Objective-c – How to wait past dispatch_async before proceeding

grand-central-dispatchobjective c

I have a series of dispatch_async that I am performing and I would like to only update the UI when they are all done. Problem is the method within dispatch_async calls something in a separate thread so it returns before the data is fully loaded and dispatch_group_notify is called before everything is loaded.

So I introduce a infinite loop to make it wait until a flag is set.
Is this the best way? See code below.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();

for (...) {
  dispatch_group_async(group, queue, ^{
    __block BOOL dataLoaded = NO;

    [thirdPartyCodeCallWithCompletion:^{
       dataLoaded = YES;
    }];

    // prevent infinite loop
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), 
    queue, ^{
          dataLoaded = YES;
    });

    // infinite loop to wait until data is loaded
    while (1) {
       if (dataLoaded) break;
    }
  }

  dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    //update UI
  });
}

Best Answer

You're already aware of dispatch groups. Why not just use dispatch_group_wait(), which includes support for a timeout? You can use dispatch_group_enter() and dispatch_group_leave() rather than dispatch_group_async() to make the group not done until the internal block for the third-party call with completion is finished.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();

for (...) {
  dispatch_group_enter(group);
  dispatch_async(queue, ^{
    [thirdPartyCodeCallWithCompletion:^{
       dispatch_group_leave(group);
    }];
  }
}

dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, NSECS_PER_SEC));
dispatch_async(dispatch_get_main_queue(), ^{
  //update UI
});

The use of dispatch_group_wait() does make this code synchronous, which is bad if run on the main thread. Depending on what exactly is supposed to happen if it times out, you could use dispatch_group_notify() as you were and use dispatch_after() to just updates the UI rather than trying to pretend the block completed.

Update: I tweaked my code to make sure that "update UI" happens on the main queue, just in case this code isn't already on the main thread.

By the way, I only used dispatch_async() for the block which calls thirdPartyCodeCallWithCompletion: because your original used dispatch_group_async() and I wasn't sure that the hypothetical method was asynchronous. Most APIs which take a completion block are asynchronous, though. If that one is, then you can just invoke it directly.

Compare method

Either you implement a compare-method for your object:

- (NSComparisonResult)compare:(Person *)otherObject {
    return [self.birthDate compare:otherObject.birthDate];
}

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];

NSSortDescriptor (better)

or usually even better:

NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
                                           ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];

You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.

Blocks (shiny!)

There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
    return [a.birthDate compare:b.birthDate];
}];

Performance

The -compare: and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor as the latter relies on KVC. The primary advantage of the NSSortDescriptor method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView by clicking on the header row.

Ios – How to make a UITextField move up when the keyboard is present – on starting to edit

You will only need a ScrollView if the contents you have now do not fit in the iPhone screen. (If you are adding the ScrollView as the superview of the components just to make the TextField scroll up when keyboard comes up, then it's not needed.)
The standard way to prevent the TextFields from being covered by the keyboard is to move the view up/down whenever the keyboard is shown.

Here is some sample code:

#define kOFFSET_FOR_KEYBOARD 80.0

-(void)keyboardWillShow {
    // Animate the current view out of the way
    if (self.view.frame.origin.y >= 0)
    {
        [self setViewMovedUp:YES];
    }
    else if (self.view.frame.origin.y < 0)
    {
        [self setViewMovedUp:NO];
    }
}

-(void)keyboardWillHide {
    if (self.view.frame.origin.y >= 0)
    {
        [self setViewMovedUp:YES];
    }
    else if (self.view.frame.origin.y < 0)
    {
        [self setViewMovedUp:NO];
    }
}

-(void)textFieldDidBeginEditing:(UITextField *)sender
{
    if ([sender isEqual:mailTf])
    {
        //move the main view, so that the keyboard does not hide it.
        if  (self.view.frame.origin.y >= 0)
        {
            [self setViewMovedUp:YES];
        }
    }
}

//method to move the view up/down whenever the keyboard is shown/dismissed
-(void)setViewMovedUp:(BOOL)movedUp
{
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:0.3]; // if you want to slide up the view

    CGRect rect = self.view.frame;
    if (movedUp)
    {
        // 1. move the view's origin up so that the text field that will be hidden come above the keyboard 
        // 2. increase the size of the view so that the area behind the keyboard is covered up.
        rect.origin.y -= kOFFSET_FOR_KEYBOARD;
        rect.size.height += kOFFSET_FOR_KEYBOARD;
    }
    else
    {
        // revert back to the normal state.
        rect.origin.y += kOFFSET_FOR_KEYBOARD;
        rect.size.height -= kOFFSET_FOR_KEYBOARD;
    }
    self.view.frame = rect;

    [UIView commitAnimations];
}


- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    // register for keyboard notifications
    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillShow)
                                             name:UIKeyboardWillShowNotification
                                           object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(keyboardWillHide)
                                             name:UIKeyboardWillHideNotification
                                           object:nil];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    // unregister for keyboard notifications while not visible.
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                             name:UIKeyboardWillShowNotification
                                           object:nil];

    [[NSNotificationCenter defaultCenter] removeObserver:self
                                             name:UIKeyboardWillHideNotification
                                           object:nil];
}

Best Answer

Related Solutions

Ios – How to sort an NSMutableArray with custom objects in it

Compare method

NSSortDescriptor (better)

Blocks (shiny!)

Performance

Ios – How to make a UITextField move up when the keyboard is present – on starting to edit

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