Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
Sean Costello teaches how to code a Reverb
I bought Desktop ValhallahDSP reverb on the recommendation of @jonmoore and became aware of Sean Costello's creativity with Reverb coding. His DSP code is used in the IOS EOS 2 Reverb.
He has decided to blog his advice to anyone interested in understanding the options available to code a reverb. Of course, the advice can be generically applied to music app development.
TL;DR - use the Juce framework and for DSP code use C++.