How to use CoreMIDI on iOS?

Question

I have been unable to find much information on CoreMIDI for iOS. Is it even possible to play a MIDI sound by sending a message to the device itself. Does the iPhone or iPad have a MIDI device installed or do you have to have a device connected to interface with?

Answer 1

This is a couple of years too late, but it may help someone else out there like it helped me. This website was instrumental in helping me read MIDI data from an external MIDI keyboard. The connections are the trickiest parts, but this tutorial will walk you through it.

Here's the class that I created.

MIDIController.h

#import <Foundation/Foundation.h>

@interface MIDIController : NSObject

@property NSMutableArray *notes;

@end

MIDIController.m

#import "MIDIController.h"

#include <CoreFoundation/CoreFoundation.h>
#import <CoreMIDI/CoreMIDI.h>

#define SYSEX_LENGTH 1024
#define KEY_ON 1
#define KEY_OFF 0

@implementation MIDIController

- (id)init {
    if (self = [super init]) {
        _notes = [[NSMutableArray alloc] init];
        [self setupMidi];
    }
    return self;
}

- (void) setupMidi {
    MIDIClientRef midiClient;
    checkError(MIDIClientCreate(CFSTR("MIDI client"), NULL, NULL, &midiClient), "MIDI client creation error");

    MIDIPortRef inputPort;
    checkError(MIDIInputPortCreate(midiClient, CFSTR("Input"), midiInputCallback, (__bridge_retained void *)self, &inputPort), "MIDI input port error");

    checkError(connectMIDIInputSource(inputPort), "connect MIDI Input Source error");

}

OSStatus connectMIDIInputSource(MIDIPortRef inputPort) {
    unsigned long sourceCount = MIDIGetNumberOfSources();
    for (int i = 0; i < sourceCount; ++i) {
        MIDIEndpointRef endPoint = MIDIGetSource(i);
        CFStringRef endpointName = NULL;
        checkError(MIDIObjectGetStringProperty(endPoint, kMIDIPropertyName, &endpointName), "String property not found");
        checkError(MIDIPortConnectSource(inputPort, endPoint, NULL), "MIDI not connected");
    }

    return noErr;
}

void midiInputCallback(const MIDIPacketList *list, void *procRef, void *srcRef) {
    MIDIController *midiController = (__bridge MIDIController*)procRef;

    UInt16 nBytes;
    const MIDIPacket *packet = &list->packet[0]; //gets first packet in list

    for(unsigned int i = 0; i < list->numPackets; i++) {
        nBytes = packet->length; //number of bytes in a packet

        handleMIDIStatus(packet, midiController);

        packet = MIDIPacketNext(packet);
    }
}



void handleMIDIStatus(const MIDIPacket *packet, MIDIController *midiController) {
    int status = packet->data[0];
    //unsigned char messageChannel = status & 0xF; //16 possible MIDI channels

    switch (status & 0xF0) {
        case 0x80:
            updateKeyboardButtonAfterKeyPressed(midiController, packet->data[1], KEY_OFF);
            break;
        case 0x90:
            //data[2] represents the velocity of a note
            if (packet->data[2] != 0) {
                updateKeyboardButtonAfterKeyPressed(midiController, packet->data[1], KEY_ON);
            }//note off also occurs if velocity is 0
            else {
                updateKeyboardButtonAfterKeyPressed(midiController, packet->data[1], KEY_OFF);
            }
            break;
        default:
            //NSLog(@"Some other message");
            break;
    }

}

void updateKeyboardButtonAfterKeyPressed(MIDIController *midiController, int key, bool keyStatus) {
    NSMutableArray *notes = [midiController notes];

    //key is being pressed
    if(keyStatus) {
        [notes addObject:[NSNumber numberWithInt:key]];
    }
    else {//key has been released
        for (int i = 0; i < [notes count]; i++) {
            if ([[notes objectAtIndex:i] integerValue] == key) {
                [notes removeObjectAtIndex:i];
            }
        }
    }
}

void checkError(OSStatus error, const char* task) {
    if(error == noErr) return;

    char errorString[20];
    *(UInt32 *)(errorString + 1) = CFSwapInt32BigToHost(error);
    if(isprint(errorString[1]) && isprint(errorString[2]) && isprint(errorString[3]) && isprint(errorString[4])) {
        errorString[0] = errorString[5] = '\'';
        errorString[6] = '\0';
    }
    else
        sprintf(errorString, "%d", (int)error);

    fprintf(stderr, "Error: %s (%s)\n", task, errorString);
    exit(1);
}


@end

Additional Notes

midiInputCallback Function

• midiInputCallback is the function that is called when a MIDI event occurs via a MIDI device (keyboard)
  NOTE: This is where you can start handling MIDI information

handleMIDIStatus function

• handleMIDIStatus takes the MIDI packet (which contains the information about what was played and an instance of MIDIController
  NOTE: You need the reference to MIDIController so that you can populate properties for the class...in my case I store all played notes, by MIDI number, in an array for use later on

• when the status is 0x90, which means a note has been triggered, if it has a velocity of 0, it is considered not played...I needed to add this if statement because it wasn't functioning properly
  NOTE: I only handle key on and key off events, so you would augment the switch statement to handle more MIDI events

updateKeyboardButtonAfterKeyPressed Method

• This is a method that I used to store notes that are played and I remove notes from this array once the key has been released

I hope this helps.

Answer 2

You should give a look at pete goodliffe's blog and he generously provides an example project. It helped me a lot to start programming CoreMIDI.

Now about your questions, on iOS, mostly CoreMIDI network sessions are used. The participants to a same "Network Session" send messages to each others.

For example, you configure a network session on your Mac (using Audio MIDI Setup tool) and you can connect your iOS devices to it. This way, you can sent messages from iOS to your OSX host and vice versa.

CoreMIDI network sessions relies on the RTP protocol to transport MIDI messages and Bonjour to discover hosts.

Next to that, CoreMIDI can also handle MIDI interface connected to the system, but iOS devices don't have physical MIDI interface by default. You have to buy external hardware if you want connect directly your iPhone to a synthesizer. However, the iPad can be connected to an USB class compliant Midi interface via the camera kit.

Another thing, on a standalone iOS device, you can send use the local CoreMIDI session to send or receive messages from/to another CoreMIDI compatible application.