Core Graphics Drawing, Bad Performance Degredation - ios

Question

I am using the following core graphics code to draw a waveform of audio data as I am recording. I apologize for tons of code, but here it is (I found it here):
//
// WaveformView.m
//
// Created by Edward Majcher on 7/17/14.
//
#import "WaveformView.h"
//Gain applied to incoming samples
static CGFloat kGain = 10.;
//Number of samples displayed
static int kMaxWaveforms = 80.;
#interface WaveformView ()
#property (nonatomic) BOOL addToBuffer;
//Holds kMaxWaveforms number of incoming samples,
//80 is based on half the width of iPhone, adding a 1 pixel line between samples
#property (strong, nonatomic) NSMutableArray* bufferArray;
+ (float)RMS:(float *)buffer length:(int)bufferSize;
#end
#implementation WaveformView
- (void)awakeFromNib
{
[super awakeFromNib];
self.bufferArray = [NSMutableArray array];
}
-(void)updateBuffer:(float *)buffer withBufferSize:(UInt32)bufferSize
{
if (!self.addToBuffer) {
self.addToBuffer = YES;
return;
} else {
self.addToBuffer = NO;
}
float rms = [WaveformView RMS:buffer length:bufferSize];
if ([self.bufferArray count] == kMaxWaveforms) {
//##################################################
// [self.bufferArray removeObjectAtIndex:0];
}
[self.bufferArray addObject:#(rms * kGain)];
[self setNeedsDisplay];
}
+ (float)RMS:(float *)buffer length:(int)bufferSize {
float sum = 0.0;
for(int i = 0; i < bufferSize; i++) {
sum += buffer[i] * buffer[i];
}
return sqrtf( sum / bufferSize );
}
// *****************************************************
- (void)drawRect:(CGRect)rect
{
CGFloat midX = CGRectGetMidX(rect);
CGFloat maxX = CGRectGetMaxX(rect);
CGFloat midY = CGRectGetMidY(rect);
CGContextRef context = UIGraphicsGetCurrentContext();
// Draw out center line
CGContextSetStrokeColorWithColor(context, [UIColor whiteColor].CGColor);
CGContextSetLineWidth(context, 1.);
CGContextMoveToPoint(context, 0., midY);
CGContextAddLineToPoint(context, maxX, midY);
CGContextStrokePath(context);
CGFloat x = 0.;
for (NSNumber* n in self.bufferArray) {
CGFloat height = 20 * [n floatValue];
CGContextMoveToPoint(context, x, midY - height);
CGContextAddLineToPoint(context, x, midY + height);
CGContextStrokePath(context);
x += 2;
}
if ([self.bufferArray count] >= kMaxWaveforms) {
[self addMarkerInContext:context forX:midX forRect:rect];
} else {
[self addMarkerInContext:context forX:x forRect:rect];
}
}
- (void)addMarkerInContext:(CGContextRef)context forX:(CGFloat)x forRect:(CGRect)rect
{
CGFloat maxY = CGRectGetMaxY(rect);
CGContextSetStrokeColorWithColor(context, [UIColor greenColor].CGColor);
CGContextSetFillColorWithColor(context, [UIColor greenColor].CGColor);
CGContextFillEllipseInRect(context, CGRectMake(x - 1.5, 0, 3, 3));
CGContextMoveToPoint(context, x, 0 + 3);
CGContextAddLineToPoint(context, x, maxY - 3);
CGContextStrokePath(context);
CGContextFillEllipseInRect(context, CGRectMake(x - 1.5, maxY - 3, 3, 3));
}
#end
So as I am recording audio, the waveform drawing gets more and more jittery, kind of like a game that has bad frame rates. I tried contacting the owner of this piece of code, but no luck. I have never used core graphics, so I'm trying to figure out why performance is so bad. Performance starts to degrade at around 2-3 seconds worth of audio (the waveform doesn't even fill the screen).
My first question is, is this redrawing the entire audio history every time drawRect is called? If you look in the drawRect function (marked by asterisks), there is a variable called CGRect x. This seems to affect the position at which the waveform is being drawn (if you set it to 60 instead of 0, it starts at x=60 pixels instead of x=0 pixels).
From my viewController, I pass in the audioData which gets stored in the self.bufferArray property. So when that loop goes through to draw the data, it seems like it's starting at zero and working its way up every time drawRect is getting called, which means that for every new piece of audio data added, drawRect gets called, and it redraws the entire waveform plus the new piece of audio data.
If that is the problem, does anyone know how I can optimize this piece of code? I tried emptying the bufferArray after the loop so that it contained only new data, but that didn't work.
If this is not the problem, are there any core graphics experts that can figure out what the problem is?
I should also mention that I commented out a piece of code (marked with ### at signs) because I need the entire waveform. I don't want it to remove pieces of the waveform at the beginning. The iOS Voice Memos app can hold a waveform of audio without performance degradation.