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What started out as a gift to his girlfriend has turned into quite the journey for 22-year-old Matt Moss.

“In about 3 weeks we did about 5 million sign ups,” explained Moss, who I met up with recently in San Francisco.

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Moss created an app called Locket Widget as an effortless way to exchange photos with his girlfriend, Ava.

“We met at UCSB and she was going back to school in the fall but I had graduated,” explained Moss.

So, he coded the app, which takes advantage of a relatively new iOS feature called Widgets. Locket puts a virtual photo frame on the iPhone’s home screen. Up to 10 trusted friends or family members can send snapshots to the widget, which will instantly show up on your home screen when you unlock your phone.

“The idea of being able to send pictures to each other’s home screen seemed really appealing, just a nice way to keep in touch throughout the day,” said Moss.

One viral TikTok video later and suddenly, the app was getting downloaded thousands, then millions of times. Locket Widget went to the top of the App Store charts in over 30 countries. It is a free download.

“So every time you open your phone you get this nice kind of surprise that comes up at you… which is a new picture of your kid or your girlfriend… or a family member,” said Moss.

Snapshots must be taken on the fly, through the widget. You can’t select a photo from your camera roll.

“When someone sends you something it’s very natural to just tap the widget and in two clicks send the photo right back,” explained Moss.

When I asked about privacy, Moss said it’s front and center. The app doesn’t spam your friends with invite requests and steps are taken to ensure that your photos remain secure.

The wildest part: Moss told me that he learned to code from – get this – YouTube tutorials!

Right now, he isn’t monetizing the app, in fact, it’s costing him money to keep the servers going that allow the pictures to be exchanged. Somehow, he’s managed to hire a few people to help him deal with the app’s sudden growth spurt.

Moss is no stranger to Apple’s products and services. He’s been coding apps for six years and was even a WWDC student scholar, students chosen by Apple to attend their yearly developer event for free. A previous app he created was called Hawkeye Access, which lets people with motor impairments browse the web using eye movements.

His advice to budding coders: “Build things that you really want and if you really like it there’s a good chance that other people will as well.”