Looking at the features...

Take note of the hardware features present on the iPhone or iPad. All iOS devices have a circle home button located below the touch-sensitive screen, volume controls found on the side of the device, a headphone jack, and a sleep button on the top of the device. On the iPhone and iPad, a switch can be found that toggles silent mode on and off.

Several other features can be found on each device, depending on the model. A rear-facing camera has been prevalent on the iPhone since the first model in 2007. A front-facing camera has become a common addition since the introduction of the iPhone 4 and Facetime in 2010. Recent iPod touch and iPad devices have also received front and rear cameras. All devices besides the first generation iPod touch contain a microphone for recording audio. With the addition of the iPhone 4S, Apple included its system-wide Siri voice dictation software, which is available to all applications through inclusion of the standard keyboard.

An ambient light sensor and accelerometer can be found on every iOS device. Bluetooth has been a staple of every iPhone and iPad, with iPod touches receiving the feature in 2008 models and beyond. A gyroscope is a new addition to iOS devices, being introduced with the iPhone 4 in the summer of 2010.

Music, photos, and applications are all held on the device in flash ROM storage. The amount of storage on each device can fluctuate anywhere between 8 GB to 64 GB and is not expandable after a user has purchased the device.

Unlike other touch capable phones or tablets, iOS devices contain no programmable hardware buttons for developers or designers to work into an application. Instead, every interaction between the user and the application must be implemented through either a software interaction with the touch screen or by utilizing the accelerometer or gyroscope for user interaction.

We've just begun to scratch the surface on the functionality available to iOS designers. Here are a few more tips that we can use to best understand the basic principles of iOS user interface development.

Understanding our audience

Fully understanding the hardware specifications and limitations for the device we're looking to target is absolutely essential. The more we know about who will be using our application and what device features they have, the better we can fine tune our user interface in order to provide a great experience for the end user.

Before we ever begin to map out any interface design, we should take the time to sit and consider where and when people will be using our application.

Imagine we are looking to build an application to help bus riders in Chicago keep track of stop locations. In this app, it wouldn't make sense to use the accelerometer and have the app update the user's location when the device is shaken, because the constant stop and go nature of a bus ride could accidentally trigger an update and frustrate the user.

Dealing with download caps

Even though Apple's cap of a 2 GB file size per application is usually more than enough space for any app, it's also important to remember that many wireless carriers across the world place a limit on the size of applications that can be downloaded over the wireless data network.

When determining a target audience and use case for an application, it's important to see what app size limitations are in place for the target market.

Using our example of an application to help bus riders find detailed stop information, it would be detrimental to have our app size be above the wireless download cap placed by the carrier, because users would be unable to download the application while walking around the city looking for a bus stop.