How to traverse the class hierarchy through a variable assignment in Cocoa Touch?

Question

I have created a UIViewController class called MyViewController with a UIImageView in its XIB file. I then import this class into another class. I make an instance of the class and I change the image in the UIImageView using code:

myViewController.myImageView.image = [UIImage imageNamed:@"myImage.png"];

This works swimmingly. My app is essentially an image viewer. I wanted to cache next and previous images by preloading a subview with an image. When I place myViewController into a variable like this:

UIViewController *pager = myViewController;

And attempt to use the variable to set the image for the UIImageView like this:

pager.myImageView.image = [UIImage imageNamed:@"myImage.png"];

I get an "error: request for member 'myImageView' in something not a structure or union". I've tried doing it using square brackets:

[[[pager myImageView] setImage:[UIImage imageNamed:@"myImage.png"]];

I get "warning 'UIViewController' may not respond to '-myImageView' ". How do I access the hierarchy to get to myImageView? I've used a @class in the header and a #import and I've synthesized the class instance. The only solution I have so far is a hack, by doing this:

UIImageView *pager = myViewController.myImageView;
pager.image = [UIImage imageNamed:@"myImage.png"];

Which doubles the amount of variables I have, one for the UIImageView variable, and one for the UIViewController variable. Thanks in advance to anyone that can help out.

Answer 1

quick answer:

You'll need to change the line:

UIViewController *pager = myViewController;

to:

MyViewController *pager = myViewController;

To be able to access the custom properties you added in your MyViewController class.

explanation:

Your MyViewController class probably looks something like this (simplified):

// MyViewController.h:
@interface MyViewController : UIViewController
{
    UIImageView * myImageView;
}
@property (readonly) UIImageView * myImageView;
@end

// MyViewController.m:
@implementation MyViewController
@synthesize myImageView;
@end

You have extended the UIViewController class by adding a member variable and a property (myImageView) to the original class.

When you use a pointer to the base class (UIViewController) to access your variable, you are telling the compiler to treat that variable as if it was a plain old UIViewController, and a plain old UIViewController has no concept of a myImageView. This is what causes the compiler errors.