Because the cornerstone of the iPhone design was a large multi-touch display, Apple's engineering team had to create an entirely new framework for how consumers interacted with their phones.
"When creating the iPhone, there were so many completely unsolved problems that we had to tackle," Forstall explained. "Every single part of the design had to be rethought for touch."
As one example, Forstall said that Apple had to engineer scrolling on the iPhone to work not only when a user's finger moved vertically up and down, but also when a user's thumb would move in an arc-like trajectory.
"And so we had to figure out a way", Forstall said, "with this new form of input, with this touch and multi-touch input, to scroll something in the way the user would want it, even though there is imprecise input here."
Forstall explained that the amount of work that went into building the original iOS interface was "immense," adding that he "devoted years of my life to this," and that it was "very, very difficult."
While the way we interact with smartphones today seems highly intuitive, the user experience that we may now take for granted was the result of a lot of hard work, creative engineering, and thoughtful consideration as to how people would ideally interact with their device.
As a quick example, most iPhone users are likely familiar with the "tap to zoom" feature when browsing the web. Note, it might also sound familiar because it was one of the patents Apple asserted against Samsung in its recent trial.
Well as it turns out, the idea for "tap to zoom" came from Forstall and was borne out of the heavy testing he was doing on early iPhone prototypes.
When browsing the web, Forstall noticed that he was constantly "pinching to zoom" so that he could read text on the screen. And so it dawned upon him that it'd be much more efficient "for the iOS to take of this automatically with a single double tap."
"The team went back and worked really hard to figure out how to do that," Forstall said
And so slowly, many of the basic features that we now associate with the modern day smartphone were put together. A large buttonless screen capable of displaying the full web, a multi-touch display with gesture support and more.
Design team considered curved glass on front and back of new phones
Just as important to the iPhone's software was the design of the device itself. Apple has long prided itself on innovative industrial design and the work that went into creating the original iPhone design really reflects that.
When long time Apple designer Christopher Stringer took the stand in August, we learned quite a bit more about Apple's secretive industrial design group.
Sign up for CIO Asia eNewsletters.