The Spectre Modular Midi Sequencer is an iPad app based on analog modular sequencers. Use virtual patch cables and the array of modules featured in this app to build sequences.
4x 8 step sequencers, 2x clock dividers, 2x sequential switches, 2x logic modules and 2 independent midi outputs with optional sample and hold feature.
Each output jack can be routed to multiple inputs (aka stackable). Sequencers are adjustable from 2 to 8 steps and can be combined for interesting patterns.
Note, octave, velocity and gate length are available for each step. CV (pitch) and gates (or triggers) can be combined independently to create midi notes.
This app was wrote completely in Objective-C, using Apple's Core libraries. Unlike a fixed sequencer, whose steps are predefined, this system is completely open and modular. Thus, each input must always be listening for a signal and be able to process it.
The way I solved the problem of "listening" was by passing a function pointer per connection to every module. Each module maintained an array of these pointers, and simply iterated over each one to determine the needed action.
Here's a short video demonstrating basic functionality.
This app has been discontinued, and is no longer available.