The arrival of the Apple Watch took its time, but in 2015 it was time for us to become familiar with a new kind of device, for which the user experience had once again made a complete rethink of the interface necessary. The screen was too small for conventional icons above conventional labels, too small for any kind of menus accessible from a status bar, and generally too small for anything approaching a simple migration of UI elements from the touchscreen of the iPhone. In terms of size, the closest we had was the iPod Nano, but even that 2.5 inch screen allowed a much more conventional interaction with the user than was the case with the Watch.

And once again, the Cupertino design gurus did not disappoint. The home screen, while being recognizably the offspring of the iPhone home Screen, had morphed into a rearrangeable matrix (or Carousel) of circular icons that resized as they approached the center of the screen, icons that for the first time had to say all they had to say without the use of a text label, or, it should be noted, without recourse to text within the icon itself —the calendar icon's weekday label really is at the outer edge of readability and should be viewed more as a very neat bit of decoration rather than an attempt to add meaningful textual information to the icon.

A look at the calendar icon on this home screen will, perhaps, make the point better than words:

In addition to the usual gestures of iOS touchscreens, the Watch's touchscreen gained what Apple dubbed the Force Touch, adding a layer of input over the familiar one without adding any extra visual elements, the use of which is accompanied by a satisfying tapping sensation produced by the Taptic Engine.

Ah yes, the Taptic Engine. Mobile phones have been buzzing away in our pockets and on our table tops for a long time now, with varying degrees of urgency (or sometimes irritation), and other companies' watch offerings have adopted that particular method of notification, as one would expect they would, though with little change in approach. But the Apple Watch—and this really isn't sales hyperbole—takes haptic feedback to a whole new level. The variety, subtlety, and outright aesthetic quality of the Taptic Engine, as Apple has called its tiny stroke of genius, literally has to be experienced to be understood.

So one begins to get an idea of how much we, as Apple Watch developers, have our work cut out for us in terms of rethinking how we will both present information to, and get information from our users, and how their experience in working (and playing) with our apps will be very different to what they, and we, are accustomed to.

The Apple Watch comes in two sizes, referred to by the height of the casing, 38mm and 42mm.

Onto this tiny canvas, Apple have managed to squeeze an impressively sharp and vibrantly colored, pressure sensitive touchscreen, with resolutions of 272 × 340 px on the smaller model, and 312 × 390 px on the larger, giving 290 pixels per inch (PPI)—also often referred to as dots per inch (DPI)—enough resolution to satisfy even the most fastidious graphics geek.

The screens of the least expensive model, the Sport model, are, according to Apple:

Hands up anyone who can tell us about aluminosilicate glass?

All other models are equipped with Sapphire screens; this sounds a little less technical but it is the second hardest transparent material on earth after diamonds. So don't store one in your pocket together with any diamond jewelry.

The 18 hours of battery life claimed by Apple has proved to be a perfectly reasonable estimate, assuming the pattern of engagement that most of us are anticipating at the moment, and even under heavy use and testing during development, the watch is able to go a full working day without a recharge. The user can, when the remaining battery charge becomes perilously low, activate a low power mode that will restrict the watch to telling the time, and your app stays shut out in the cold. We will, of course, later have a look at what power conservation considerations you will want to be including when designing your own apps to avoid this situation.

Apple does the heavy lifting around designing and coding for a single device, one that just happens to come in two sizes, so you will only be working on a single UI design, and only writing one set of source code.

Just to round off the vital statistics, the Watch has 512MB of RAM and 8GB of storage, but the companion app on the iPhone (and there always is one) will store most of your larger resources, images and video, for example, if you have any.

Based on and in many respects similar to iOS, watchOS is the operating system that powers the Watch. It is orientated towards conserving power and the economic use of resources in memory, while offering a rich and engaging experience for the user, and most developers will find leveraging their previous iOS programming experience easy and intuitive, while adapting to the optimizations of many common programming tools and techniques. You will grow accustomed to alternate ways of providing information to the user, for example using tables to display lists, which, while visually similar to those on an iPhone, are handled quite differently at a code level to tables in iOS.

watchOS was revealed to the world along with the Watch in April 2015, and was upgraded to watchOS 2 in the following October. From our perspective as developers, version 2.0 was a huge leap forward, not only as the Watch gained the ability to function away from the iPhone, significantly increasing its scope of use, but also because, just as critically, we gained access to many of the features of watchOS and its hardware that were hitherto reserved for Apple's own apps, most notably the Taptic Engine, Digital Crown, accelerometer, heart rate sensor, speaker, and microphone, and the addition of custom Complications (some of which we will be looking at shortly).

WatchKit is the Apple framework that will provide you with the technologies that you will use to create your apps. Think of WatchKit as a vast library of code that all developers will need, code that has been written, tested, and optimized for you by the very people that built both the hardware and its operating system, ensuring a maximum level of uniformity, compatibility, code safety, security, and convenience for the developer. There is, to take one example, a massive amount of work for your app involved in redrawing a button when its title changes, work that will be almost identical every time that button changes its appearance in any way, but you as a developer will only write one line of code to get all that boiler plate set up.

This you will likely know already from UIKit, with which iOS apps spring to life.

The main app is the only part of this quartet that is always present in the bundle that gets installed on the watch by the phone app, the other three are optional, and whether to include them or not will depend entirely on what makes sense for your app. This itself adds a new dimension to your app design, as information can be spread across these various channels, partly duplicated, or some combination of both. While the scope and contents of the main app are directly comparable to the equivalent iOS apps, it is worth taking a brief look at what the other three do, how they do it, and what likely use cases they will be associated with.

As the name might suggest, the Glance is meant as a brief, and one-way, encounter. The user is presented with a single screen of information, and is offered no interaction other than a single tap, which will take her to the main app. It is an interaction initiated by the user, swiping up from the clock screen. In the images below, you can see that the Glance (left) presents information in a format that is easy to read quickly, containing only essential information, while the main app presents more than just the most immediate data, but requires more than just a casual glance (hence the name) from the user:

Depending on the nature of your app, you may find your users consulting this screen more often than your main app, so it pays to regard the Glance as more than just a stripped down extension of your main app and to give it a level of attention commensurate with its importance. Those who enjoy rising to the challenge of doing a lot with limited resources will likely find this a most pleasing area of design and implementation.

It might seem strange to give the name Complication to a UI element, particularly given the emphasis Apple give to the simplicity of using the Watch, but the term actually stems from the tradition of watch-making, and refers to any information, typically dates and weekdays, presented by a watch beyond simply the time of day.

We can see in these images how some watch faces offer different numbers of Complications:

A Complication, as implemented on the Apple Watch, is your chance to present a very small amount of information on the watch face itself, if the user has selected your app from a list of those that offer information in this format. If implemented by yourself and activated by the user, this is also the most direct route to your app, since it involves just a solitary tap on the complication to launch your main app.

This is the one interface that can initiate an exchange between the user and your app. Modelled on and similar in many ways to what we already know from iOS Notifications, your app can draw attention to itself and present to the users a number of options, including launching your main app, or choosing from a set of actions to be undertaken.

The Notification view (left) can be a scrollable view (center) and may offer the user the chance to choose actions, tapping the app icon launches the main app (right).

Once again, depending on your app's functionality, this may be the interface that your user sees and touches the most. Apple's own Messages app is such a case, in which a user may read all of his messages as they come in, but only react to a limited number of them.

Possibly more thanks to its snappy name than anything else, one of the most widely reported hardware features of the Watch has been the Digital Crown, which functions both as a push button, comparable in use to the iPhone's Home button, and as a method for selecting or entering data into your app. While not exactly a radical development, the royal scroll wheel really does make entering data, selecting options, and navigating around an interface very much quicker and more precise than tapping and swiping what are frequently extremely small UI elements on the screen and also avoids covering a large part of the screen content while doing so; this is the inevitable result of using finger touch gestures.

And if the battery gets a bit low, you could try using it to wind up the watch. Just kidding.

Arousing much less interest than the Crown, and slightly more conventional, is the humble Side Button located below it, which is, well, a button. Everyone needs a button, right? Perhaps to bring up the Friends Screen, with which to send messages and initiate phone calls. It's a fine button.

Oh, and your app can't use it.

In addition to the standard taps (or touches), and double taps, with which to select options, activate buttons, and so on, in much the same way as she would on any other iOS touchscreen, the user can also deploy the Force Touch, a harder and slightly longer press on the screen, which is acknowledged by watchOS with a satisfying pulse (see Taptic Haptic, below), and can be interpreted by an app in whichever way is appropriate.

Wikipedia offers the definition of haptic technology as

Haptic feedback has long been a part of gaming controllers and other devices, including smartphones, made by many manufacturers. Apple's take on this, the Taptic Engine, is definitely one of the industry's classiest implementations of this technology, and employs a linear actuator (look it up, if you like) to produce a quite stunning range of buzzing and tapping sensations on the wrist, that users will have available to them all day, no matter what they are wearing (dresses and smartphones, no pockets, now what?), and no matter where they are, making the Watch into a device that can communicate meaningfully with the user with minimal distraction, maximum discretion (you can't hear the Taptic Engine next to you at all), and one that can initiate that communication (beyond just beeping) in a way that is simply not possible on a phone, which may be on a table in front of the user, in a jacket pocket on a coat hook somewhere, or on a table somewhere quite different.

WatchKit provides a number of preset taptics, which the developer is encouraged to use according to the conventions suggested by the naming of each preset, so that users will, over time, become accustomed to each haptic notification and know intuitively and without needing to look at the watch, what kind of information the watch is offering to convey.

Whether a subtle and gentle tap on the wrist, or an insistent, repeated buzzing sensation, as inaudible as it is invisible, the Taptic Engine represents not only state of the art haptics, but also points the way to a whole new genre of communication from a device to the user. To a great extent, what becomes of this is down to what designers and developers can dream up.

While the audio output of the Watch is limited to a tiny but impressively loud and clear speaker, one which is loud enough to conduct a telephone conversation in an averagely loud city street (so no headphone output), the audio input of the device is both surprisingly high quality (call recipients are unaware that a conversation is being had from the watch) and pleasingly flexible, in that it gets piped into Apple's voice-controlled assistant Siri (yes she lives on the Watch too) where it can be used to dictate text and save audio files, among many other things.

Alternatively, the user can connect to a Bluetooth headset, which is a more suitable arrangement for that jog around the park.

The Heartrate Sensor, in enabling users to monitor one of the most important aspects of their physical health, is undoubtedly an important feature of the Apple Watch. The ability to share heartbeats with another Watch owner perhaps less so.

And it is because of this wide open scope for development and yet-to-be-discovered potential that we also need to be able to view the Apple Watch as not simply an extension of the iPhone (though it certainly is that), but also as an autonomous device in its own right, one that can collect data from a number of sources (whether the user, the phone or the web), process that data natively, present it visually or otherwise, at its own initiative, and even store that data onboard or online. Since the release of watchOS 2, the limitations of what we can do independently of the phone are defined more by our imagination than by actual barriers inherent in the technology, and the Watch fits in perfectly with the tendency of apps to be increasingly spread across devices and platforms. Most users of social networks, for example, will regularly use more than one platform to catch up on what's going on and update their status, typically a web browser on a laptop and a phone app, but this trend is being broadened across a whole range of software categories, as users increasingly expect to be able to use apps across several devices according to whatever is appropriate to their situation.

And so we have an operating system, watchOS, that is both a part of its larger sibling, iOS, and a separate OS in its own right, that perfectly mirrors the relationship between the physical watch, and its larger counterpart in our pockets. The relationship between WatchKit and UIKit is the same. Our success in designing and developing for this platform will likely depend to a significant extent on our ability to embrace this dual personality.

If that sounds like an exciting challenge, and a stimulating environment in which to expand your skills as a developer, then this book is for you.