An example of using -[NSCondition waitUntilDate:].

NSCondition-WaitUntilDate-Example.m

//
//  NSCondition-WaitUntilDate-Example.m
//  NSConditionExample
//
//  Created by Prachi Gauriar on 12/23/2013.
//  Copyright (c) 2013 Prachi Gauriar.
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//  THE SOFTWARE.
//

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[])
{
    const NSTimeInterval QLCTimeoutInterval = 4.0;
    
    @autoreleasepool {
        dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_group_t group = dispatch_group_create();
        
        NSCondition *condition = [[NSCondition alloc] init];
        __block BOOL predicate = NO;
        
        // Block that sets the predicate and then signals
        dispatch_group_async(group, queue, ^{
            // Sleep time is between 1 and 2*QLCTimeInterval seconds
            uint32_t sleepTimeInterval = arc4random_uniform(2 * QLCTimeoutInterval) + 1;
            NSLog(@"Sleeping for %u seconds.", sleepTimeInterval);
            sleep(sleepTimeInterval);
            
            [condition lock];
            NSLog(@"Signaling.");
            predicate = YES;
            [condition signal];
            [condition unlock];
        });
        
        // Block that waits for the predicate to be set
        NSDate *startDate = [NSDate date];
        NSDate *timeoutDate = [NSDate dateWithTimeIntervalSinceNow:QLCTimeoutInterval];
        dispatch_group_async(group, queue, ^{
            NSLog(@"Waiting for signal.");
            BOOL signaled = NO;
            
            [condition lock];
            // Keep waiting until either the predicate is true or we timed out
            while (!predicate && (signaled = [condition waitUntilDate:timeoutDate])) {
            }
            
            NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];
            [condition unlock];
            
            NSLog(@"%@ after %.2f seconds.", signaled ? @"Signaled" : @"Timed out", elapsedTime);
        });
        
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    }
    
    return 0;
}