Why setNeedsDisplay required when initWithNibName used - ios

Question

I have custom UIViewController class called MSPageViewController with and associated nib file. I have an IBOutlet which is a UIImageView called pageImage.
Now, I want to use this view controller in another UIViewController which will display a series of my custom MSPageViewController in a UIPageViewController. So, I use the following code:
// alloc and init my custom view controller
MSPageViewController *page1 = [[MSPageViewController alloc] initWithNibName:#"MSPageViewController" bundle:nil];
// I must call this or, the image that I set below will always be null
// why? I guess it's because the view hasn't been drawn yet because it hasn't been displayed, so I need to force the redraw - but this is my question. Is this is the right approach?
[page1.view setNeedsDisplay];
// set the image
page1.pageImage.image = [UIImage imageNamed:#"tutorialPage1.png"];
// make my array of view controllers, it expects an array because could be double-sided
NSArray *viewController = [NSArray page1];
// pass the array that contains my custom view controller
[self.pageController setViewControllers:viewController direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];
So am I doing this right? I have to force the redraw so that my outlets exist when I try to assign to them?

Answer 1

It's not the setNeedsDisplay part that you "need", it's the self.view part. By accessing the view property you are forcing the view controller to actually load the NIB. I guess that as a side effect of this, the pageImage property is populated as well (and was nil before you called self.view).
So, just calling self.view; instead of [self.view setNeedsDisplay]; should be enough.

Answer 2

As others have noted, pageImage (a UIImageView?) is likely not loaded from the nib yet when you're accessing it.
If you have a custom getter for pageImage, you could do the following:
- (UIImageView*) pageImage
{
[self view];
return _pageImage; // assuming the property backing ivar is synthesized as _pageImage.
}
But my personal preference would be to not expose the imageview itself and just expose a property for image. Then you can set the image to the viewcontroller regardless of it's loaded state, and internally set it to the imageView once the view loads:
- (void) setImage: (UIImage*) image
{
_image = image;
if ( self.isViewLoaded )
{
self.pageImage.image = image;
}
}
- (void) viewDidLoad
{
[super viewDidLoad];
self.pageImage.image = self.image;
}

Answer 3

If you moved some code to the viewDidLoad method you would be guaranteed that the view had been drawn.