This suite of six MIDI sequencer tools run as AU-only plug-ins within a suitable AU-host. Rozeta also makes use of one of the new features Apple has snuck rather quietly into the iOS 11 update: the ability for AU plug-ins to send MIDI Out data.
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Flagship sequencer apps are so packed with features that it could almost be a full-time job learning the package of your choice. If your needs are simpler, why not make it easy on yourself and investigate one of many options that are more entry-level or cut-down, yet still remarkably capable?