iOS Helping understand management of Views (& release)

[dantastic]

I just came across this *problem* as I created a UIView with a date picker and a uitoolbar. I'd have the view slide up and down the screen. Not exactly sure how to go about this I found some code on dem internets and did what any budding coder does, cmd+c, cmd+v.

After some tuning to suit my needs the code works perfectly.

Being a good memory citizen I went on the usual quest to make sure there were no lose ends when I started asking myself - Why does this work???

@interface {
	UIDatePicker *datePicker;
	UIView *pickerView;
}
@property (nonatomic, retain) UIDatePicker *datePicker;
@property (nonatomic, retain) UIView *pickerView;

...

- (void) loadDatePicker {

	datePicker = [[UIDatePicker alloc] initWithFrame:CGRectMake(0, 44, 320, 250)];  
	[datePicker addTarget:self
	               action:@selector(changeDate:) forControlEvents:UIControlEventValueChanged];
    ...
	UIToolbar *toolBar = [[UIToolbar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)];
   ...
	pickerView = [[UIView alloc] initWithFrame:CGRectMake(0, 480, 320, 260)];

	[pickerView addSubview:toolBar];
	[pickerView addSubview:datePicker];
	[toolBar release];
	[datePicker release];
	[self.view addSubview:pickerView];
	[pickerView release];

So far so good, I get what is happening here. I clean up all the objects I allocate and I simply dump a view into the subview. In my understanding, the contents of this subview should be somewhat orphaned from the objects that was used to create them? Here's what I don't get:

- (void) changeDate:(id)sender {
	// should this not crash my app??
	cellLabel.text =  [datePicker date];	
}

If I have already released 'datePicker', howcome the above works?
What is the correct way of doing this?

 

[robbieduncan]

If I have already released 'datePicker', howcome the above works?
What is the correct way of doing this?

This is the problem with copy/pasting code: you have no idea what you have, how it works and have learnt nothing. The answer, in this case, is that when you add a view to another view using addSubview the superview retains it's children.

 

[dantastic]

Ok, so even though released the reference to 'datePicker' is still perfectly valid until I call my function to clean up the subviews:

  ...
		if (subview.tag == 78) {
			for (UIView *subSubview in [subview subviews]) {
				[subSubview removeFromSuperview];
			}
			[subview removeFromSuperview];
		}

Will this properly get rid of the references for me or do I have to do anything else as well?

Should I be releasing where I am or should I release when I remove the subviews?

 

[robbieduncan]

Removing the subview will cause the parent view to release it. If you have a variable pointing at it (and this release by the parent causes the retain count to drop to 0) then your pointer will end up pointing at freed memory: bad.

 

[dantastic]

Right, So I'm actually OK so.

I've an OK button that when called slides the uiview out of view and then removes all subviews. I don't reference any of these fellas again until a new instance of the entire viewcontroller is allocated.

Cheers mate!