class inheritance in objective C

Question

I have a parent class like this :

@interface SGBaseTableViewCell : UITableViewCell

+ (CGFloat)defaultHeight;
...

@end

The SGBaseTableViewCell is the parent class for all my custom UITablelViewCell

@implementation SGBaseTableViewCell : UITableViewCell

+ (CGFloat)defaultHeight {

    static CGFloat defaultHeight = 0.0;

     static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        SGBaseTableViewCell *cell = [[self class] newDefaultCell]; // newDefaultCell will just load the cell from a xib
        defaultHeight = cell.height;
    });
    return defaultHeight;
 }
@end

I would like that each custom cell will returns it's height. The problem of my code is that it will always return the same height for each cell ( will return the first cell's height).

Is there a solution that each cell will return it's height without overriding in the child class the defaultHeight method ?

PS : I know that i can override the defaultHieght method in each subclass to return the appropriate height, but i would like to know if i can do it juts in the base class ?

Tnaks

When you mean "the same height for each cell", you mean for each subclass of your cell or for each cell in a tableView (you may have cells with multi-lined labels) ? — tgyhlsb
– tgyhlsb, Commented Jul 2, 2014 at 9:48
What happens if you use UITableViewCell *cell = [[self class] newDefaultCell];? — trojanfoe
– trojanfoe, Commented Jul 2, 2014 at 9:51
@Tanguy of corse you can, in a class method self is the Class — samir
– samir, Commented Jul 2, 2014 at 9:55
Then I have no more idea :/ Sadly I always override my +(CGFloat)height in cell subclass. — tgyhlsb
– tgyhlsb, Commented Jul 2, 2014 at 12:08
@Tanguy i have answered my question, if you are interested :) — samir
– samir, Commented Jul 3, 2014 at 13:08

samir · Accepted Answer · 2014-07-03 13:07:38Z

1

My final solution (proposed in an other forum) :

+ (CGFloat)defaultHeight
{
    static dispatch_once_t onceToken;
    static NSMutableDictionary *heights;

    dispatch_once(&onceToken, ^{ heights = [NSMutableDictionary new]; });

    @synchronized(self)
    {
        NSString *key = NSStringFromClass(self);
        NSNumber *h = heights[key];
        if (h) return [h floatValue];
        SGBaseTableViewCell *cell = [self newDefaultCell];
        [heights setValue:@(cell.height) forKey:key];
        return cell.height;
    }
}

answered Jul 3, 2014 at 13:07

samir

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tgyhlsb · Accepted Answer · 2014-07-10 00:51:27Z

0

I found this article from RayWenderlich: Dynamic tableViewCell auto-layout

It seems to be a good alternative solution and you won't have to override any method for each subclass.

answered Jul 10, 2014 at 0:51

tgyhlsb

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