Question

Can someone please educate me why the following does not work? The button never gets set to selected.

[self.boldButton setSelected:isBold];

If I replace the above with an if else statement it works fine. I can also change the setSelected values to 1 or 0, instead of YES or NO and it still works fine.

if (isBold)
{
    [self.boldButton setSelected:YES];
}
else
{
    [self.boldButton setSelected:NO];
}

So I have a working project, but I don’t understand why these two implementations don’t deliver the same results. Thanks.

FWIW – I test for bold with another method. Though if the test were flawed, I don’t see how the second approach could work, while the first still doesn’t.

- (BOOL)isBold
{
    CTFontRef fontRef = (CTFontRef)CFBridgingRetain(self);
    CTFontSymbolicTraits symbolicTraits = CTFontGetSymbolicTraits(fontRef);
    return (symbolicTraits & kCTFontTraitBold);
}

Answer 1

BOOL is defined like this in <objc/objc.h>:

typedef signed char     BOOL;

That means a BOOL can actually hold any value in the range -128 through 127 (inclusive).

-[UIControl setSelected:] works roughly like this:

#define kSelectedBitPosition 10
#define kSelectedBit (1 << kSelectedBitPosition)

- (void)setSelected:(BOOL)selected {
    if (((self->_controlFlags >> kSelectedBitPosition) & 1) == selected) {
        return;
    } else {
        self->_controlFlags = (self->_controlFlags & ~kSelectedBit)
            | ((selected & 1) << kSelectedBitPosition);
        [self setNeedsDisplay];
    }
}

(I disassembled the simulator version of UIKit with Hopper to figure that out.)

So, notice two things:

• The if statement condition can only be true if selected == 0 or selected == 1. It will never be true if selected has any other value.

• The assignment statement (that updates _controlFlags) only uses bit 0 (the 1’s bit) of selected. So, for example, if selected == -2, which is logically true in C and has every bit set except bit 0, the assignment statement will still not turn on the bit in _controlFlags.

This means that you must pass 0 or 1 to -[UIControl setSelected:]. No other value will work reliably.

The shortest way to convert all non-zero values to 1 in C is by applying the ! operator twice:

[self.boldButton setSelected:!!isBold];

However, it would probably be better to fix your -isBold method to return a “safe” BOOL instead:

- (BOOL)isBold {
    CTFontRef fontRef = (CTFontRef)CFBridgingRetain(self);
    CTFontSymbolicTraits symbolicTraits = CTFontGetSymbolicTraits(fontRef);
    return !!(symbolicTraits & kCTFontTraitBold);
}