Take the 2-minute tour ×  What the API uses for work with plug
devices, something like this app: square iphone scanner
?CocoaTouch, some from Foundation?  share|improve this question
add comment  3 Answers  up vote 3 down vote accepted  Square
works through the headphone jack, so it doesn't use the External
Accessory framework. External accessories on the iPhone are
notoriously difficult to do, which is why barely any
manufacturers are creating products.   I can count on one finger
the number of third party devices I've seen that communicate
through the dock connector on the iPhone.  share|improve this
answer add comment   Oops. Square doesn't use ExternalAccessory,
but some kind of custom interface through the headphone jack, as
explained by @kubi.  The relevant interface looks like this:
@interface SKSquareInterface : NSObject {     ? delegate;     ?
queue;     ? state; @property(assign) ?
XXEncryptedProperty_6b644; @property(assign) ?
XXEncryptedProperty_6a1bc; @property(assign) ?
XXEncryptedProperty_77a2