Building Cocoa Applications: A Step by Step Guide

Working with Multiple Threads

GraphPaper is a tricky application because it has to do three things at the same time:

1. Respond to user events.

2. “Listen” for data from Evaluator and graph it when it arrives.

3. Send data to Evaluator.

Handling (i) and (ii) at the same time is no problem: we saw how to do that in the MathPaper application in Chapter 11. The NSApplication object’s event loop, which watches for user events, will also watch for data on a file descriptor^[38] using the NSTask, NSPipe, and NSFileHandle classes. The problem is (iii) — sending data to the Evaluator process. Doing this concurrently with (i) and (ii) presents a problem in Cocoa that has to do with the way that operating systems handle pipes.

Unix Pipes and Evaluator

When two programs are connected with a Unix pipe , the operating system allocates a buffer to address the possibility that the program at the “write end” of the pipe might send data before the program at the “read end” of the pipe is ready to accept it. Of course, each pipe buffer is only so big, and thus if the program at the read end of the pipe doesn’t read the data fast enough, the pipe buffer gets filled. If the pipe buffer is filled and the program that is writing tries to keep sending data down the pipe, that program will be blocked until the pipe buffer has some empty space.

It is much faster to send data to Evaluator than it is for Evaluator to process the data and send it back, so it’s reasonable to assume that any process sending data to Evaluator ...