Question

Seems these are the only methods to put a NSThread to sleep

* sleepForTimeInterval:

* sleepUntilDate:

Would it be bad practice what I am asking?

Answer 1

Do you want your thread to just stop until some other thread tells it to start up again? If so, you could use an NSConditionLock. An NSConditionLock is similar to a condition variable. It has a couple of basic methods, lockWhenCondition, and unlockWithCondition, and lock. A typical usage is to have your background thread waiting on the condition lock with “lockWhenCondition:”, and the in you foreground thread to set the condition, which causes the background thread to wake up. The condition is a simple integer, usually an enumeration.

Here’s an example:

enum {
    kWorkTodo = 1,
    kNoWorkTodo = 0
}

- (id)init {
    if ((self = [super init])) {
        theConditionLock = [[NSCoditionLock alloc] initWithCondition: kNoWorkTodo];
        workItems = [[NSMutableArray alloc] init];
    }
}

- (void)startDoingWork {
    [NSThread detachNewThreadSelector:@selector(doBackgroundWork) toTarget:self withObject:nil];
}

- (void)doBackgroundWork:(id)arg {
    while (YES) {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSArray *items = nil;
        [theConditionLock lockWhenCondition:kWorkTodo]; // Wait until there is work to do
        items = [NSArray arrayWithArray:workItems]
        [workItems removeAllObjects];
        [theConditionLock unlockWithCondition:kNoWorkTodo];
        for(id item in items) {
            // Do some work on item.
        }
        [pool drain];
    }
}

- (void)notifyBackgroundThreadAboutNewWork {
    [theConditionLock lock];
    [workItems addObject:/* some unit of work */];
    [theConditionLock unlockWithCondition:kWorkTodo];
}

In this example, when startDoingWork is called doBackgroundWork: will start on a background thread, but then stop because there isn’t any work to do. Once notifyBackgroundThreadAboutNewWork is called, then doBackgroundWork: will fire up and process the new work, and then go back to sleep waiting for new work to be available, which will happen the next time notifyBackgroundThreadAboutNewWork is called.