or negative) of the constants. It helps to test these out in a playground rather than going by memory because the signs aren’t always intuitive. Even better, write routines that automates your Auto Layout tasks because if you debug once (and add proper tests), you never have to think about it again.

Mac playgrounds are inherently superior to iOS ones as they don’t run a simulator and are faster and more responsive. That is to say, you don’t have to quit and relaunch Xcode quite so often. If you are debugging iOS layouts though, and your playground hangs when starting the simulator or running your code, learn to quit, kill the core simulator service, and relaunch Xcode. It will save you a lot of time.

I have a one liner in my utilities to deal with the service:

% cat ~/bin/simfix
killall -9 -v

Most of the time a single quit, simfix, and launch will get things moving again with recalcitrant iOS playgrounds. Be aware that malformed format strings and other auto layout issues won’t produce especially helpful feedback, especially compared to the runtime output of compiled projects. If you know what you’re doing, you can set up a simple layout testbed in playgrounds with less overhead and time than, for example, a one view project but at a cost.

Stick to projects, and avoid playgrounds, when testing even mildly complex code-based layouts. You cannot, at this time (at least as far as I know), control which simulator will be used in the playground so it’s not great for checks on a multitude of simulator geometries. The tl;dr is that playgrounds work for general layout routines. Prefer compiled projects for specific tasks.

Beta 3 Playground Workarounds

Adding Resources and Sources folders to Playgrounds

Until they’re fixed, you may have to add them by hand.

  1. Right-click/control-click.
  2. Show package contents
  3. You can add new Resources and Sources files at the top level
  4. Alternatively, navigate down to individual pages (which are finally in Beta 3!) by showing their package contents and add them there

Fortunately under Beta 3 you don’t have to manually add pages, as the new page functionality is finally back.

Creating New Playgrounds

They’re no longer listed in the File > New dialog. Instead choose File > New > Playground (Command-Shift-Option-N) or open “Welcome to Xcode” (Command-Shift-1) and click “Get started with a playground”.

Most of the obvious alternatives (like Command-Control-N, which creates new workspaces) are already taken, but if you don’t mind using the menu for that, I think it’s a nicer key binding for “New Playground”. If you want to mess with this open prefs (Command-comma), type playgrounds into the search field, and edit the key binding for “New > Playground”

Don’t forget that new playground page is “Command-Option-N”.

The Unexpected Joy of Vector Images in iOS 11

A few years ago, I griped about That Vector Thing, which was the way that Xcode 6 handled PDF vector assets and could not extend them to arbitrary use in UIImages.

Enter WWDC 2017. In Session 201 “What’s New in Cocoa Touch”, Apple described Asset Catalogs with PDF-backed vector images. All you have to do is tick the “Preserve Vector Data” checkbox.

After chatting with some colleagues about whether this would actually work as promised, I dragged up a 29×29 vector PDF image.

And I added it as a 1x image in an asset catalog. Notice the Resizing checkbox to the right. It is normally unchecked by default.

I then built a super-simple single view testbed to test things out.

override func viewDidLoad() {
    let imageView = UIImageView()
        .translatesAutoresizingMaskIntoConstraints = false
    imageView.contentMode = .scaleAspectFit
    ["H:|[v]|", "V:|[v]|"].forEach { format in
            .constraints(withVisualFormat: format, 
                options: .init(rawValue:0), metrics: nil, 
                views: ["v": imageView]).forEach { 
            $0.isActive = true 
    let image = UIImage(named: "Biff")
    imageView.image = image

Here’s what that 29×29 image looks like running on an iPhone 7+ in the simulator. The 1x image is being rendered on a 3x destination, at a greatly magnified size. Its vector data ensures the image renders without losing detail or clarity. Compare it to the same asset that does not preserve vector data and my original test from 2014:


Click above to see full size screenshot originals. Below is a comparison shot from the simulator at the largest size.

I expect there are minor performance hits in scaling and rendering the vector image compared to loading a standard PNG or JPEG, but I didn’t get around to measuring the costs.

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The problem with Swift Playground localization

Starting in Swift Playgrounds 2, you can now use localized strings to guide the narration of your interactive lessons. As the screenshot above demonstrates, you can used localizable markup to provide the most appropriate text for titles, introductory text, and feedback.

However, what you can’t do is localize Swift members. Your French and Chinese consumers must tell Byte to moveForward(), not avancer() or 向前移动().

One of the guiding principles of the Swift language is demonstrated in its embrace of unicode for identifier symbols. This approach accommodates programmers and programming styles from many languages and cultures.

Xcode 9 has introduced major advances in code refactoring. It seems an obvious win to allow that technology to be applied to Swift Playgrounds 2, enabling identifier localization.

That’s because identifiers play such a key role in Swift Playgrounds. Unlike standard development tasks, where it’s unnecessary to create idiomatic APIs like IUContrôleurDeNavigation, the point of Swift Playgrounds is to teach and instruct. It uses small, limited, controlled API exposure, nearly all custom and supporting of the teaching story.

The anthropomorphized Byte character acts as a stand-in for the learner coder. And in doing so, it should communicate with commands that this coder identifies with, turnLeft and moveForward, not incomprehensibleForeignPhrase1 and evenMoreConfusingForeignPhrase2.

I think this is an opportunity waiting to happen, and I can’t imagine it would be all that hard to implement given the expansive identifier suite and the limited API visibility presented in a typical playgroundBook.

What do you think? Is it too much to ask for a localizable.Members.plist?

Simulating a second finger during drag

You can drag and drop in the iOS simulator by clicking and holding an item. The item “pops” and you can then drag it to a destination. Today, an Apple engineer shared a neat way to free up a “second finger” during this process.

Pause and press the control key. This pins an item mid-drag, enabling you to use the Mac cursor as another touch. You can then retrieve your drag by grabbing the paused item and conclude your drop.

Apple open sources key file-level transformation Xcode components

Ted Kremenek writes on the swift-dev list:

This afternoon at WWDC we announced a new refactoring feature in Xcode 9 that supports Swift, C, Objective-C, and C++.  We also announced we will be open sourcing the key parts of the engine that support file-level transformations, as well as the compiler pieces for the new index-while-building feature in Xcode.

We will be releasing the sources in stages, likely over the next few weeks:

– For the refactoring support for Swift, there are some cleanups we’d like to do as well as some documentation we’d like to author before we push these sources back.  Argyrios Kyrtzidis and his team from Apple will be handling that effort.

– For the refactoring support for C/C++/Objective-C, these are changes we’d like to work with the LLVM community to upstream to the LLVM project.  These will likely be first staged to the swift-clang repository on GitHub, but that is not their intended final destination.  Duncan Exon Smith and his team from Apple will be handling that effort.

– We’ll also be open sourcing the compiler support for indexing-while-building, which include changes to both Clang and Swift.   Argyrios and his team will be driving that effort.  For the clang changes they will likely be first staged to swift-clang, and then discussed with the LLVM community to upstream them to mainline Clang.

– Finally, we will be open sourcing the remaining pieces of the Swift migrator.  Argyrios and his team will be handling the push back of changes there, and those changes will only be impacting the swift repository.

As usually, we’ll also be pushing back changes to have Swift work with the latest Apple SDKs.  We’re expecting that push back to happen early next week.  When that happens we will temporarily lock commit access to the repositories.  Details about that will be sent out later in a later email.  Until then, the downloadable toolchains from will continue to work with Xcode 8.3.2.  After we do the push back the downloadable toolchains will be moved to be baselined on the Xcode 9.0 betas.  This shift is necessary as changes to the overlays depend on the latest SDKs.

Xcode Autocomplete Frustrations

A year after it debuted, Xcode’s enhanced autocomplete features continue to struggle with overly liberal matches:

In this example, several of the matching text results display few commonalities with my search phrase . There’s really no reason that “fatale” should match CFDataGetLength(theData: CFData!).

It shouldn’t be hard to create heuristics that count the number of matched chunks, their distance from each other, to build a score related to whether the match is chunky (a good thing for keywords) and singular (another good thing for discerning developer intent).

Successful autocompletion promotes good matches and discards inappropriate ones. “upper” should score high on CFStringUppercase and low on CGScreenUpdateOperation and CSSMERR_TP_INVALID_CERTGROUP_POINTER.

That’s not the only problem with autocomplete. Image literal completion is a big problem. Xcode often prioritizes images over code APIs. When starting to type “picker”, Xcode should not suggest “picture-of-lovely-cat”. Here are some real world examples of this issue:

One developer told me that while typing in for closures, that eighty percent of the time, he gets a random autocompleted image literal instead of the keyword he’s shooting for:

Surely, this is an obvious place to introduce autocomplete preferences that allow you to exclude literals from the API list. The auto complete for image literals should act more like colors, offering an Image Literal entry point to a image picker instead of clogging the API name space:

It would certainly get rid of those inappropriate in matches.

Thanks Olivier Halligon, Andrew Campoli, and everyone else who gave me feedback and direction for this post.