CODEWITHSUNDEEP
iphoneiossynchronizationgrand-central-dispatch
ivanzoid
ivanzoid

Reputation: 6062

Wait for multiple events in iOS?

What is equivalent of Win32's WaitForMultipleObjects function in iOS?

This is roughly what I want:

NSCondition *condition1;
NSCondition *condition2;
NSCondition *condition3;

wait_for_conditions([NSArray arrayWithObjects: condition1, condition2, condition3, nil], 
^{
    // Some code which must be executed when all conditions were fired
});

// in some other places of program:
[condition1 signal];

// ...
[condition2 signal];

// ...
[condition3 signal];

What are the ways to accomplish this in iOS?

Edit: I'm not bound to usage of NSCondition, any other synchronization things will be ok (I've just googled and found NSCondition).

Upvotes: 2

Views: 1540

Answers (3)

ivanzoid
ivanzoid

Reputation: 6062

OK, I ended up using my own solution using GCD's groups of blocks notifications. I've also used serial queue (instead of concurrent) which guarantees we create only 1 additional thread.

@interface WaitCondition : NSObject
    - (void) fire;
@end

@implementation WaitCondition {
    BOOL fired;
    NSCondition *condition;
}

- (id) init
{
    if ((self = [super init])) {
        condition = [NSCondition new];
    }
    return self;
}

- (void) fire
{
    [condition lock];
    [condition signal];
    fired = YES;
    [condition unlock];
}

- (void) wait
{
    [condition lock];
    while (!fired) {
        [condition wait];
    }
    [condition unlock];
}

@end

void Dispatch_NotifyForConditions(NSArray *conditions, dispatch_block_t completion)
{
    dispatch_queue_t queue = dispatch_queue_create("notify_queue", NULL);
    dispatch_group_t group = dispatch_group_create();

    for (WaitCondition *condition in conditions)
    {
        NSCAssert([condition isKindOfClass:[WaitCondition class]], @"Dispatch_NotifyForConditions: conditions must contain WaitCondition objects only.");
        dispatch_group_async(group, queue, ^{
            [condition wait];
        });
    }

    dispatch_group_notify(group, queue, completion);

    dispatch_release(queue);
    dispatch_release(group);
}

Upvotes: 0

Richard J. Ross III
Richard J. Ross III

Reputation: 55583

What about this?

void async(void (^block)())
{
    [NSThread detachNewThreadSelector:@selector(invoke) toTarget:[block copy] withObject:nil];
}

__attribute__((sentinel(NULL)))
void wait_for_conditions(void (^block)(), NSCondition *condition, ...)
{
    va_list args;
    va_start(args, condition);

    NSMutableArray *conditions = [NSMutableArray array];

    do {
        [conditions addObject:condition];
    } while ((condition = va_arg(args, NSCondition *)));

    va_end(args);

    NSCondition *overallCondition = [NSCondition new];

    for (int i = 0; i < conditions.count; i++) {
        NSCondition *cond = [conditions objectAtIndex:i];

        async(^{
            [cond lock];
            [cond wait];
            [cond unlock];

            [overallCondition lock];
            [overallCondition signal];
            [overallCondition unlock];
        });
    }

    for (int i = 0; i < conditions.count; i++) {
        [overallCondition lock];
        [overallCondition wait];
        [overallCondition unlock];
    }

    if (block)
        block();
}

Obviously this has the drawback of effectively doubling your threads, but I don't know if there is an easier way of accomplishing this.

Upvotes: 1

CReaTuS
CReaTuS

Reputation: 2585

You can create a NSNotifications for each condition with unique notification name. Then for each notification will call the same function.

Upvotes: 1

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