How do I send strings between my osx app and arduino continuously? - objective-c

Question

I made a cocoa application that generates a list of instructions for an arduino uno to execute. Because the list is too large to fit at one time within the arduino's memory, I am trying to send the arduino an individual instruction at a time.
Arduino:
void setup(){
Serial.begin(9600);
}
void loop(){
if(Serial.available() > 0){
String in = Serial.readString();
delay(10);
Serial.print(in);
}
Serial.print("A");
}
Cocoa App
(Code after the serial port is open and working)
- (void)incomingTextUpdateThread: (NSThread *) parentThread {
// mark that the thread is running
readThreadRunning = TRUE;
const int BUFFER_SIZE = 1;
char byte_buffer[BUFFER_SIZE]; // buffer for holding incoming data
long numBytes=0; // number of bytes read during read
// assign a high priority to this thread
[NSThread setThreadPriority:1.0];
// this will loop unitl the serial port closes
while(TRUE) {
// read() blocks until some data is available or the port is closed
numBytes = read(serialFileDescriptor, byte_buffer, BUFFER_SIZE); // read up to the size of the buffer
if(numBytes>0) {
if(byte_buffer[0] == 'A'){
Coordinate *c = [coordinates objectAtIndex:coordinateIndex];
[self writeString:[self formateCoordinateString:c]];
coordinateIndex++;
}
}
if(coordinateIndex == coordinates.count){
close(serialFileDescriptor);
break;
}
}
readThreadRunning = FALSE;
}
I run the arduino code first and it prints a bunch of 'A's in the serial console. However once I start the cocoa app, it stops printing 'A's and doesn't do anything.
When I set a breakpoint within the while loop, the arduino starts printing the 'A's again. I continue to step within the while loop, and the "instruction" string is sent correctly to the arduino.
My issue is that this only works when I set this break point. Otherwise both my cocoa app and arduino app go into a stand-still.
Thanks for any and all advice! Please feel free to ask for clarification.