Category Archives: Programming

How to use child View Controllers in Swift 4.0 programmatically

I’ve just released my Learn to read Korean app for iPhone. It uses a number of child View Controllers in the home screen. While child View Controllers are not a new thing, it was a new experience for me, and I greatly recommend them to reduce the clutter of your View Controllers.

Here’s a photo of my home screen:

The main view controller consists of a vertical UIScrollView and multiple horizontal scrolling UICollectionViews below. While it’s possible to do it all in one massive View Controller, it’s much better to delegate UICollectionView events to their individual child View Controllers.

The good news is that using child UIViewControllers is super easy. You can use your Storyboard or do it programmatically in your UIViewController files. I opted for the latter as I find it easier to reproduce across Xcode projects.

All you need to do to add a child View Controller is below. I included an optional constraints section.

// Create child VC
let childVC = UIViewController()

// Set child VC

// Add child VC's view to parent

// Register child VC
childVC.didMove(toParentViewController: self)

// Setup constraints for layout
childVC.view.translatesAutoresizingMaskIntoConstraints = false
childVC.view.topAnchor.constraint(equalTo: heroView.bottomAnchor).isActive = true
childVC.view.leftAnchor.constraint(equalTo: self.view.leftAnchor).isActive = true
childVC.view.widthAnchor.constraint(equalTo: self.view.widthAnchor).isActive = true
childVC.view.heightAnchor.constraint(equalToConstant: height).isActive = true

With multiple child VCs (each handling their own UICollectionView events), the code base becomes manageable. In each child View Controller, you can handle customization, such as background color, UILabels, UIButtons, etc.

Another tip I have is to use the UIView’s convert(_:to:) method as necessary. You may need to get the child subview’s position relative to your parent View Controller’s view (such as for an UIViewControllerTransitioningDelegate). The code for that is simple too:

// contrived example label in Child VC to get parent frame
let label = UILabel()
let childViewFrame = label.frame
let frameInParent = label.convert(childViewFrame, to: parentVC.view)

That’s all I wanted to share for today. Don’t be afraid of using child View Controllers to break up your massive View Controllers!

Vertically Scrolling UIImage programmatically

Working on a small side project, I wanted to display images in my view controller view at full device width in a vertical scrolling view. Sounds simple right? The good news is that it is. While you may want to use UITableView for more control, using UIStackView is a simpler way to get up and running fast.

For my sample code, I opted to do it programmatically as it’s easier to copy & paste code than explain what constraints to add in Xcode. Also note that the code presented here is a proof of concept, quick and dirty example (not intended for production).

The steps are easy to understand:

  1. Add a scroll view to your view
            self.scrollView = UIScrollView()
            scrollView.translatesAutoresizingMaskIntoConstraints = false
            scrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
            scrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true
            scrollView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor).isActive = true
            scrollView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor).isActive = true
  2. Add a stack view to your scroll view
            self.stackView = UIStackView()
            stackView.translatesAutoresizingMaskIntoConstraints = false
            stackView.axis = .vertical
            stackView.spacing = 23.0
            scrollView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[stackView]|", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: ["stackView": stackView]))
            scrollView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[stackView]|", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: ["stackView": stackView]))
  3. Add images to your stack view
    stackView.addArrangedSubview(image(filename: "photo1"))

For the full code, read the ViewController on GitHub

Hope this helps you if you’re trying to throw together a quick prototype of vertically scrolling images in iOs.

Prior Inclination

Earlier this year, Alex Honnold scaled El Capitan without ropes. That was an impressive & dangerous feat by Alex.

What stuck with me is this quote about Alex from Tommy Caldwell:

Alex once told me that he had never fallen completely unexpectedly—meaning without at least some prior inclination that it could happen.

That is amazing and shows that Alex is simply operating at a higher level. I would make the blanket generalization that most climbers have fallen while climbing without anticipating it.

Why this sticks out to me is when I apply it to other fields. Outdoor climbing can easily be a life or death ordeal. Software generally is not life or death during development.

Can you imagine a programmer who writes code that doesn’t crash without the programmer having some prior inclination? That sounds impossible right? Or a slow development cycle.

I’m not saying that programmers need to be able to anticipate every crash ever. But if someone were able to never have their code crash without prior inclination, that’s awesome.

Supporting the iPhone X with Storyboard

There are a ton of guides out there for updating your app(s) to support the iPhone X.

If you create your view programmatically, you can use iOS 11’s safeAreaLayoutGuide. If your app targets iOS 10 or below, you can use the availability condition, #available().

With the Storyboard, one thing I appreciate from Apple is making the safe area layout guide backwards deployable.

Apple told us in WWDC 2017 Session 412 that Storyboards using safe areas are backwards deployable. This means you can switch to using the safe area layout guide in Interface Builder even if you still target iOS 10 and older.


I don’t always use the storyboard for my layouts, but for apps that I need to update, this backwards deployability helps a lot.

CLI Cut Visual Option

Something I came across recently was command line text manipulation with a CSV. The way that the list option is passed in is cool.

For demonstration purposed, we have a contrived text document “dummy.txt” that happens to be delimited by the % character. The contents inside the file are:


To get the first column of data, you can run

cut -d% -f1 dummy.txt

which gives you:


If you wanted to save the output, the standard command line “>” comes in handy.

To get the columns up to (and including the) 2nd column, you can run

cut -d% -f-2 dummy.txt

which gives you:


To get the 2nd & 3rd columns, inclusive, you can run

cut -d% -f2-3 dummy.txt

which gives you:


To get the 3rd column onward to the last column, you can run

cut -d% -f3- dummy.txt

which gives you:


The examples above are just for this demo, but I think the hyphen syntax in the list fields option is easy to learn and visually clear (for a CLI interface).

Multiple UIDynamicAnimators

In past apps, I tended to have one UIDynamicAnimator in my ViewController and that was that. UIDynamicAnimator allows you to use UIDynamics / effects on your UIViews.

The issue that I ran into was that removeBehavior(_:), which “Removes a specified dynamic behavior from a dynamic animator“, didn’t seem to work. I would keep track of specific UIDynamicBehavior instances and pass them as the argument for removeBehavior(_:) but it didn’t appear to remove the behavior.

What does work is calling removeAllBehaviors() on the UIDynamicAnimator. This is fine if you only have one UIView. But most likely, you have multiple UIViews & behaviors. Calling remove all on the only animator isn’t a good idea. That could leave UIViews frozen out of place.

Recently, I released a fun weekend app, Fun Faces. Browsing stack overflow, it occurred to me to use multiple UIDynamicAnimators. One for each UIView I wanted to animate. This worked for my use case, where calling removeAllBehaviors() doesn’t interrupt the other UIView’s behaviors (if any).

Using multiple UIDynamicAnimators isn’t an answer if you have multiple UIViews under the same animator with UICollisionBehavior or other effects that let the UIViews interact with each other.

Using CoreMotion deviceMotion to keep image level example (Xcode 8.3, Swift 3.1)

I’ve been playing around with CoreMotion since it is frankly so cool. I’ve followed NSHipster’s CMDevice​Motion post, but I made some changes to use the latest Swift v3.1. Below is sample code for using both the gyroscope and accelerometer to keep an image level when you rotate your phone.

//  ViewController.swift
//  Created by Rex on 4/22/17.

import UIKit
import CoreMotion

class ViewController: UIViewController {

    let interval = 0.01
    let imageFilename = "bg.jpg"
    let imageWidth = CGFloat(800)
    let imageHeight = CGFloat(1200)
    let manager = CMMotionManager()
    var imageView: UIImageView?

    override func viewDidLoad() {

        guard manager.isDeviceMotionAvailable else { return }
        manager.deviceMotionUpdateInterval = interval
        let queue = OperationQueue()
        manager.startDeviceMotionUpdates(to: queue, withHandler: {(data, error) in
            guard let data = data else { return }
            let gravity = data.gravity
            let rotation = atan2(gravity.x, gravity.y) - .pi

            OperationQueue.main.addOperation {
                self.imageView?.transform = CGAffineTransform(rotationAngle: CGFloat(rotation))
    func setImageView() {
        if let img = UIImage(named: imageFilename) {
            let iv = UIImageView(image: img)

            // center the image
            let x = (self.view.frame.width/2)-(imageWidth/2)
            let y = (self.view.frame.height/2)-(imageHeight/2)
            iv.frame = CGRect(x: x, y: y, width: imageWidth, height: imageHeight)
            self.imageView = iv

The setup is simple. Create a new Single View Application project in Xcode. You’ll need to add a JPG to the Assets.xcassets folder in the project. Replace the Viewcontroller with the code below and make sure to update the image filename, width, and height constants.

The code hopefully is straightforward. We make sure the CMMotionManager’s device motion is available. Then, we add the imageview (as the only UIView element we’re adding to the screen). We use an OperationQueue to process the rotation calculation off the main queue. Then we update the imageview with a transform on the main queue.

iOS 10 Locales and Currency Symbols – Sample App

While working on adding localization to my tip calculator, one thing that seems obvious in retrospect is the difference between a device’s language & region. iOS lets you set the language & region separately. For example, you might want to read text in English, but you could be in Asia. This is relevant to tipping since you could travel to a country where tipping is expected, but the country your phone’s language is associated with doesn’t traditionally tip.

While exploring locales and currency symbols, I whipped together a basic demo app that lets you scroll between all the known locales and their currency symbol in iOS 10. This is pretty useful since you can quickly see what the currencySymbol is for each known iOS locale.

Below is the full implementation of very hacked together (quick and dirty) code. All you need to do:

  • Create new Single View Application project in Xcode
  • Replace the ViewController.swift with below (written for Swift 3)
  • Run the app in Xcode
import UIKit

class ViewController: UIViewController, UITableViewDelegate {
    let cellIdentifier = "Cell"
    let currentLocaleHeight = CGFloat(80)
    let locales = Locale.availableIdentifiers.sorted { $0.localizedCaseInsensitiveCompare($1) == ComparisonResult.orderedAscending }
    override func viewDidLoad() {
        let tableView: UITableView = UITableView()
        tableView.frame = CGRect(x: 0, y: currentLocaleHeight, width: view.frame.width, height: view.frame.height)
        tableView.dataSource = self
        tableView.delegate = self
    func addCurrentLocaleLabel() {
        let local = Locale.current.identifier
        let width = view.frame.width
        let label = UILabel(frame: CGRect(x: 0, y: 0, width: width, height: currentLocaleHeight))
        label.text = "Current locale: " + local
        label.textAlignment = .center

extension ViewController: UITableViewDataSource {
    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return locales.count
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: .value1, reuseIdentifier: cellIdentifier)
        let localeString = locales[indexPath.row]
        let numberFormatter = NumberFormatter()
        numberFormatter.locale = Locale(identifier: localeString)
        cell.textLabel?.text = localeString
        cell.detailTextLabel?.text = numberFormatter.currencySymbol
        return cell


Note that the “¤” symbol means the currency is unspecified.

Using Fastlane Snapshot to generate screenshots with UIPickerViews

This week, I released an update for my Tip Solver calculator to add Chinese localization. I had to generate 5 screenshots for 5 devices (iPhone and iPad) across 3 languages. In the time that I spent automating the process with Fastlane Snapshot, I could have easily done it manually in way less time. But the good news is that I’ve set myself up to painlessly generate screenshots for new languages. Snapshot takes some time to run, but it’s still a huge improvement over generating screenshots manually.

It took me a lot longer than I would have liked to setup my Snapshot process due to my usage of UIPickerViews. Tip Solver makes heavy usage of UIPickerView and I ran into many issues with UITest.

Your mileage may vary, but I found I had to do the following to be able to use UITest and UIPickerViews:

  • disable Ads (which run over the network)
  • drastically reduce the number of UIPickerView rows (in numberOfRowsInComponent)
  • use titleForRow instead of viewForRow for UITest running

The last one (using titleForRow) was a complete non starter since I rely on heavy UIPickerView visual customization. Generating screenshots with incorrect picker views defeats the whole point of the exercise.

I tried using the Xcode’s UITest recorder, but I ran into many issues. One glaring issue is that while recording, I was able to swipe the UIPicker up, but when I played it back, it ended up swiping up the Control Center (instead of adjusting the UIPicker). There is a method (adjustToPickerWheelValue), but I found that it only works with titleForRow (which I don’t use). What I would like is an expansion of the XCUIElement API to add a simple increment/move up or down once.

My final solution (aka work around) was to use a combination of Fastlane launch arguments & brute forcing the UIView (via UIViewController viewDidAppear) to generate my screenshots. My work around isn’t ideal, but it gets the job done.

In my Fastlane Snapfile, I was able to define arguments:

 "-screenshot 1",
 "-screenshot 2",
 "-screenshot 3",
 "-screenshot 4",
 "-screenshot 5"

In my ViewController (running Swift 3), I was able to handle them accordingly:

let screenshot = UserDefaults.standard.string(forKey: "screenshot")
if screenshot == "1" {
    // do something
} else if screenshot == "2" {
    // do something
} else if screenshot == "3" {
    // do something
} else if screenshot == "4" {
    // do something
} else if screenshot == "5" {
    // do something

Once everything is setup, generating screenshots was simply running snapshot on the command line.

I’m sure there’s room for improvement in the code (using an enum, etc.), but I left it at that since it’s only for screenshot generation.

If you’ve made it all the way down here, thanks for reading. I just wanted to share my experience with UITest and UIPickers. UITest probably needs more love from Apple as it was not pleasant to work with.


How to open phone app from iMessage extension and pass data

This weekend, I spent some time digging into iOS 10 iMessage app extensions.

Creating an iMessage app or sticker pack is relatively easy. Here are a couple of resources for getting started: tutsplus and medium. Apple also has their own example iMessage app.

For a new iMessage only app, you can choose File > New > Project and either ‘Sticker Pack Application’ or ‘iMessage Application’ in Xcode. To add an iMessage extension to your existing containing phone app, you can use File > New > Target and either ‘Sticker Pack Extension’ or ‘iMessage Extension’.

I wanted to understand the current state of user workflow between the iOS (main/phone) app and the iMessage extension app. Apple has it’s work cut out for them. The iMessage App store is an awkward modal triggered from an individual iMessage conversation. Users seem to have trouble locating the store and managing iMessage apps, so apps are getting bad reviews.

The good news is that you can launch your phone app from the iMessage app.

From the iMessage extension app, you can use this (with your own AppName):

guard let url: URL = URL(string: "AppName://?myParam=myValue") else { return }

self.extensionContext?.open(url, completionHandler: { (success: Bool) in

Calling the above code will open the phone app. One issue I ran into (under the extension scheme) is that the phone app will crash in the simulator when opened this way. Xcode shows a SIGKILL since the iMessage app connected to Xcode is quit while the phone app is being opened. This appears to only be a simulator issue.

You can access the URL params in your phone app’s App Delegate:

    func application(_ app: UIApplication, open url: URL, options: [UIApplicationOpenURLOptionsKey : Any] = [:]) -> Bool {

        return true


The bad news is that you cannot launch your iMessage app from your phone app. Apple must have been short on time, as their usual modus operandi is to launch headline features that are incomplete and hopefully iterate later if their business still cares. Radar(s) have been filed.

When asked, “Is it possible for my app to open the Messages app with my iMessage extension activated?” An Apple Staff member says, “There currently isn’t a way to do this.”

I think opening your iMessage app from your phone app would be an excellent, natural use case. User does something in the containing app, then they want to share that app’s state with a friend using iMessage. Why rely on the user going to Messages, finding your iMessage app in the correct convo, and then recreating app state to send it over?

So to recap, iMessage apps have a lot of potential as they are living apps within iMessages. They can communicate with their containing app. But I have reservations about the UX, including discoverability.