Custom UITableViewCell?

[samurle]

I'm trying to create a custom UITableViewCell, but it crashes (EXC_BAD_ACCESS) after cellForRowAtIndexPath is called.
Is there anything else required when returing a custom cell?

@interface MyTableViewCell : UITableViewCell {
    UILabel *myLabel1;
    UILabel *myLabel2;
}
@property (nonatomic,retain) UILabel *myLabel1;
@property (nonatomic,retain) UILabel *myLabel2;
@end

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

  static NSString *cellIdentifier = @"MyCell";

  MyTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

  if (cell == nil) {
     cell = [[MyTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];
  }

  cell.myLabel1.text = @"One";
  cell.myLabel2.text = @"Two";

  return cell;
}

I don't understand why it crashes when I use MyTableViewCell instead of UITableViewCell.
Isn't this the correct way to use custom cells?

But, it seems kind of odd it should work anyway, since the method is returning UITableViewCell.

[GFish]

Originally Posted by samurle

I'm trying to create a custom UITableViewCell, but it crashes (EXC_BAD_ACCESS) after cellForRowAtIndexPath is called.
Is there anything else required when returing a custom cell?

@interface MyTableViewCell : UITableViewCell {
    UILabel *myLabel1;
    UILabel *myLabel2;
}
@property (nonatomic,retain) UILabel *myLabel1;
@property (nonatomic,retain) UILabel *myLabel2;
@end

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

  static NSString *cellIdentifier = @"MyCell";

  MyTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

  if (cell == nil) {
     cell = [[MyTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];
  }

  cell.myLabel1.text = @"One";
  cell.myLabel2.text = @"Two";

  return cell;
}

I don't understand why it crashes when I use MyTableViewCell instead of UITableViewCell.
Isn't this the correct way to use custom cells?

But, it seems kind of odd it should work anyway, since the method is returning UITableViewCell.

In the

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

you need to load the CustomCell you created... something like this...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    
    static NSString *CellIdentifier = @"Cell";
	
	MyTableViewCell *cell = (MyTableViewCell *) [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
	
    if (cell == nil) {
		NSArray *topLevelObjects = [[NSBundle mainBundle] loadNibNamed:@"MyTableViewCell" owner:nil options:nil];
		
		for (id currentObject in topLevelObjects) {
			if ([currentObject isKindOfClass:[UITableViewCell class]]) {
				cell = (MyTableViewCell *) currentObject;
				break;
			}
		}		
    }
    
    // Configure the cell.
    cell.MyLabel1.text = @"One";
    cell.MyLabel2.text = @"Two";
	
    return cell;
}

should work fine...

[samurle]

Originally Posted by GFish

In the... you need to load the CustomCell you created... something like this...
should work fine...

Thanks, but I should also mention I'm not using NIBs or the Interface Builder.
This is all being done programmatically.

And the crash occurs after the method cellForRowAtIndexPath has returned, and not in any
specific place while the method is being called.

But, after much frustration, I did manage to find the bug after I turned on Zombie Objects for
first time. The bug was in my custom initWithStyle method, where I allocate both custom labels.

For some reason, I had to use "self.myLabel1 = variable", instead of "myLabel1 = variable".
I don't understand why these two lines aren't the same. I synthesized both variables:

@synthesize myLabel1;
@synthesize myLabel2;

Is there some Objective-C rule stating that I can't use synthesized variables (setters and getters)
in constructors (initializers)?

[Duncan C]

Originally Posted by samurle

Thanks, but I should also mention I'm not using NIBs or the Interface Builder.
This is all being done programmatically.

And the crash occurs after the method cellForRowAtIndexPath has returned, and not in any
specific place while the method is being called.

But, after much frustration, I did manage to find the bug after I turned on Zombie Objects for
first time. The bug was in my custom initWithStyle method, where I allocate both custom labels.

For some reason, I had to use "self.myLabel1 = variable", instead of "myLabel1 = variable".
I don't understand why these two lines aren't the same. I synthesized both variables:

@synthesize myLabel1;
@synthesize myLabel2;

Is there some Objective-C rule stating that I can't use synthesized variables (setters and getters)
in constructors (initializers)?

self.myLabel1 = variable;

and

myLabel1 = variable;

are indeed different. The first form uses the property. That code ends up calling the setter method for the property. If myLabel1 is set up as a retained property, the setter will retain the value assigned to it.

The second form does a simple assignment to the instance variable.

When you create a property for an ivar, you should really use the property to access the variable and stop using the ivar directly.

[samurle]

Originally Posted by Duncan C

self.myLabel1 = variable;

and

myLabel1 = variable;

are indeed different. The first form uses the property. That code ends up calling the setter method for the property. If myLabel1 is set up as a retained property, the setter will retain the value assigned to it.

The second form does a simple assignment to the instance variable.

When you create a property for an ivar, you should really use the property to access the variable and stop using the ivar directly.

Thanks, but I think I still misunderstand some fundamental fact about synthesized variables.

So, you're saying I can't use them for setting?

I thought that was the reason why I synthesize them in the first place.

[Duncan C]

Originally Posted by samurle

Thanks, but I think I still misunderstand some fundamental fact about synthesized variables.

So, you're saying I can't use them for setting?

I thought that was the reason why I synthesize them in the first place.

The @synthesize directive asks the compiler to create a getter and a setter for a property you defined. That's all it does.

The statement:

[/code]

myLabel1 = variable;

changes the value of the underlying instance variable without invoking the setter. The code

[/code]

self.myLabel1 = variable;

on the other hand, invokes the setter, and does whatever housekeeping the setter is set up to do (including retaining the new object, if the property is a retained property.)

If you create a property, you should use the self.property syntax (or the alternate [self property] and [self setProperty: value] syntax) and not refer to the instance variable directly, unless you know what you're doing and why.

Note that if you are writing a custom setter or getter (instead of using the @synthesized version) then you have to refer to the instance variable directly, to avoid endless recursion. That's an exception. There are a few more, but until you have a reason to access the instance variable directly, and know why you're doing it, don't.

[samurle]

Thanks, I have a better understanding now.