Scratch Pad

Proper use of threads will do wonders for your program’s responsiveness. It will also do wonders in making your code harder to follow. What if you have a thread churning along in the background and you’d like to know when that thread has produced a value before continuing on with your work? Maybe you have a thread called ComputeWinningStockThread that takes all the data from the stock market, applies some magic formula and spits out stocks that are going to make you millions in random time intervals.

1. Start ComputeWinningStockThread in background.
2. Until ComputeWinningStockThread has picked a stock, continue checking to see if it has.

This is the poor man’s version. It is also called the busy loop or polling. And it uses up valuable resources that your iPhone could be using for other things.

The proper way to handle this is with the NSConditionLock. You can use NSConditionLock to have one thread wait for notification from another thread before continuing. Or, in other words, it blocks until a condition is met (hence, the name conditional lock).

In Objective-C, this is the gist of what you would have to do.

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- (void) ComputeWinningStockThread {
    while (YES) {
        [myLock lock];
        winningStock = [self someSecretAlgorithm];        
        [myLock unlockWithCondition:0];
    }
}

Then maybe in another message, you are calling this thread to earn your millions.

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NSConditionLock *myLock = [[NSConditionLock alloc] initWithCondition:0];
[NSThread detachNewThreadSelector:@selector(ComputeWinningStockThread) toTarget:self withObject:nil];
 
// This will block until it is unlocked in the thread
[myLock lockWhenCondition:1];
NSLog(@"I have a winning stock! %@", winningStock);

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Development, Objective-C, iPhone Development Busy Loop, iPhone, Multithreading, NSConditionLock, NSThread, Polling, Threading, Waiting