Communication Between Unrelated Objects - objective-c

Question

Background
I'm creating a rhythm app for MIDI drumming using SpriteKit on Mac OS. Note objects are the ones in green and blue below. The notes move from right to left on the screen. As the notes get closer to the "play bar" in red, I want to create an outline of the notes next to the play bar to indicate to the user that the notes are getting close to the play bar and that it's time to play soon.
The Problem
I need each Note object to notify another class called Sequencer when they get a certain distance from the play head so that it can start the note outline animation for the closest note. The notes know how far they are from the play head, so that's not a problem. Would delegation be a candidate for this situation? I have tried to understand the purpose of delegation, but I can never figure out how to implement it into my specific game-related situation.
EDIT: Code added
GameScene.m
#implementation GameScene
- (void)didMoveToView:(SKView *)view {
...
[self initSequence];
[self initNoteProcessor];
[self initPlayBar];
...
}
...
//keeps track of a collection of notes and vertical line measure markers and
//responsible for drawing them in scene and moving them left
-(void) initSequence {
_sequence = [[Sequencer alloc] initWithWidth:1150];
[_sequence setPosition:CGPointMake(130, 0)];
[self addChild:_sequence];
}
...
//responsible for handling incoming MIDI input from a keyboard and checking to see
//if notes are played on time, early or late
-(void) initNoteProcessor {
_noteProcessor = [[NoteProcessor alloc] initWithSequence:_sequence];
}
...
//creates a vertical line that indicates when notes need to be played
-(void) initPlayBar {
_playBar = (SKSpriteNode*)[self childNodeWithName:#"playBar"];
_playBar.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:_playBar.frame.size];
_playBar.physicsBody.affectedByGravity = NO;
[_playBar.physicsBody setDynamic:YES];
[_playBar.physicsBody setAllowsRotation:NO];
[_playBar.physicsBody setCategoryBitMask:kPlayBarCategory];
[_playBar.physicsBody setCollisionBitMask:kNoCategory];
[_playBar.physicsBody setContactTestBitMask:kNoteCategory | kLineCategory];
[[NSUserDefaults standardUserDefaults] setDouble:_playBar.position.x forKey:#"playBarXPosition"];
}
NoteProcessor.m
#implementation NoteProcessor
-(instancetype) initWithSequence:(Sequencer*)sequence {
if (self = [super init]) {
_sequence = sequence;
_noteActiveIndex = 0;
[self initActiveNoteIndices];
[self initNoteIndicator];
//when user plays key on keyboard, notify this class
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(midiPlayed:) name:#"MidiPlayedNotificationKey" object:nil];
}
return self;
}
//keeps track of the next note of each pitch that is incoming
-(void) initActiveNoteIndices {
_activeNoteIndicesByPitch = [NSMutableArray array];
for (int i = 0; i < kTotalKeys; i++) {
[_activeNoteIndicesByPitch addObject:[NSNumber numberWithInteger:0]];
}
}
//checks position of note and decides whether they are on time, early or late
-(void) updateCurrentNoteStatus {
...
}
//handles notifications from MIDI keyboard when user plays a key
-(void) midiPlayed:(NSNotification*)notification {
//stuff happens here
}
#end
Sequencer.m
#implementation Sequencer
-(instancetype) initWithWidth:(double)width {
if (self = [super init]) {
...
[self drawNotes];
}
return self;
}
//draws all of the notes on screen
-(void) drawNotes {
NSMutableArray *parsedNotes = [_midiParser getNoteData];
float distancePerBeat = [self getGridArea] / [self getMeasuresDisplayed] / 4;
float xPos = 0;
for (int i = 0; i < [parsedNotes count]; i++) {
NSArray *currNote = [parsedNotes objectAtIndex:i];
double location = [[currNote objectAtIndex:0] doubleValue];
double duration = [[currNote objectAtIndex:1] doubleValue];
int midiValue = [[currNote objectAtIndex:2] intValue];
Note *note = [[Note alloc] initWithLocation:location Duration:duration MidiValue:midiValue];
xPos = distancePerBeat * (location - 1);
float yPos = kNoteHeight[note.midiValue - 36];
[note setPosition:CGPointMake(xPos, yPos)];
[note setZPosition:kNoteZPosition];
NSMutableArray *noteList = [_notes objectAtIndex:midiValue-36];
[noteList addObject:note];
[self addChild:note];
}
}
//this method is called 60 times per second
//each call moves 1/60th of the distance it would move in 1/60th of a second
-(void) moveLeftAfterElapsedTime:(double)elapsedTime {
double timePerBeat = 60 / [self getBpm];
double timePerMeasure = timePerBeat * 4;
double lengthPerMeasure = [self getGridArea] / [self getMeasuresDisplayed];
double distancePerFrame = lengthPerMeasure / (timePerMeasure * 60);
double x = elapsedTime * distancePerFrame * 60;
[self moveAllNotesLeftByDistance:x];
}
//actually does all of the moving of the notes
-(void) moveAllNotesLeftByDistance:(double)distance {
for (int i = 0; i < [_notes count]; i++) {
NSMutableArray *noteList = [_notes objectAtIndex:i];
for (int j = 0; j < [noteList count]; j++) {
Note *note = [noteList objectAtIndex:j];
double xPos = note.position.x - distance;
double y = note.position.y;
[note setPosition:CGPointMake(xPos, y)];
}
}
}