Question

I want to reuse the same UIView object as a table header in different UIViewControllers.

I have an ArticleViewController class;

@interface ArticleViewController : UIViewController {
    UIView *headerView;
}

- (UIView *)headerView;

@end

I then implement the headerView get accessor to lazy load the object;

#import "ArticleViewController.h"

@implementation ArticleViewController

- (UIView *)headerView {

    if(headerView)
        return headerView;

    float w = [[self view] bounds].size.width;

    CGRect headerFrame = CGRectMake(0, 0, w, 64);
    CGRect labelFrame = CGRectMake(8, 8, w - 16, 48);

    UILabel *headerText = [[UILabel alloc] initWithFrame:labelFrame];
    [headerText setNumberOfLines:0];
    [headerText setFont:[UIFont boldSystemFontOfSize:14.0]];
    [headerText setBackgroundColor:[UIColor groupTableViewBackgroundColor]];

    headerView = [[UIView alloc] initWithFrame:headerFrame];
    [headerView setBackgroundColor:[UIColor groupTableViewBackgroundColor]];
    [headerView addSubview:headerText];

    return headerView;

}

@end

In another view controller I want to reuse the same headerView object so I’ve declared my interface thus;

@interface CitationViewController : UIViewController {
    UIView *headerView;
}

@property (nonatomic, retain) UIView *headerView;

@end

I use [citationViewController setHeaderView:headerView]; to assign my headerView within ArticleViewController before I push the citationViewController onto my UINavController.

Everything works fine and I get the same header showing when the new view loads. The problem is when I pop the CitationViewController off the UINavController and go back to the old view I’ve lost the headerView object in ArticleViewController.

I’ve tried just passing a pointer to the pointer but I was having trouble getting the **UIView and &headerView to compile. I thought I could just have one object in memory and both views have their own pointers to it. I didn’t get far and went to find another way.

I use the view in a header for a UITableView and I thought it might be this problem; UITableView section header and section footer not updating (redraw problem) but reloading data didn’t fix it.

I then realised that I haven’t incremented the retain count for the headerView so when I pass it over to the CitationViewController and that decrements the retain count I’ll dealloc the object. So then I added a [headerView retain] call before calling the setter but that doesn’t seem to keep the header showing when I reload the old view.

Do I need to have some kind of retain pattern in my get accessor? The custom getter examples always have primative types or simple objects, is the problem because there is another UIView object within headerView as a subview?

I also considered changing the @property (retain) to have assign or copy but got stuck as I don’t implement the copyWithZone protocol and wasn’t sure how to.

I’ve been reading on all these different aspects and understand them in isolation but don’t seem to be able to unite them as a coherent whole. What have I misunderstood in all this?

Answer 1

Each view object can only ever appear in one view at a time. When you add a view to another view, it will automatically be removed from any other view it’s in.

You either need to have one instance of headerView per view, or add extra code to ensure the single object is moved around between views. The former would be the common approach (most view objects don’t take up enough memory to worry about) while the latter would be highly unusual.