Jobs soon figured out that there was a way to have the best of both worlds.
He would permit outsiders to write apps,
but they would have to meet strict standards, be tested and approved by Apple, and be sold only through the iTunes Store.
It was a way to reap the advantage of empowering thousands of software developers while retaining enough control
to protect the integrity of the iPhone and the simplicity of the customer experience.
"It was an absolutely magical solution that hit the sweet spot," said Levinson.
"It gave us the benefits of openness while retaining end-to-end control."
The App Store for the iPhone opened on iTunes in July 2008; the billionth download came nine months later.
By the time the iPad went on sale in April 2010, there were 185,000 available iPhone apps.
Most could also be used on the iPad, although they didn't take advantage of the bigger screen size.
But in less than five months, developers had written twenty-five thousand new apps that were specifically configured for the iPad.
By July 2011 there were 500,000 apps for both devices, and there had been more than fifteen billion downloads of them.