Foo.m

@interface Foo()
@property(nonatomic, strong) dispatch_group_t lock;
@end

@implementation Foo

-(id)init
{
    self = [super init];
    
    if(!self){
        return nil;
    }
    
    _lock = dispatch_group_create();
    dispatch_group_enter(_lock);
    //do work
    dispatch_group_leave(_lock);
    
return self;
}

-(void)someMethod
{
    dispatch_group_wait(self.lock, DISPATCH_TIME_FOREVER);
    
}

@end