Note: If you got here because you googled “WCErrorCodePayloadUnsupportedTypes” I made a page called How to fix WCErrorCodePayloadUnsupportedTypes Error when using sendMessage which explains it better.

I’m working on an app to help me stay on an intermittent fasting routine. I wrote about it a little in Icon-first Development.

Fast-o-Mat is an iPhone app, but I want an Apple Watch complication to give me quick access to when my fast begins or ends. To do that, I need to get data from the phone to the watch.

I had never done this before, and I didn’t have the first idea of how it is done in modern iOS/watchOS development.

Here was my process

1. Do a few google searches to find out the basics. I learn that this is called Watch Connectivity.
2. Try to make sure that this is the modern way of doing things, since Apple changes things a lot and watch development generally change a lot in 2019. It is.
3. Look for a tutorial. I pick this Hacking With Swift one because they are usually pretty good. (Here is Part II, the Watch app, if you need it)

Then, at this point, all I do is look for the import and the basic classes I need and see how far I get from just basic iOS knowledge.

This tutorial is good at facilitating that.

1. The code samples are easy to skim for
2. There isn’t much pre-amble before we start getting into it
3. It’s focussed on just the code we need for Watch Connectivity

So, this is very unlike my idea for vague tutorials, but I am not really a new learner.

There isn’t a new concept here for me to learn on my own—I understand the concept of asynchronous message sending. I just need to know what framework and classes to use for this specific task.

The issue is that this same tutorial is what a new learner would find as well.

I believe a they would get this all working by following the instructions step-by-step, but would they have learned it beyond that? Could they troubleshoot?

One thing that is not clear from the API or this tutorial is that Any doesn’t really mean Any in the message parameter to sendMessage

func sendMessage(_ message: [String : Any], replyHandler: (([String : Any]) -> Void)?, errorHandler: ((Error) -> Void)? = nil)

I decided to just use one of my types there. It’s a struct with two TimeInterval parameters.

The documentation says

A dictionary of property list values that you want to send. You define the contents of the dictionary that your counterpart supports. This parameter must not be nil.

And running it says:

errorHandler: NO with WCErrorCodePayloadUnsupportedTypes

And now I see that “property list” values are things that you can store in a plist (so, not my struct, just simple types or NSArray or NSDictionary). And yada yada yada, it’s a little more complicated when you want to do this for real.

This is all to say, sometimes you just want the code (like me) and sometimes you are trying to learn a new concept from first principles, and the same tutorial can’t deliver both (or should even try).