ObjC: ARC releasing dynamically created view controller too early - ios

Question

I have a view controller, named AllThingsViewController that dynamically creates other view controllers, named ThingViewController, and adds their top level view to a UIScrollView. (I'm writing proprietary code so I've changed the names of my classes, but the structure of my code is exactly the same.)
Here's what its loadView method contains:
NSArray *things = [[ThingDataController shared] getThings];
if ([things count] == 0) {
// code in this block is not relevant as it's not being executed...
} else {
for(unsigned int i = 0; i < [things count]; ++i) {
ThingViewController *thingViewController = [[ThingViewController alloc] init];
[thingViewController loadView];
[scrollView addSubview:thingViewController.topView];
thingViewController.topView.frame = CGRectNewOrigin(thingViewController.topView.frame,
0, thingViewController.topView.frame.size.height*i);
[thingViewController displayThing:thing[i]];
}
}
ThingViewController's loadView method looks like this:
- (void)loadView
{
NSArray *topLevelObjs = nil;
topLevelObjs = [[NSBundle mainBundle] loadNibNamed:#"ThingView" owner:self options:nil];
if (topLevelObjs == nil)
{
NSLog(#"Error: Could not load ThingView xib\n");
return;
}
}
When my app starts up everything displays correctly, until I try to tap one of the buttons that exists in the xib being loaded by ThingViewController, at which point it crashes due to an exception: "unrecognized selector sent to instance". It seems that ARC is releasing my ThingViewController instances too early.
Looking at my code, I figured it was because they weren't being held on to anything, so I created an NSMutableArray as an instance variable in my AllThingsViewController class, and started adding the ThingViewControllers to it thusly:
NSArray *things = [[ThingDataController shared] getThings];
if ([things count] == 0) {
// not being executed...
} else {
for(unsigned int i = 0; i < [things count]; ++i) {
ThingViewController *thingViewController = [[ThingViewController alloc] init];
[thingViewController loadView];
[scrollView addSubview:thingViewController.topView];
thingViewController.topView.frame = CGRectNewOrigin(thingViewController.topView.frame,
0, thingViewController.topView.frame.size.height*i);
[thingViewController displayThing:thing[i]];
[allThingsViewControllers addObject:thingViewController];
}
}
However, it didn't change anything, even though those objects are being added to the array. Finally, just to confirm that this is ARC releasing it early, I changed "thingViewController" to be an instance variable in AllThingsViewController and changed:
ThingViewController *thingViewController = [[ThingViewController alloc] init];
to be:
thingViewController = [[ThingViewController alloc] init];
Sure enough, the last item in the scrollable list doesn't crash when I tap its buttons, but the other ones do, because its ThingViewController isn't being deallocated.
I'm still relatively new to ARC, but after a bunch of Googling I have no idea how to fix this. What do I do?

Answer 1

Couple of things.
Problem 1:
This looks like the cause of your bug:
[allBillViewControllers addObject:billViewController];
It should be:
[allBillViewControllers addObject:thingViewController];
Right?
Problem 2
You are not properly adding the view controller to your view hierarchy. It should be this:
[self addChildViewController:childViewController];
[childViewController.view setFrame:targetFrame];
[scrollView addSubview:childViewController.view];
[childViewController didMoveToParentViewController:self];
And similarly when removing a child view controller:
[childViewController willMoveToParentViewController:nil];
[childViewController.view removeFromSuperview];
[childViewController removeFromParentViewController];
Problem 3
Never call loadView explicitly on a view controller. It gets called by UIKit whenever you access the view property of a view controller.
Problem 4
You must add the view of the child view controller to your scroll view, not an arbitrary subview topView in its view hierarchy. Refactor your ThingViewController class to make this simpler for yourself. :-)

Answer 2

Let's look at your code:
for(unsigned int i = 0; i < [things count]; ++i) {
ThingViewController *thingViewController = [[ThingViewController alloc] init];
[thingViewController loadView];
[scrollView addSubview:thingViewController.topView];
thingViewController.topView.frame = CGRectNewOrigin(thingViewController.topView.frame,
0, thingViewController.topView.frame.size.height*i);
[billViewController displayThing:thing[i]];
[allBillViewControllers addObject:billViewController];
}
After each loop of the for loop executes, nothing will have a strong reference to the ThingViewController. Thus, it gets released and destroyed.
If ThingViewController is a subclass of UIViewController, then it should be made a "child view controller" of the scrollview's view controller. I recommend reading the section on from the View Controller Programming Guide on creating custom container view controllers (i.e., a view controller that encapsulates and displays other view controllers).