Chris' Blog.

My occasional thoughts on iOS development, developers careers, trying to make an income from the App Store, and updates on life in general.

Transitioning bugs


This post attempts to work around one of the bugs triggered by setting modalPresentationStyle to Custom. However, there are numerous other bugs caused by this:

  • It doesn't remove the 'from' view controller's view from the window.
  • It doesn’t call will/did disappear on the ‘from’ view controller.
  • If you present a modal, rotate, then dismiss - the original view controller is wrongly rotated.
  • Unwind segues stop working.
  • The ‘to’ view is often nil in iOS8 and needs to be added post-completion (this is the bug described below).

This may sound huge, however there is one simple workaround: set the modalPresentationStyle to FullScreen. You still get the custom transition, and everything else just works. So, if you do that, you can ignore the rest of this post...

One more gotcha: On iOS7, the from view controller has a strong reference to your transition, so make sure that your transition references the view controllers weakly, or nil them just before calling completion, to avoid a memory leak.

Original post

Hi all, I struggled half of today on a bug in iOS8 when creating custom view controller transitions. Two of my friends shortly afterwards told me they had the same problem. In the hope that I can help someone out there not waste half a day on this problem, here we go:

The problem

Say you've got a View Controller, and you want to display it modally, but you want a more interesting transition than the normal 'swoosh up, perform some action, tap close, and it swooshes down'. So you read the documentation, set modalPresentationStyle to Custom. Next, you set the modal VC's transitioningDelegate to some class that implements UIViewControllerTransitioningDelegate (a reasonable choice is that your modal VC will be its own transitioning delegate). Then, on this delegate class, you implement animationControllerForPresentedController... and animationControllerForDismissedController... to return your presentation and dismissal transitions respectively. And then you implement those two transitions, following the basic rules: Add the 'to' view to the 'container' view at some stage, and call the context's completeTransition when you're done.

So far, so good, nothing overly complicated there (it sounds harder than it actually is). And for me, running it on iOS7 bears no surprises, it works nicely. However, on iOS8...

...on iOS8, on the dismissal transition, the 'to' view controller is removed from the main window at the end of the transition. This is an issue that has even tripped up geniuses such as Ash Furrow; His radar for this issue.

So I had to come up with a solution that would work on iOS7, iOS8, and (fingers crossed) on iOS9+ regardless of whether they fix the bug or not.


- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext {
    // Get the 'from' and 'to' views/controllers.
    UIViewController *fromVC = [transitionContext viewControllerForKey:
    UIViewController *toVC = [transitionContext viewControllerForKey:
    // viewForKey is only available on iOS8+.
    BOOL hasViewForKey = [transitionContext
    UIView *fromView = hasViewForKey ?
        [transitionContext viewForKey:UITransitionContextFromViewKey] :
    UIView *toView = hasViewForKey ?
        [transitionContext viewForKey:UITransitionContextToViewKey] :
    UIView *container = [transitionContext containerView];

    // iOS8 has a bug where viewForKey:to returns nil.
    // The workaround is:
    // A) get the 'toView' from 'toVC'.
    // B) manually add the 'toView' to the container's
    // superview (eg the root window) after the completeTransition
    // call, as automatically happens on iOS7 where things work properly.
    BOOL toViewNilBug = toView==nil;
    if (!toView) { // Workaround by getting it from the view.
        toView = toVC.view;
    UIView *containerSuper = container.superview;

    // Perform the transition.
    toView.frame = container.bounds;
    toView.alpha = 0;
    [container addSubView:toView];
    [UIView animateWithDuration:kDuration delay:0
        options:UIViewAnimationOptionCurveEaseIn animations:^{

        toView.alpha = 1;

    } completion:^(BOOL finished) {

        [transitionContext completeTransition:YES];

        if (toViewNilBug) {
            [containerSuper addSubview:toView];



I found that viewForKey:UITransitionContextToViewKey returns nil on iOS8. So if it's nil, I grab the view from the 'to' view controller.

However, this bug also seems to result in the 'to' view not being moved from the container to the window when completeTransition:YES is called. On iOS7, before completeTransition is called, the 'to' view is a subview of the container; then completeTransition moves it up a level to be a subview of the container's superview (which happens to be the UIWindow).

So if viewForKey:UITransitionContextToViewKey returns nil, I fail over to toVC.view and keep track of the fact that it hit the bug. Then immediately after the completeTransition call, if the bug was hit, I add it to the container's initial superview (which happens to be the UIWindow), to mimic the iOS7 behaviour.

And we need to store the container's original superview in the containerSuper variable, because container.superview returns nil after the completeTransition call. This is because the container is just temporary, and is correctly removed at the end of the transition by UIKit.

So this code works on iOS7 as well as iOS8, and should work on iOS9 regardless of whether they fix this bug or not. But don't hold me to it.

Thanks for reading

If you're interested, I recently released a video series on making an iOS app - please check it out:

I'd love to get an email if this helps you!

IB Designable

I gave a talk at the Sydney iOS Developers Meetup a couple days ago, the topic was Xcode's new(ish) feature: IB Designable.



And here are the slides.

Hope someone gets something out of this!

App Architecture

Here's some thoughts on how you should architect your App, based upon discussions between two well-seasoned iOS developers. Let's jump in!


Model-View-Controller is a very important concept you should adhere to! Don't be intimidated by the complicated sounding name. All it means is that you should split your responsibilities between 3 broad areas of your code.

Think of it like the company where you work (unless you're self-employed, in which case you should close your laptop and go surfing now!). Your company should have a marketing department, an accounting department, and an HR department. Whenever anything needs to be done, the appropriate department is responsible for making it happen. That's the gist of it.

Views are any part of your app that is visible on screen. On iOS, this is easy: UIViews, Xibs, and possibly some helper classes/categories that do display-related things.

Models is your data and core logic of your application. You might consider this your middleware. Managers, Entities, and Service classes fall under this banner.

Controllers are anything that bridges the gap between Views and Models. These are your UIViewControllers, and occasionally some helper classes.

Note: Of course, sometimes you'll have to bend these guidelines to get things done. Don't be a hardliner!

Fetching Data

A common scenario is that you'll want to fetch data from the server and display it on screen. In our experience, this is a good way of achieving this:


I'll explain in detail:

  • A manager class is a singleton. It has an array of your entity objects. On init of this singleton object, it loads the objects from the data store.
  • On app startup, the view controller displays the current list of objects.
  • When appropriate, the view controller calls a method on the manager singleton object, signalling it would like fresh data to be fetched.
  • The manager sets a 'isLoading' property to YES.
  • The view controller uses KVO to observe the 'isLoading' is now YES, and display the loading activity indicator.
  • The manager calls a static method on the Services class, passing in a completion block. The last argument of the completion block is a 'success' BOOL.
  • The services class calls AFNetworking to make the appropriate remote call, parsing the JSON results into a strongly-typed array of entity objects, calling the completion block.
  • Your entity objects are simple NSObject subclasses with a set of properties, and little else.
  • The manager is responsible for displaying the alert to the user if the service call failed. This prevents multiple view controllers displaying multiple error messages.
  • If it succeeded, the manager stores the newly returned array of entity objects in a property.
  • Either way, the manager sets its isLoading property to NO.
  • The view controller uses KVO to observe that the objects have changed, and refreshes itself to reflect the new data.
  • The view controller also uses KVO to observe the isLoading, and stops/hides the loading indicator now that it is NO.
  • A manager singleton persists its objects to disk at some appropriate time (eg just after new data has arrived).

This works well when you may have multiple view controllers who are interested in a common set of data from the same manager. This method will elegantly prevent the multiple view controllers from kicking off multiple loads from the server. This is very important for iPad apps where you'll likely have multiple view controllers contained on the one screen, eg a UISplitViewController setup.

You may consider simplifying this workflow if eg you will only ever have one view controller querying the manager, common in simpler iPhone apps. In cases like this, you could consider using callbacks instead of KVO.


One issue with the aforementioned architecture is that KVO appears to be unidiomatic in Swift. You need to inherit NSObject, sprinkle @objc around the place, and other tricky things to make it work. It appears likely that a future version of Swift will bring us a successor to KVO, but in the meantime here's what I recommend you consider:

Lets borrow C#'s concept of Events and combine it with Swift's weak delegates to get a simple effective solution. Basically we're trying to create a manager which can have multiple weakly-referenced delegates. Here's how it would look:

/// Since there can be multiple delegates, they can only
/// be used to signifying events (output), not for
/// supplying data for the manager (input).
@objc protocol MyManagerDelegate {
    func manager(manager: MyManager, isNowLoading: Bool)

@objc class MyManager {
    /// Multiple delegates.
    let delegates = NSHashTable.weakObjectsHashTable()

    // Add a delegate.
    func addDelegate(delegate: MyManagerDelegate) {

    /// Example of how you'd call the multiple delegates.
    private func tellDelegatesIsNowLoading(loading: Bool) {
        for object in delegates.allObjects {
            let delegate = object as MyManagerDelegate
            delegate.manager(self, isNowLoading: loading)

I'll leave it up to your discretion whether you think the above is worse or better than KVO in Swift. I'll concede that it is indeed debatable ;)

Update: Better alternatives may be possible.

Data Stores

As for saving your data to 'disk', you've got a lot of options. From my experience, the good options are as follows: (in order of simple to complicated)

  • NSCoding + plists
  • YapDatabase (simple object store)
  • Sqlite + FMDB
  • Realm (CoreData done right, still can't pass objects between threads)
  • CoreData

Only you can really decide which of the above are right for your app, as it depends on many factors.

I'd recommend starting with NSCoding for simple apps, moving to YapDatabase if you need further functionality, and Realm if you've got a big enterprisey app with a team full of smart people who can handle the complication (eg threading implications) of a proper ORM.

Kudos belongs to Tom from BareFeetWare for the discussion which led to this post!

You can see older posts in the right panel, under 'archive'.


iOS8 View Controller transitioning bug 17 Apr 2015

IB Designable 18 Mar 2015

iOS App Architecture 2 Mar 2015

Video Course Launch 14 Feb 2015

Video Course Pre-launch 8 Feb 2015

Blogging Platforms 13 Jan 2015

Mobile in 2014 - Year in Review 11 Jan 2015

Secret Keys talk 16 Nov 2014

Dimmi 11 Nov 2014

Project setup in Xcode6 22 Oct 2014

Uploading to an S3 bucket from iOS 15 Oct 2014

iOS8 App Testing Roundup 28 Sep 2014

Storing obfuscated secret keys in your iOS app 16 Sep 2014

Getting Core Location / CLLocationManager to work on iOS8 14 Sep 2014

Accessing the response body in failure blocks with AFNetworking 2 10 Sep 2014

How to allow your UITextFields to scroll out of the way of the keyboard 8 Sep 2014

How to subclass UIButton in iOS7 and make a UIButtonTypeSystem 4 Sep 2014

New season 1 Aug 2014

House finished 17 Jun 2014

WebP decoding on iOS 9 Feb 2014

Moving on again 22 Jan 2014

Lossy images for retina iPads - JPEG vs WebP 30 Nov 2013

Career options I wish I knew about when I was younger 20 Oct 2013

Positivity and your friends 7 Oct 2013

Tactility 26 Jul 2013

WWDC-induced narcolepsy 15 Jul 2013

Back on rails 31 May 2013

Full circle 6 May 2013

Programmatic UI on iOS 3 May 2013

Screencasts and positivity 8 Apr 2013

Year of positivity 14 Mar 2013

iOS Dev State of the Union 6 Feb 2013

Adventures with IAPs 3 Feb 2013

No longer a Googler 23 Dec 2012

Localising iPhone apps with Microsoft Translator 8 Dec 2012

Fight back (app biz update 13) 12 Nov 2012

Sent to the backburner (app biz update 12) 25 Oct 2012

Lisi Schappi 7 Oct 2012

Today's happy plateau (app biz update 11) 26 Aug 2012

First week's sales of Today (app biz update 10) 19 Aug 2012

Today launch! And a difficult decision made... (app biz update 9) 15 Aug 2012

Approved! (app biz update 8) 5 Aug 2012

Creating a graph in Objective-C on the iPhone 3 Aug 2012

Hurry up and wait (app biz update 7) 30 Jul 2012

Today app marketing site 27 Jul 2012

Today app submitted 25 Jul 2012

UIAlertView input wrapper 24 Jul 2012

Mentoring 23 Jul 2012

This is too hard! (app biz update 6) 20 Jul 2012

Perspectives (app biz update 5) 9 Jul 2012

4th starting-my-own-biz update 1 Jul 2012

ScrumFox landing page 28 Jun 2012

Server Scope landing page 27 Jun 2012

Telstra Calls and Data Usage 26 Jun 2012

Service History + Dropbox 26 Jun 2012

Impromptu Presenter 26 Jun 2012

Fertility Tracker 26 Jun 2012

Baby Allergy Tracker 26 Jun 2012

Starting my own business, update 3 22 Jun 2012

Starting my own business, update 2 17 Jun 2012

Starting my own business - First update 10 Jun 2012

I must be crazy 6 Jun 2012

Finding your location on an iPhone 7 May 2012

A generous career 4 May 2012

Skeleton Key Cocoaheads presentation 3 May 2012

CHBgDropboxSync - Dropbox auto-sync for your iOS apps 1 May 2012

That book about that Steve Jobs guy 30 Apr 2012

Another app marketing idea 23 Apr 2012

Sweet grouped tables on the iPhone 17 Apr 2012

Skeleton Key App 11 Apr 2012

Another app marketing idea... 5 Apr 2012

Quickly check for any missing retina graphics in your project 3 Apr 2012

Skeleton Key Password Manager with Dropbox 2 Apr 2012

RC Boat motor finally mounted 2 Apr 2012

Promoting apps presentation slides 1 Apr 2012

How i just wasted a month on my latest app, and how you don't need to 26 Mar 2012

The Finishing Line 20 Mar 2012

Using Launchd to run a script every 5 mins on a Mac 20 Feb 2012

Generating AES256 keys from a password/passphrase in ObjC 20 Feb 2012

Indie iPhone app marketing, part 2 19 Feb 2012

My App Manifesto: Syncing + Dropbox + YAML = Awesome 15 Feb 2012

Indie iPhone App Marketing part 1 7 Feb 2012

Perspectives 2 Feb 2012

Accountability and Free Will 1 Feb 2012

Badassery 31 Jan 2012

Sacrifice 30 Jan 2012

Lead Yourself First 29 Jan 2012

How to ping a server in Objective-C / iPhone 26 Jan 2012

iOS Automated Builds with Xcode4 16 Jan 2012

Xcode 4 - Command line builds of iPhone apps 15 Jan 2012

Guest post by Jason McDougall 13 Jan 2012

Scouts, Games and Motivation 10 Jan 2012

2011 Re-cap 8 Jan 2012

Ruby script to increment a build number 4 Jan 2012

Turning 30? All ideas, no execution? 18 Dec 2011

CHDropboxSync - simply sync your iOS app's documents to Dropbox 14 Dec 2011

Deep-enumerating a directory on the iphone, getting file attributes as you go 10 Dec 2011

Getting a date without the time component in objective-c 6 Dec 2011

Memory management in Objective-C 4 Dec 2011

Starting small 29 Nov 2011

Dictionary Types Helper 29 Nov 2011

Observer Pattern in Objective-C 16 Nov 2011

Why you should give presentations 13 Nov 2011

How to get a programming or design job in Sydney 9 Nov 2011

Custom nav bar / toolbar backgrounds in iOS5 8 Nov 2011

Stuck 27 Oct 2011

Dead easy singletons in Obj-C 19 Oct 2011

JSON vs OCON (Objective-C Object Notation) 18 Oct 2011

In defence of Objective-C 16 Oct 2011

Update the MessagePack objective-c library to support packing 12 Oct 2011

Icons 11 Oct 2011

How to host a site on Amazon AWS S3, step-by-step 7 Oct 2011

Drawing a textured pattern over the default UINavigationBar 6 Oct 2011

Markdown Presentations 1 Oct 2011

More MegaComet testing: Ruling out keepalives 15 Sep 2011

MegaComet test #4 - This time with more kernel 14 Sep 2011

Building People 10 Sep 2011

Half way there: Getting MegaComet to 523,000 concurrent HTTP connections 5 Sep 2011

Making a progress bar in your iPhone UINavigationBar 22 Aug 2011

Hacker News Reader 20 Aug 2011

How to programmatically resize elements for landscape vs portrait in your iphone interface 16 Aug 2011

MegaComet testing part 2 3 Aug 2011

Australian Baby Colours 28 Jul 2011

Boat prop shaft 25 Jul 2011

Megacomet with 1 million queued messages 24 Jul 2011

Installed the strut and rudder 18 Jul 2011

Painted the inside of the boat 17 Jul 2011

Fuzzy iphone graphics when using an UIImageView set to UIViewContentModeCenter 13 Jul 2011

My 3 Data and Calls Usage 11 Jul 2011

Reading a line from the console in node.js 10 Jul 2011

Trim whitespaces on all text fields in a view controller 9 Jul 2011

Final finish 9 Jul 2011

MessagePack parser for Objective-C / iPhone 30 Jun 2011

Lacquering the starboard side 25 Jun 2011

What do do with EXC_ARM_DA_ALIGN on an iPhone app 23 Jun 2011

Lacquering the hull 23 Jun 2011

Staining the boat 22 Jun 2011

NSMutableSet with weak references in objective-c 20 Jun 2011

Iphone gesture recogniser that works for baby games 20 Jun 2011

Image manipulation pixel by pixel in objective C for the iphone 19 Jun 2011

Baby Allergy Tracker 12 Jun 2011

Power sanding the deck 10 Jun 2011

Planing the edge of the deck 2 Jun 2011

Figured out the deck 2 Jun 2011

Boat bulkheads 2 Jun 2011

Simulating iOS memory warnings 31 May 2011

Putting a UIButton in a UIToolbar 29 May 2011

How to allow closing a UIActionSheet by tapping outside it 29 May 2011

Finding the currently visible view in a UITabBarController 24 May 2011

Random Chef 17 May 2011

Centered UIButton in a navigation bar on the iphone 16 May 2011

Little Orchard 13 May 2011

Boat update 13 May 2011

How to get the current time in all time zones for the iphone / obj-c 12 May 2011

Design portfolio 10 May 2011

Tricks with grand central dispatch, such as objective-c's equivalent to setTimeout 9 May 2011

How to make an iphone view controller detect left or right swipes 5 May 2011

Centered section headers on a UITableView 5 May 2011

Christmas in may 4 May 2011

Finished trimming the boat (its floatable now!) and got some parts 29 Apr 2011

How to make a multiline label with dynamic text on the iphone and get the correct height 27 Apr 2011

Forcing an image size on the image in a table view cell on an iphone 20 Apr 2011

Git on the Mac 19 Apr 2011

Build a url query string in obj-c from a dictionary of params like jquery does 12 Apr 2011

Rendering a radial gradient on the iphone / objective-c 11 Apr 2011

Skinning the port side of the boat 8 Apr 2011

Skinning the side of the boat 5 Apr 2011

Sending a UDP broadcast packet in C / Objective-C 5 Apr 2011

How to talk to a unix socket / named pipe with python 4 Apr 2011

Skinning the bottom of the boat 31 Mar 2011

Service discovery using node.js and ssdp / universal plug n play 30 Mar 2011

Extremely simple python threading 29 Mar 2011

New rescue boat 26 Mar 2011

HttpContext vs HttpContextBase vs HttpContextWrapper 5 Nov 2010

Simple C# Wiki engine 30 Sep 2010

Simple way to throttle parts of your Asp.Net web app 29 Sep 2010

How to implement DES and Triple DES from scratch 4 Aug 2010

How to use sessions with Struts 2 30 Jul 2010

How to use Cookies in Struts 2 with ServletRequest and ServletResponse 30 Jul 2010

Using Quartz Scheduler in a Java web app (servlet) 27 Jul 2010

Javascript date picker that Doesn't Suck!(tm) 27 Jul 2010

Using Oracle XE with Hibernate 20 Jul 2010

A simple implementation of AES in Ruby from scratch 29 Jun 2010

Asp.Net Forms authentication to your own database 28 May 2010

AS2805 (like ISO8583) financial message parser in C# 7 May 2010

Ruby hex dumper 4 May 2010

Using Spring to manage Hibernate sessions in Struts2 (and other web frameworks) 13 Jan 2010

Emails in C#: Delivery and Read receipts / Attachments 12 Jan 2010

Using Java libraries in a C# app with IKVM 16 Dec 2009

Learning Java tutorial 27 Nov 2009

Using generic database providers with C# 17 Nov 2009

Scheduled task executable batch babysitter 29 Oct 2009

Working with query strings in Javascript using Prototype 30 Sep 2009

Still fighting with String.Format? 9 Sep 2009

How I'd build the next Google 24 Aug 2009

Getting IIS and Tomcat to play nicely with isapi_redirect 24 Aug 2009

Using the new ODP.Net to access Oracle from C# with simple deployment 11 Aug 2009

C# Cryptography - Encrypting a bunch of bytes 14 Jul 2009

Sorting enormous files using a C# external merge sort 10 Jul 2009

Reconciling/comparing huge data sets with C# 9 Jul 2009

Some keyboard-friendly DHTML tricks 10 Jun 2009

How to figure out what/who is connected to your SQL server 18 Mar 2009

Adding a column to a massive Sql server table 16 Mar 2009

Multithreading using Delegates in C# 10 Mar 2009

Using C# locks and threads to rip through a to-do list 6 Feb 2009

Using threads and lock in C# 3 Feb 2009

Setting a form name and onsubmit using form_for in rails 2 14 Jan 2009

Compressing using the 7Zip LZMA algorithm in C# beats GZipStream 14 Jan 2009

Bare minimum HTML that validates 22 Dec 2008

MS Sql Server 2005 locking 17 Dec 2008

Simple Comet demo for Ruby on Rails 19 Nov 2008

Geocoding part 2 - Plotting postcodes onto a map of Australia with C# 24 Oct 2008

Using evolutionary algorithms to make a walkthrough for the light-bot game with C# 20 Oct 2008

How to tell when memory leaks are about to kill your Asp.Net application 16 Oct 2008

C# version of isxdigit - is a character a hex digit? 15 Sep 2008

Geocoding part 1 - Getting the longitude and latitude of all australian postcodes from google maps 26 Aug 2008

Converting HSV to RGB colour using C# 14 Aug 2008

Opening a TCP connection in C# with a custom timeout 11 Aug 2008

Oracle Explorer - a very simple C# open source Toad alternative 31 Jul 2008

Linking DigitalMars' D with a C library (Mongrel's HTTP parser) 23 Jun 2008

Connecting to Oracle from C# / Winforms / without tnsnames.ora 16 Jun 2008

A simple server: DigitalMars' D + Libev 6 Jun 2008

Travelling from Rails 1 to Rails 2 9 Apr 2008

Rocketsale 9 Apr 2008

Online Rostering System 9 Apr 2008

DanceInforma 9 Apr 2008

Client Analyser 9 Apr 2008

How good is this??? 16 Dec 2007

Using RSS or Atom to keep an eye on your company's heartbeat 10 Nov 2007

Easy Integrated Active Directory Security in ASP.Net 24 Oct 2007

First post 17 Oct 2007