indexloop.swift

var messages: [Message] = []

func dispatch(_ message: Message) {
  messages.append(message)
  dispatchToPlugins()
}

func dispatchToPlugins() {
  while messages.count > 0 {
    for plugin in plugins {
      plugin.dispatch(message: messages[0]) 
    }
    messages.remove(at:0)
  }
}

Thread1:
dispatch(message1)

Thread2:
dispatch(message2)