Ios – How to create a deadlock in Grand Central Dispatch

grand-central-dispatchiosobjective cxcode

In Apple docs, it says:

Important: You should never call the dispatch_sync or dispatch_sync_f
function from a task that is executing in the same queue that you are
planning to pass to the function. This is particularly important for
serial queues, which are guaranteed to deadlock, but should also be
avoided for concurrent queues.

How do you write the code to do exactly this?

Best Answer

An intentional deadlock on a certain queue:

dispatch_queue_t queue = dispatch_queue_create("my.label", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
    dispatch_sync(queue, ^{
        // outer block is waiting for this inner block to complete,
        // inner block won't start before outer block finishes
        // => deadlock
    });

    // this will never be reached
});

It's clear here that the outer and inner blocks are operating on the same queue. Most cases where this will occur is in places where it's less obvious what queue the caller of the dispatch_sync is operating on. This usually occurs in a (deeply) nested stack where you're executing code in some class that was originally launched on a certain queue, and by accident you call a dispatch_sync to the same queue.

Related Solutions

Ios – How to change the name of an iOS app

Go to Targets in Xcode.
Build Settings on your project's target (your current development name).
Search for Product Name under Packaging. Change its value to what you want your new project name to be.

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

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

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