R – When are released objects finally destroyed

deallocmemoryobjective cpool

When you release an object in Objective-C (assuming its release count is 1) its release count is decremented to 0 and the dealloc method called. Is the object destroyed right there and then after the [super dealloc], or is it added to the pool and destroyed when the pool is drained?

I would assume that released objects are destroyed at the end of dealloc (when [super dealloc] is called) I know autorelease variables are added to the pool, just wanted to be sure what happens to normal released objects.

cheers -gary-

Best Answer

First off, Objective-C the programming language does not have any concept of memory management. Memory management is built into Foundation (the common framework of Cocoa for Mac OS X and Cocoa Touch on iPhone OS). Foundation adds a rootclass NSObject that implements alloc, retain, release and autorelease as convenience wrappers on top of class_createInstance() and object_dispose() functions from the Objective-C run-time.

Since Objective-C is memory management agnostic, adding garbage collection and making all memory management methods on NSObject no-ops was quite easy. But there is no garbage collection on iPhone OS and legacy Mac OS X and there we instead use a reference counting scheme in Cocoa.

An object is created when calling the alloc class method on NSObject or NSProxy from Foundation. These default implementations will call class_createInstance() so that you never need to manually.

An object "dies" when dealloc is run on the root class NSObject. This is when the memory for the object on the heap is released by calling object_dispose(), you will never need to call this function yourself as long as you inherit from NSObject or NSProxy from Foundation.

Autoreleased objects are not treated any special as far as the run-time is concerned, an autoreleased object is just as alive as any other object. What happens when you autorelease an object is approximately;

-(id)autorelease; {
  [NSAutoreleasePool addObject:self];  // retain count +1
  [self release];                      // retain count -1
  return self;
}

Calling autorelease will not decrement the retain count, it will just transfer ownership of the object from the caller to the current autorelease pool. Later when the current autorelease pool goes away, it will call release on all of the objects it owns, and any object no longer owned by anything else is released.

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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