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Re: SMDoubleSlider usability on GNUstep


From: Fred Kiefer
Subject: Re: SMDoubleSlider usability on GNUstep
Date: Tue, 20 Feb 2018 16:17:27 +0100

David‘s analysis sounds correct to me. I will change that code when I am back 
home next week.

Fred

Unterwegs

> Am 20.02.2018 um 15:55 schrieb David Chisnall <address@hidden>:
> 
>> On 20 Feb 2018, at 14:30, Yavor Doganov <address@hidden> wrote:
>> 
>> I think this condition is always false.  _cell.has_valid_object_value
>> is NO and _object_value is nil.  So it jumps to NSCell.m:269 and
>> SMDoubleSliderCell's -stringValue is called which calls -stringHiValue
>> which in turn calls -doubleHiValue and from there the infinite
>> recursion is in place.
>> 
>> At least this is what I observe in the debugger.
> 
> Ah, that makes sense - I’d missed that in the trace.  So now the question is 
> what happens when you call -doubleValue on an NSCell in Cocoa when it has a 
> string value?  Here’s a little test program that finds out:
> 
> #import <Cocoa/Cocoa.h>
> 
> @interface Proxy : NSProxy
> {
>    @public
>    id obj;
> }
> @end
> 
> @implementation Proxy
> - (BOOL)respondsToSelector: (SEL)aSelector
> {
>    return [obj respondsToSelector: aSelector];
> }
> - (NSMethodSignature*)methodSignatureForSelector: (SEL)aSelector
> {
>    return [obj methodSignatureForSelector: aSelector];
> }
> - (void)forwardInvocation: (NSInvocation*)anInvocation
> {
>    NSLog(@"%@ set to proxy", anInvocation);
>    [anInvocation invokeWithTarget: obj];
> }
> @end
> 
> @interface Test : NSActionCell @end
> @implementation Test
> - (float)floatValue
> {
>    NSLog(@"-floatValue called\n");
>    return [super floatValue];
> }
> - (NSString*)stringValue
> {
>    NSLog(@"-stringValue called\n");
>    return [super stringValue];
> }
> @end
> 
> int main(void)
> {
>    @autoreleasepool
>    {
>        Test *t = [Test new];
>        Proxy *p = [Proxy alloc];
>        p->obj = @"0.23";
>        [t setObjectValue: t];
>        NSLog(@"Querying");
>        NSLog(@"%f", [t floatValue]);
>    }
> }
> 
> The output is:
> 
> 2018-02-20 14:46:39.917 a.out[85231:11731363] Querying
> 2018-02-20 14:46:39.917 a.out[85231:11731363] -floatValue called
> 2018-02-20 14:46:39.917 a.out[85231:11731363] 0.000000
> 
> 
> So, from this we learn that Cocoa’s NSCell implementation doesn’t call any 
> methods on either itself or the object to find the floating point value.  
> This is a bit odd, but at the very least we should fix GNUstep’s NSCell to 
> query the object and not itself to find the string value.  The correct fix is 
> probably:
> 
> - (float) floatValue
> {
>  if (_cell.has_valid_object_value == YES)
>  {
>    if ([_object_value respondsToSelector: @selector(floatValue)]))
>    {
>      return [_object_value floatValue];
>    }
>    if ([_object_value respondsToSelector: @selector(stringValue)]))
>    {
>      return [[_object_value stringValue] floatValue];
>    }
>  }
>  return 0;
> }
> 
> And apply similar fixes to the other *Value methods in NSCell.
> 
> You can hack around this brokenness by including the above method in a 
> category on NSCell in your application (though please remember to remove it 
> once GNUstep is fixed!).
> 
> David
> 
> 
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