Binding: Instance variable loses value.

Question

Hi all,
Just making my first steps into Objective-C, I've done a lot of C, C++ and C# on win & linux. Anyhow, I've been hacking away happily and discovered a peculiar behaviour, and I'm not sure if it's my code or some obscure bug in Xcode.
1. I have a Check Box(NSButton) directly bound to a BOOL instance variable called "checkValue" using KVC. The containing class is a custom NSView subclass.
2. I manually implemented the setter according to KVC naming rules "setCheckValue".
3. When debugging the UI (check box) calls the setter perfectly with the correct value, which shows up in the NSLog output.
4. I hooked up the action for the check box as well, and inside the action handler the instance variable reports the value correctly. Everything looks fine.
5. Now here's the rub. I put a mousedown event handler into the class as well, and it is called flawlessly when I click on the custom view. However, "checkValue" does not report the correctly set value.
So, how can a class instance variable, which is set and reports correctly elsewhere in the same scope, all of the sudden take on a nonsense value in a event handler? If a variable has a value set, it should be the same everywhere within the same scope!
Below is simplified code extracted from the original project for clarity. It produces exactly the same behaviour as the more complex project without the distracting code.
#import "CheckBoxHandler.h"
@implementation CheckBoxHandler
@synthesize checkValue;
//Apparently this is required for binding in NSView based classes
//The same behaviour occurs even if you remove this.
+(void)initialize
[self exposeBinding:@"checkValue"];
-(id)initWithFrame:(NSRect)frameRect
self = [super initWithFrame:frameRect];
if(!self)
return nil;
[self setCheckValue:YES];
return self;
//manual impelementation of KVC setter
-(void)setCheckValue:(BOOL)v
checkValue = v;
NSLog(@"Setter Called: checkValue is %d", checkValue);
//The action works fine as evidenced by Log output
//Check and uncheck the box a few times.
-(IBAction)checkBoxAction:(id)sender
NSLog(@"UI State:%d Ivar State: %d", [checkBox state], checkValue);
//Mouse clicking on the view calls into this event.
//The problem is the value reported by [self checkValue] OR checkValue directly
//do not match the UI state. In fact it always reports some nonsense value.
-(void)mouseDown:(NSEvent *)event
NSLog(@"Inside Mousedown: checkValue is: %d", [self checkValue]);
@end
I have a workaround, but it really bends my head when something doesn't behave as expected!

Answer 1

I have synthesized against "checkValue", although I manually implemented the setter so I could set a breakpoint and observe the value. I thought [self checkValue] and self.checkValue both simply call the same getter, just different syntax.
The setting of the value using the UI checkbox happens outside of the event call chain. I can check and uncheck the checkbox and the value changes correctly. The checkbox is in a separate space on the window, so clicking it does not activate mousedown on the view, this is correct behaviour. Clicking on the view does activate the mousedown event, this is done after I've set the checkbox so the value has been set well before the mousedown event. Thus it should already have the correct value in the event.
For completeness this is the interface file:
#import <Cocoa/Cocoa.h>
@interface CheckBoxHandler : NSView {
IBOutlet NSButton *checkBox;
BOOL checkValue;
-(IBAction)checkBoxAction:(id)sender;
@property (assign, readwrite) BOOL checkValue;
@end
I'll change everything to self.checkValue just for the excercise. Ok, done. checkValue still reports a value of 1 in the mousedown event regardless of the value set elsewhere.