Snapchat caught screenshots by feeling a finger lift

The summer after school ended, Evan Spiegel, the chief executive of Snapchat, and Bobby Murphy, his co-founder, were working from Spiegel's dad's house. They spent the days sending disappearing photos back and forth, testing the odd little product they had just made and having fun with it.

Everyone they showed it to had the same objection. People told them the idea made no sense, because a photo cannot really disappear. As Spiegel recalled the reaction: "You can always take a screenshot." So what was the point.

Spiegel said the pushback sent the team thinking about a fix. In his view, for disappearing photos to feel safe enough to use, the sender had to know if the other person had kept one. That notice, he said, became an early breakthrough that gained traction and "helped Snapchat spread as a new way of communicating."

At the time, Apple gave developers no signal when a viewer saved an image. There was no built-in warning an app could simply request, so Snapchat had to find another clue.

It found one in the way Snapchat then worked. To keep a photo open, the viewer had to press a finger down and hold it on the screen. Spiegel said he and Bobby realized that taking a screenshot interrupted that touch.

The phone reported that the finger had lifted off the screen. By watching for that brief touch event, Snapchat could infer that a screenshot had happened, even without help from Apple. It then sent a notice back to the sender that the recipient had taken one.

The fix did not stop saving. People did not mind so much if someone kept a photo, Spiegel said, they just wanted to know. Two builders working from Spiegel's dad's house had built a workaround for the flaw everyone said made the idea pointless.