Question

I am trying to figure out how to update of UI elements (UIImageViews) based upon the content of user event callbacks that are invoked when a user event is passed during MIDI playback. More specifically, these user events contain note data (e.g. played note is 60, aka middle C) that is passed into the user callback function.

I want to update my UIImageViews based upon what note is played. I have tried to access the UIImageViews from within the callback but since it doesn’t have direct access to the ViewController and since it runs on a thread other than the main one, I have been advised to do it a different way.

So, what I would like to do is to create a separate controller that can relay information from the callback function to the UI but I have no idea who to go about doing that. I’ve posted the code for my ViewController below. It includes the callback function and all of the associated code for setting up the user events and other view related stuff.

I’m working in Xcode 4.3.1 with iOS 5 and I am using ARC.

PracticeViewController.h

#import <UIKit/UIKit.h>
#import "Lesson.h"
#import "Note.h"
#import <AudioToolbox/AudioToolbox.h>

@interface PracticeViewController : UIViewController

@property (strong, nonatomic) Lesson *selectedLesson;
@property (strong, nonatomic) IBOutlet UINavigationItem *practiceWindowTitle;
@property MusicPlayer player;

//Outlets for White Keys
@property (strong, nonatomic) IBOutlet UIImageView *whiteKey21;
// […]
@property (strong, nonatomic) IBOutlet UIImageView *whiteKey108;

//Outlets for Black Keys
@property (strong, nonatomic) IBOutlet UIImageView *blackKey22;
// […]
@property (strong, nonatomic) IBOutlet UIImageView *blackKey106;

// Key Highlight Images
@property (strong, nonatomic) UIImage *highlightA;
@property (strong, nonatomic) UIImage *highlightB;
@property (strong, nonatomic) UIImage *highlightC;
@property (strong, nonatomic) UIImage *highlightD;
@property (strong, nonatomic) UIImage *highlightE;
@property (strong, nonatomic) UIImage *highlightF;
@property (strong, nonatomic) UIImage *highlightG;
@property (strong, nonatomic) UIImage *highlightH;

- (IBAction)practiceLesson:(id)sender;

@end

PracticeViewController.m

#import "PracticeViewController.h"

@interface PracticeViewController ()

@end

@implementation PracticeViewController
@synthesize blackKey22;
// […]
@synthesize blackKey106;
@synthesize whiteKey21;
// […]
@synthesize whiteKey108;

@synthesize selectedLesson, practiceWindowTitle, player, highlightA, highlightB, highlightC, highlightD, highlightE, highlightF, highlightG, highlightH;

// Implement the UserEvent structure.

typedef struct UserEvent {
    UInt32 length;
    UInt32 typeID;
    UInt32 trackID;
    MusicTimeStamp tStamp;
    MusicTimeStamp dur;
    int playedNote;
} UserEvent;

// Implement the UserCallback function.

void noteUserCallback (void *inClientData, MusicSequence inSequence, MusicTrack inTrack, MusicTimeStamp inEventTime, const MusicEventUserData *inEventData, MusicTimeStamp inStartSliceBeat, MusicTimeStamp inEndSliceBeat)
{       
    UserEvent* event = (UserEvent *)inEventData;
    UInt32 size = event->length;
    UInt32 note = event->playedNote;
    UInt32 timestamp = event->tStamp;
    NSLog(@"Size: %lu Note: %lu, Timestamp: %lu", size, note, timestamp);
}

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }

    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.practiceWindowTitle.title = selectedLesson.titleAndSubtitle;

    // Load in the images for the glow.
    highlightA = [UIImage imageNamed:@"glow_whiteKeysA.png"];
    highlightB = [UIImage imageNamed:@"glow_whiteKeysB.png"];
    highlightC = [UIImage imageNamed:@"glow_whiteKeysC.png"];
    highlightD = [UIImage imageNamed:@"glow_whiteKeysD.png"];
    highlightE = [UIImage imageNamed:@"glow_whiteKeysE.png"];
    highlightF = [UIImage imageNamed:@"glow_whiteKeysF.png"];
    highlightG = [UIImage imageNamed:@"glow_blackKey.png"];
    highlightH = [UIImage imageNamed:@"glow_whiteKeysH.png"];

    // Create player, sequence, left/right hand tracks, and iterator.

    NewMusicPlayer(&player);
    MusicSequence sequence;
    NewMusicSequence(&sequence);
    MusicTrack rightHand;
    MusicTrack leftHand;
    MusicEventIterator iterator;

    // Load in MIDI file.

    NSString *path = [[NSString alloc] init];
    path = [[NSBundle mainBundle] pathForResource:selectedLesson.midiFilename ofType:@"mid"];
    NSURL *url = [NSURL fileURLWithPath:path];
    MusicSequenceFileLoad(sequence, (__bridge CFURLRef)url, 0, kMusicSequenceLoadSMF_ChannelsToTracks);

    // Get the right and left hand tracks from the sequence.

    int rightHandIndex = 0;
    //int leftHandIndex = 1;

    MusicSequenceGetIndTrack(sequence, rightHandIndex, &rightHand); //Get right hand.
    //MusicSequenceGetIndTrack(sequence, leftHandIndex, leftHand); //Get left hand.

    //Iterate through the right hand track and add user events.

    Boolean hasNextEvent = false;
    Boolean hasEvent = false;

    NewMusicEventIterator(rightHand, &iterator);
    MusicEventIteratorHasCurrentEvent(iterator, &hasEvent);
    MusicEventIteratorHasNextEvent(iterator, &hasNextEvent);

    while (hasNextEvent == true) {
        MusicTimeStamp timestamp = 0;
        MusicEventType eventType = 0;
        const void *eventData = NULL;
        int note;
        MusicTimeStamp duration;

        MusicEventIteratorGetEventInfo(iterator, &timestamp, &eventType, &eventData, NULL);

        if (eventType == kMusicEventType_MIDINoteMessage) {
            MIDINoteMessage *noteMessage = (MIDINoteMessage *)eventData;
            note = noteMessage->note;
            duration = noteMessage->duration;
            UserEvent event;

            event.length = 0;
            event.length = sizeof(UserEvent);
            event.playedNote = note;
            event.tStamp = timestamp;

            MusicEventUserData* data = (MusicEventUserData *)&event;
            MusicTrackNewUserEvent(rightHand, timestamp, data);
        }

        MusicEventIteratorHasNextEvent(iterator, &hasNextEvent);
        MusicEventIteratorNextEvent(iterator);
    }

    MusicSequenceSetUserCallback(sequence, noteUserCallback, NULL);

    MusicPlayerSetSequence(player, sequence);
    MusicPlayerPreroll(player);
}

- (void)viewDidUnload
{
    [self setPracticeWindowTitle:nil];
    [self setWhiteKey21:nil];
    // […]
    [self setWhiteKey108:nil];
    [self setBlackKey22:nil];
    // […]
    [self setBlackKey106:nil];
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return YES;
}

- (IBAction)practiceLesson:(id)sender {
    MusicPlayerStart(player);
}
@end

Answer 1

You were on the right track with your other question. I don’t know why you think you need a whole different approach.

In your callback, make sure to do any work that touches UI on the main thread, using -performSelectorOnMainThread: or dispatch_async with dispatch_get_main_queue. Probably, any code that uses PracticeViewController should go on the main thread.

e.g.

void noteUserCallback (void *inClientData, MusicSequence inSequence, MusicTrack inTrack, MusicTimeStamp inEventTime, const MusicEventUserData *inEventData, MusicTimeStamp inStartSliceBeat, MusicTimeStamp inEndSliceBeat)
{   
    PracticeViewController* pvc = (__bridge PracticeViewController *)inClientData;

    dispatch_async(dispatch_get_main_queue(), ^{
        [pvc.whiteKey21 setImage:pvc.highlightA];
    });
    ...
}