Remote Selfie shutter are really low cost Bluetooth 3 buttons that are really easy to hack for making different applications.
These button can be found on ebay, amazon or aliexpress like following this link for less that 6$. The button is paired to the smartphone and then allow to execute photo shoot without the need of any third party application.
Once you have connected your button to your smartphone, the device is identified as a keyboard. For taking photo, it uses the existing shortcuts like the “sound up” key for iPhone and the “enter” key for the android version. So basically this is a 2 key keyboard and you can capture these key easily from your smartphone app. Depending on the model you are using the behavior is different. As an exemple, the green button you see on top is sending the two keys one after the other. So for each click you have two keys sent : “sound up” and “enter”
This other remote selfie is working differently as it is sending the two keys depending on the button you are clicking. You can click up to 10 time a second and it can be detected so you can easily detects double clics, triple clics, long clics. In fact if you keep your finger on the button you will have on message (on key) every 100ms.
On iPhone, bypassing the system “sound up” behavior is not the more easy thing but the use of the “enter” key is more easy.