RxSwift DelegateProxy with required methods

Once you’ve sipped from the Reactive Kool-Aid it can become emotionally painful to have to implement callback methods the old-fashioned way when you hit a delegate-style API that doesn’t have a prebuilt Observable wrapper.

Max Alexander did a great tutorial post on how to implement your own DelegateProxy for such an API, but unfortunately it won’t work for delegates that that have required methods. A common mistake is to implement the required delegate method in your DelegateProxy class, forward the  call using self._forwardToDelegate, and then call rx_delegate.observe() as before. This will result in the fairly misleading message:

Delegate proxy is already implementing `xxx:`, a more performant way of registering might exist.

The right way to implement a DelegateProxy for a delegate protocol with required methods involves doing the following (using the Google Identity Toolkit here as an example):

  1. First implement the DelegateProxy as per Max’s instructions, i.e.:
    class RxGIDSignInDelegateProxy: DelegateProxy, GIDSignInDelegate, DelegateProxyType {
    
        static func currentDelegateFor(object: AnyObject) -> AnyObject? {
            let signin = object as! GIDSignIn
            return signin.delegate
        }
    
        static func setCurrentDelegate(delegate: AnyObject?, toObject object: AnyObject) {
            let signin = object as! GIDSignIn
            signin.delegate = delegate as? GIDSignInDelegate
        }
    }
    
  2. Next declare a 1ocal PublishSubject property in your proxy, and publish Next & Error events to the subject within the delegate method. You can see we’re also forwarding to the traditional delegate (if one exists), and for completeness’ sake, sending Completed on deinit.
        let signInSubject = PublishSubject<GIDGoogleUser>()
    
        func signIn(signIn: GIDSignIn!, didSignInForUser user: GIDGoogleUser!, withError error: NSError!) {
            if let u = user {
                signInSubject.on(.Next(u))
            } else if let e = error {
                signInSubject.on(.Error(e))
            }
            self._forwardToDelegate?.signIn(signIn, didSignInForUser: user, withError: error)
        }
    
        deinit {
            signInSubject.on(.Completed)
        }
    
  3. At this point, we can implement our rx_* extension method on the relevant class. PublishSubject is a subclass of Observable, so we can just return this value directly.
    extension GIDSignIn {
        public var rx_delegate: DelegateProxy {
            return proxyForObject(RxGIDSignInDelegateProxy.self, self)
        }
    
        public var rx_userDidSignIn: Observable<GIDGoogleUser> {
            let proxy = proxyForObject(RxGIDSignInDelegateProxy.self, self)
            return proxy.signInSubject
        }
        
        // implement your optional delegate methods with `rx_delegate.observe`
    }
    

That’s it! Observe away without sullying your code with delegate callbacks.

Swift ErrorTypes – A Modest Proposal

After the WWDC announcement, when I got over my initial knee-jerk ‘Exceptions – ick!’ reaction, I came around to being a supporter of the Swift 2 error handling model. While it would be nice to be able to desugar the implicit result type, and there’s a glaringly obvious hole with async error handling, for general failable synchronous calls it works well. However, there’s one aspect that really bothers me. Consider the following code:

enum MathError: ErrorType {
    case DivideByZeroError
}

func divide(l: Int, _ r: Int) throws -> Int {
    guard r != 0 else { throw MathError.DivideByZeroError }
    return l / r
}

func calculate() {
    do {
        let result = try divide(10, 2)
        print("Result = \(result)")
    } catch MathError.DivideByZeroError {
        print("Can’t divide by zero!")
    } catch {
        // this should never happen
        break
    }
}

Errors thrown by a function are completely untyped, so Swift is unable to infer that the first catch clause is exhaustive – this means you ALWAYS have to include a catch-all at the end of your do block if you want to completely handle the error. This isn’t just a case of redundant code – if you add an error case later on, you’re going to suddenly (and silently) hit your ‘This should never happen’ handler, which is probably not what you’d expect (and seems out of step with the way switch statements work).

The other problem is that we have no idea from the function signature what errors we should be trying to catch. Apple introduced a Throws: keyword in the doc comments, but relying on the original developer to keep comments up to date can be… ineffective.

A number of people have proposed that throwing functions should be declared with a list of all the types they can throw, rather like Java’s checked exceptions. This will nicely solve our divide function example:

func divide(l: Int, _ r: Int) throws MathError -> Int {
    guard r != 0 else { throw MathError.DivideByZeroError }
    return l / r
}

func calculate() {
    do {
        let result = try divide(10, 2)
        print("Result = \(result)")
    } catch MathError.DivideByZeroError { // this catch is exhaustive
        print("Can’t divide by zero!")
    }
}

Which is great – we now have a sensible exhaustive catch statement, we’ll get helpful compile errors if we add a MathError case and forget to handle it, and the function definition is clearer and self-documenting. However, this is a fairly simple case, and the problems with checked exceptions were always much more evident with larger & more modular codebases – for instance, we’d start to see functions like:

func openDatabase(path: String) throws libuvError, URLFormatError, ZipArchiveError, SQLiteError -> Database {

Technically in this case we’d still be able to write an exhaustive catch without using a catch-all, but I can give a rock-solid guarantee that no-one ever will. A function signature like this leaks internal implementation details, and the exposed error types are very unlikely to be useful to API consumers.

Additionally, this can have an invasive impact on the rest of the code – consider the scenario where these error types are annotated on all functions that call openDatabase, then all the functions that call those functions, and so-on up the line until we have a monster catch (all) block. Now imagine we update our database code and add a new ErrorType, or change the zip library and that ErrorType is now different. We could end up having to touch a dozen files to cover a semantically unimportant change. It’s also worth noting that the Swift team have explicitly said they don’t want “pedantic lists of possible error types, like Java”.

Now this is a somewhat pathological case (although I have seen a lot of Java code written like this in the past!) and hopefully ‘good developers’ wouldn’t write code this way. The way this problem should be solved would be either:

  1. Erase the typed error information that’s not useful to consumers and just throw an untyped ErrorType – this brings us back full-circle – or
  2. Translate the internal errors into a meaningful new ErrorType, such as:
enum DatabaseError: ErrorType {
    case InvalidPath(message: String)
    case CorruptDatabase(message: String)
}

Ultimately, the more I’ve thought about this, the less useful a list of multiple ErrorTypes seems to be. However, I think there’s still value in being able to (optionally) annotate a single custom ErrorType, as in the case of the MathError or a consolidated DatabaseError from our openDatabase call. So my Modest Proposal is stated thus:

// allow us to do this:
func myFunction() throws -> Int

// or this:
func myFunction() throws CustomError -> Int

// but not this:
func myFunction() throws CustomErrorOne, CustomErrorTwo -> Int

A single custom ErrorType is much closer to the underlying language model, neatly complements the Swift 2.1 function covariance, delivers most of the benefits of typed errors, and limits the damage that slovenly error propagation can do to the codebase.

Using Xcode Playgrounds for Presentations

Playground Presentation

Playgrounds are primarily designed as a learning tool, but with Xcode 7’s multiple pages and expanded rich markup support, they’re also really great for presentations – they allow you to mix slide-like text & image content with interactive code (for coding demos). Because everyone loves coding demos!

I used this technique last week at /dev/world/2015, and thought I’d share a few tips if you’re interested in doing your own playground presentations.

  1. Don’t try to put too much content on the one page; otherwise you’ll spend half the presentation scrolling up & down.
  2. Add a key binding to the ‘Show Rendered Markup’ menu option – you’ll be using it a lot!
  3. Use the image markup (//: ![[Alt Text]](image_name.jpg)) to show images from your ‘Resources’ folder – e.g. diagrams you’d normally put on a slide.
  4. Don’t use a dark text theme if you’ll be presenting on a projector – a light theme will be more readable. Also, don’t forget to change the Console font size in your presentation theme!
  5. Use custom Xcode snippets rather than typing in code – it’s much quicker & less error-prone, but your audience will still feel like the code is coming together in front of them. Another option is to pre-fill most of the code and just add the interesting bits live.
  6. If you’re using a Swift file in ‘Sources’ to hide supporting code, note that types must be public to be visible to the playground.
  7. Swift code in ‘Sources’ can’t link to custom frameworks, so you’ll need to put your supporting code in a separate framework if you want to do this. ‘Sources’ is best used for basic helper functions.
  8. Implementing CustomPlaygroundQuickLookable on your types can add a lot of fun to your presentation if it’s something that lends itself to a visual.

Lastly, be prepared for the playground to crash (because it will!)

My /dev/world talk is available online if you’d like to see the playground presentation in action.

AWS Powershell: “A parameter cannot be found that matches parameter name ‘Credentials’.”

I spent ages debugging this one a few months ago, and just hit it again, so I thought I’d share to save others some time.

If you have an older AWS powershell script, you may hit this error when running AWS Powershell cmdlets, particularly if using a cross-account role – e.g.:

$aws_role = Use-STSRole -RoleArn $arn -ExternalId $externalid -Region $region
$aws_creds = $aws_role.Credentials

Get-S3Bucket -Credentials $aws_creds -BucketName $bucket -Region $region
# will throw "A parameter cannot be found that matches parameter name 'Credentials'."

The problem is that at some point, the AWS Powershell cmdlets renamed the ‘Credentials’ parameter to ‘Credential’ (no trailing s). Running the script after upgrading AWS Powershell manifests the error. To compound this, I assume because of the way they’ve implemented the shared parameters, Get-Help doesn’t actually show the Credential parameter at all. I trawled through the release notes and was unable to find the version at which the parameter changed, or even if there was any warning.

The fix is obviously to rename your -Credentials parameters to -Credential, and then shake your fist in the general direction of Amazon.

Watch App Development Blog – Week 4 *cough* 5

 I’m blogging my progress in developing an Apple Watch App. Read the previous instalments here.

So, um, I missed a week. Sorry about that. Sad face.

Step 4: The Pebble

We don’t have access to any Apple Watch hardware yet, so it’s difficult to get a good feel for how you will use your app in context. I was given a Pebble for Christmas though, and thought it may be worthwhile getting the core information displayed on a Pebble app so I can experience and analyse the usage flow behind a watch app with this data.

Pebble: the Palm Pilot of Smartwatches

First, my impressions of the pebble:

  • Battery life is not too bad, I get about a week of normal usage. It doesn’t compare favourably to several years of battery life on a traditional watch (like my trusty Tag 2000), but it should soundly spank most newer-generation smart watches, including the Apple Watch, based on the rumours to date.
  • The screen is awful if you’re used to a high-resolution smartphone display. The B&W 144 x 168 display looks like 90s tech.
  • The Watch itself is plasticky (lasted a whole day before it copped a small but visible scratch on the face), and too large and ungainly for most wrists.
  • The app/watchface marketplace is fairly limited & most of the apps have a ‘hobbyist’ feel to them. I don’t necessarily mean that in a negative manner – it’s great to see the enthusiasm, but without a good mechanism to monetise apps there’s not the same level of investment & innovation that I see in the iOS App Store or the Play Store.
  • The hardware is fairly slow and anaemic.
  • Development for the pebble is difficult. The Pebble C API is very low level, requires a lot of careful memory management, can only run on the device, and can’t be debugged. They’ve released a JavaScript API to try to make the experience a bit better, though I haven’t tried it.
  • If you’re using your pebble with an iPhone, the experience is less than seamless due to Apple’s bluetooth accessory restrictions. Some functionality  (e.g. network access) stops working if the Pebble iOS app has been terminated from the background. Third party iOS apps have a single Pebble connection to share, and communication can only be initiated from the phone.

To me, the product is very reminiscent of the early Palm Pilots – clunky B&W screen, an awkward developer experience, small hobbyist developer community etc. The future’s yet to be written, but the Pebble will need to undergo radical and ruthless improvement to keep pace with the latest smartwatches.

The Pebble App

The general concept behind the watch app is a simple display showing the departure times for the closest station. This should provide the ‘in context’ component of the most important watch app functionality. The initial UI design (pictured) includes the nearest station, and the destination, pattern, and minutes remaining for the next four departures from the station.

Pebble Mockup

The simplest way to manage communication between the phone app & the watch app is the AppSync API. The general semantics of the API involve syncing a dictionary of shared data between the phone & watch; it also makes data storage on the watch more convenient. The downside is that this requires a specific key for each individual data element synced to the watch – i.e. specific numbered departures rather than a variable array of scheduled trains.

With that in mind, the keys were defined thus:

#define KEY_STATION 0
#define KEY_DEST_1 1
#define KEY_TIME_1 2
#define KEY_DEST_2 3
#define KEY_TIME_2 4
#define KEY_DEST_3 5
#define KEY_TIME_3 6
#define KEY_DEST_4 7
#define KEY_TIME_4 8

The main issue I ran into with AppSync was a storage limitation. The sample code includes a 30 byte sync buffer which is insufficient for most data sync requirements, but it wasn’t immediately obvious that’s what the error DICT_NOT_ENOUGH_STORAGE was referring to. Upping the buffer to 128 bytes solved the issue. That should be enough for anybody.

Once the data was synced, I update the UI using the following function:

static void drawText() {
  const Tuple *tuple;
  if ((tuple = app_sync_get(&s_sync, KEY_STATION))) {
    if (tuple->value->cstring[0] == 0) {
      text_layer_set_text(station_text_layer, "Waiting for data");
    } else {
      text_layer_set_text(station_text_layer, tuple->value->cstring);
    }
  }
  for (int i = 0; i < 4; i++) {
    if ((tuple = app_sync_get(&s_sync, i * 2 + 1))) {
      if (tuple->value->cstring[0] == 0) {
        text_layer_set_text(dest_text_layers[i], "");
      } else {
        text_layer_set_text(dest_text_layers[i], tuple->value->cstring);
      }
    }
    if ((tuple = app_sync_get(&s_sync, i * 2 + 2))) {
      if (!tuple->value->int32) {
        text_layer_set_text(time_text_layers[i], "");
      } else {
        time_t departure_time = tuple->value->int32;
        time_t current_time = time(NULL);
        int minutes = (departure_time - current_time)/60;
        char time_str[5];
        snprintf(time_str, 5, "%dm", minutes);
        text_layer_set_text(time_text_layers[i], time_str);
      }
    }
  }
}

…where dest_text_layers and time_text_layers are four element arrays containing references to the text layers on the watch UI.

Can you spot the bug? If you haven’t done much work with embedded systems it’s not obvious. Critically, the documentation for text_layer_set_text says:

The string is not copied, so its buffer most likely cannot be stack allocated, but is recommended to be a buffer that is long-lived, at least as long as the TextLayer is part of a visible Layer hierarchy.

time_str is not copied when passed to text_layer_set_text; the effect being that it goes out of scope and is never displayed on the watch face. The solution is a set of string buffers referenced statically – I used a static char pointer array, and malloced/freed the buffers in window_load/window_unload.

// at the top of the file
#define TIME_LABEL_LENGTH 5
static char *time_strings[4];

// in the window_load() function
  for (int i = 0; i < 4; i++) {
    time_strings[i] = malloc(sizeof(char[TIME_LABEL_LENGTH]));
  }

// drawtext() changes to:
  snprintf(time_strings[i], TIME_LABEL_LENGTH, "%dm", minutes);
  text_layer_set_text(time_text_layers[i], time_strings[i]);

The iOS App

Pebble integration doesn’t require a a substantial amount of code – drag in the frameworks and pull a PBWatch reference from PBPebbleCentral.defaultCentral().lastConnectedWatch(). Because I want to be able to show the number of minutes until a train leaves, I changed the earlier code from a HH:MM string to a ZonedDate/NSDate in the Haskell & Swift code. I then implemented Pebble communication using the following (dest/pattern is abbreviated to economise on transfer bandwidth & Pebble display size):

    func updatePebble(station: Station, _ times : [Departure]) {
        var pebbleUpdate : [NSNumber: AnyObject] = [
            KeyStation : station.name,
        ]
        for i: Int in 0..<4 {
            let destKey = NSNumber(int: Int32(i * 2 + 1))
            let timeKey = NSNumber(int: Int32(i * 2 + 2))
            pebbleUpdate[destKey] = times[i].shortDescription
            pebbleUpdate[timeKey] = NSNumber(int32: Int32(times[i].time.timeIntervalSince1970))
        }
        self.watch?.appMessagesPushUpdate(pebbleUpdate, withUUID: appUUID, onSent: { (w, Dict, e) in
            if let error = e {
                println("Error sending update to pebble: \(error.localizedDescription)")
            } else {
                println("Sent update to pebble!")
            }
        })
    }

There was a problem though: the Pebble showed around -470 minutes for each train (i.e. 8 hours out – suspicious, as local time is +8:00). Turns out Pebble has no concept of timezone at all. The docs spin this as: “Note that the epoch is adjusted for Timezones and Daylight Savings.” Not sure that qualifies as epoch time, but it was clear the conversion is meant to happen on the phone. The following code sorted the issue:

  let adjustedEpoch = Int(times[i].time.timeIntervalSince1970) + NSTimeZone.localTimeZone().secondsFromGMT
  pebbleUpdate[timeKey] = NSNumber(int32: Int32(adjustedEpoch))

Glorious 1 bit UI

The result (I have a promising future as a watch model). I’ll give it a good workout near the train station over the next week.

Pebble Running

As always, the code is available here, here and here.

A last note: The fact that I’m on week 5 of my watch app development journey and am yet to touch WatchKit is not lost on me. I should hopefully start hitting WatchKit code this week. With a bit of luck.

 

The operation couldn’t be completed. (SSErrorDomain error 100.)

If you’re trying to test iOS App Store receipt validation, and you perform a receipt refresh using SKReceiptRefreshRequest, you are almost certainly going to come across the mysterious and enigmatic SSErrorDomain Error 100. There’s not a lot of information on the googles, so here’s what I know/suspect.

As far as I can tell, code 100 is the App Store’s way of telling you “Sorry, I have no receipt for that bundle ID for that user”. That’s unlikely to happen in production unless shenanigans are underway (a receipt is generated even for a free app ‘Get’), but it can happen often in development. The sandbox App Store appears to have the ability to generate fake receipts when requested, but all ducks need to be in a row for this to happen.

In the sandbox (Development/Ad Hoc builds):

  • If you don’t have an app record set up in iTunes Connect, you’ll get a Code 100
  • If you’re signed in with your regular Apple ID instead of a sandbox account: Code 100
  • If you’re signed in with a sandbox account associated with a different iTunes Connect account: Code 100

The story is a bit different for Apple Testflight builds – these are production builds with special handling for in-app purchases, and the App Store (currently) does NOT generate a fake original purchase receipt. I haven’t tested this myself, but from a developer report on the dev forums (login required):

  • If you have a virgin install from TestFlight, you’ll get a Code 100
  • If you’ve previously installed the App Store version of the app, you’ll get a receipt
  • If you have a virgin install from TestFlight but have made an in-app purchase, you’ll get a receipt

Hopefully this saves others some frustration.

Watch App Development Blog – Week 3

In weeks 1 & 2, I got a Transperth-scraping REST API built and deployed to an AWS-based cloud host. This week I’ll get started on:

Step 3: The iPhone App

Third party apps on Apple Watch are very limited and rely heavily on the companion iPhone app for logic, network access, location etc, so the first place to start with the Watch app is on the iPhone.

The phone app itself will need useful functionality otherwise it’s unlikely to be approved. I have a few ideas for cool features for the phone, but for the moment it can just display the live times for the nearest station. This will require:

  1. Getting the list of stations from the server
  2. Finding the nearest station based on the user’s current location
  3. Getting the live times for that station from the server.

Let’s get started. I’m going to be building the app in Swift (of course). Mattt Thompson of NSHipster/AFNetworking fame has written a Swift-only networking framework called Alamofire, so we’ll start with it.

After setting up the framework following the instructions, I created a new Swift file called ‘ApiClient’. Downloading JSON from the server using Alamofire looks like the following:

func getAllStations(f: [Station] -> ()) {
    Alamofire.request(.GET, "\(hostname)/train")
        .responseJSON { (_, _, json, _) in
           if let json = json as? [NSDictionary] {
                let s = json.map({ Station(dictionary: $0) })
                f(s)
            } else { f([]) }
    }
}

struct Station {
    let id : String
    let name : String
    let location: CLLocation

    init(dictionary: NSDictionary) {
        self.id = dictionary["id"] as String
        self.name = dictionary["name"] as String
        let lat = Double(dictionary["lat"] as NSNumber)
        let long = Double(dictionary["long"] as NSNumber)
        self.location = CLLocation(latitude: lat, longitude: long)
    }
}

I’m using a global function (to be more functional) with a callback parameter that takes an array of stations. The returned JSON array is mapped over to convert the NSDictionary instances into Station values. If anything goes wrong, an empty array is passed to the callback – this isn’t brilliant error handling and will probably change, but it’s clearer to show as-is for now.

The user’s location can then be retrieved from a CLLocationManager, and the nearest location calculated like so:

    func nearestStation(loc: CLLocation) -> Station? {
        return stations.filter({ $0.distanceFrom(loc) <= 1500 }).sorted({ $0.distanceFrom(loc) &< $1.distanceFrom(loc) }).first
    }

    // where distanceFrom is defined on Station as
    func distanceFrom(otherLocation: CLLocation) -> CLLocationDistance {
        return location.distanceFromLocation(otherLocation)
    }

This will filter out all stations greater than 1.5km away, and return the nearest of the remainder (or nil if there are no nearby stations).

From there, we can retrieve the live times from the server using the code:

func getLiveTrainTimes(station: String, f: [Departure] -> ()) {
    Alamofire.request(.GET, "\(hostname)/train/\(station)")
        .responseJSON { (_, _, json, _) in
            if let json = json as? [NSDictionary] {
                let s = json.map({ Departure(dictionary: $0) })
                f(s)
            } else { f([]) }
    }
}

Wait – this looks pretty much identical to getAllStations, just with a different URL and return type. Let’s refactor:

protocol JSONConvertible {
    init(dictionary: NSDictionary)
}

func get<T: JSONConvertible>(path: String, f: [T] ->()) {
    Alamofire.request(.GET, hostname + path)
        .responseJSON { (_, _, json, _) in
        if let json = json as? [NSDictionary] {
            f(json.map({ T(dictionary: $0) }))
        } else { f([]) }
    }
}

// we can then redefine the other request methods as:
func getAllStations(f: [Station] -> ()) {
    get("/train", f)
}

func getLiveTrainTimes(station: String, f: [Departure] -> ()) {
    get("/train/\(station)", f)
}

The UI for the app (which I won’t go through here) is just a regular UITableView with a row for each train at the nearest station. However, it’s worth considering how the app will operate at different phases of its lifecycle – I want the live times to be updated in the background (for reasons that will become apparent later).

While the app is in the foreground, the location updates will come through as normal – I have a 50m distance filter on as this is unlikely to change your nearest station. When entering the background, the app will switch to the ‘significant change’ location service – again, accuracy is not super important so the course-grained significant change should work fine.

For the live train times network requests, in the foreground these will be triggered by a 30s NSTimer, in the background using the background fetch API. I haven’t used background fetch before, but it seems like the right technology to use in the case – allow the OS to decide if the app should refresh its data, based on battery life, network connectivity, and usage patterns of the app.

The various services are switched on & off like so:

    func applicationDidBecomeActive(application: UIApplication) {
        locationManager.startUpdatingLocation()
        timer = NSTimer.scheduledTimerWithTimeInterval(NSTimeInterval(30), target: self, selector: "timerFired:", userInfo: nil, repeats: true)
        timer?.fire()
    }

    func applicationWillResignActive(application: UIApplication) {
        locationManager.stopUpdatingLocation()
        timer?.invalidate()
        timer = nil
    }

    func applicationDidEnterBackground(application: UIApplication) {
        locationManager.startMonitoringSignificantLocationChanges()
    }

    func applicationWillEnterForeground(application: UIApplication) {
        locationManager.stopMonitoringSignificantLocationChanges()
    }

This is enough to start road-testing the app functionality out on the phone, and maybe start formulating a few ideas around the best functionality to prioritise on the watch. Speaking of the watch, next week I’ll have a surprise along those lines. As before, the code is available on bitbucket.